Popular Science Monthly/Volume 6/December 1874/Odors and Life




DESCARTES, Leibnitz, and all the great minds of the seventeenth century, believed that phenomena are such interdependent parts of one whole, that they require to be explained by each other, and consequently, that a very close mutual connection should be maintained among the sciences. In their view, this was the condition of rapid advance and intelligent development. The experimental method, constant to systematic obstinacy in erecting so many barriers between the different sections of natural philosophy, has greatly hindered the completeness of whatever knowledge we possess as the result of mutual interaction among all truths. At this day, such barriers are tending to vanish of their own accord, and the science of man in his relations with external media begins to show the outlines of its plan and harmony. We have before this sketched several of its chapters, and we will endeavor now to write another, on the subject of odors.


The seat of smell, or the olfactory sense, is the pituitary membrane lining the inner wall of the nostrils. It is a mucous surface, laid in irregular wrinkles, and receiving the spreading, slender, terminal filaments of a certain number of nerves. This membrane, like all other mucous ones, constantly secretes a fluid designed to lubricate it. By the aid of the muscles covering the lower part of the nostrils, the apparatus of smelling can be dilated or contracted, precisely like that of sight. This understood, the mechanism of olfaction is quite simple. It consists in the contact of odorous particles with the olfactory nerve. These particles are conveyed by the air to the inside of the nasal cavities, and there strike upon the sensitive fibres. If the access of air is prevented, or if the nerve is altered, no sensation is produced. Experiments in physiology, in fact, have settled that the olfactory nerves (or those of the first pair) are assigned exclusively to the perception of odors. Loss of the sense of smell occurs whenever the nerves are destroyed or injured by any process, or even whenever they are merely compressed. On the other hand, it is a matter of common observation that impeding the passage of air into the nostrils is quite as effectual a way of making any sort of olfactory sensation impossible. Let us add, that the region most sensitive to odors is that of the upper part of the nasal cavities. There are, as we shall notice in proceeding, considerable differences as regards the degree of sensitiveness in this sense of smell, comparing one man with another. But it is a still more singular fact that sometimes, without apparent cause, the sense is utterly wanting. In other cases it is unaffected by the action of certain odors only, an analogous infirmity to that which students of the eye call daltonism, and which consists in the perception of certain colors only. We find in scientific annals the case of a priest who was insensible to all odors except that of a manure-heap, or that of decayed cabbage; and another, of a person to whom vanilla was entirely without scent. Blumenbach speaks, too of an Englishman, with all his senses very acute, who perceived no perfume in mignonette.

Olfaction is sometimes voluntary, sometimes involuntary. In the former case, by an act which is called scenting something, and is resorted to for the sake of a keener sensation, we first close the mouth, and then sometimes draw in a full breath, sometimes a succession of short, quick inspirations. Then the muscular apparatus edging the opening of the nostrils comes into play, to contract that orifice, and point it downward, so as to increase the intensity of the current of inhaled air. When, on the contrary, we wish to smell as little as possible, the organ becomes passive. We effect strong expirations by the nose to drive out the air that produces scent, and inhalation, instead of being performed by the nostrils, instinctively takes place through the mouth.

Scents and the sense of smell have an important share in the phenomena of gustation, that is, there is a close connection between the perception of odors and that of tastes. Physiological analysis has clearly brought out the fact that most of the tastes we perceive proceed from the combination of olfactory sensations with a small number of gustatory sensations. In reality, there are but four primitive and radical tastes—sweet, sour, salt, and bitter. A very simple experiment will convince us of this fact. If we keep the nostrils closed when tasting a certain number of sapid substances, so as to neutralize the sense of smell, the taste perceived is invariably reduced to one of the four simple savors we have just named. Then, whenever the pituitary membrane is out of order, the taste of food is no longer the same; the tongue distinguishes nothing but sweet, sour, salt, or bitter.

It is time now to begin the study of the. physiological and chemical conditions of smell, and for this we must first inquire how odorous substances behave with regard to the medium which separates them from our organs. Prévost, in an essay published in 1799 on the means of making emanations from odorous bodies perceptible to sight, was the first to bring to view the fact that certain odorous substances, solid or fluid, placed on moistened glass, or in a saucerful of water, instantly act on those molecules of the liquid which they touch, and repel them more or less, producing a vacuum. He judged that this method might serve to make odors sensible to sight, and enable us to distinguish odorous from inodorous bodies. These movements of odorous bodies on the surfaces of liquids, of which camphor particularly gives so curious an instance, have lately been studied with the greatest care by a French physiologist, with a view to establishing a theory of odors. With this purpose Liégeois has examined most of the odoriferous substances, and has ascertained that almost all of them perform various motions of circulation and displacement on the surface of water, resembling those noted with camphor. Some act precisely as camphor does. Among these are benzoic acid, succinic acid, the rind of bitter oranges, etc. With others, motion soon stops, for they are quickly surrounded by an oily film which keeps them confined. Some must be reduced to powder before the phenomenon takes place. As regards odorous liquids, it occurred to Liégeois to saturate very light and spongy seeds, themselves odorless, with them, and he then found, on throwing the seeds on water, that circulatory and displacing movements took place, as with other substances. He concluded, from a series of experiments methodically tried, that the motions in question must be attributed, not to a release of gas, acting in the manner of a recoil, but simply to the separation and rapid diffusion, within the water, of the odorous particles. The volatility of substances cannot be admitted to have any part in explaining the phenomenon. It depends wholly on the affinity of fluids for the odorous particles, and also for those of fatty matter. Liégeois found, for instance, that a drop of oil put on the surface of water, without sensibly lessening in size, emits an enormous quantity of microscopic droplets, which are diffused through the mass of the water. Aromatic essences produce a like effect. Though insoluble in water, they have a powerful tendency to disperse themselves throughout it, and water that receives a very small quantity of the odoriferous principle, in the shape of extremely fine powder, has enough to gain their perfume completely. Liégeois's experiments give proof of the most diligent labors and of praiseworthy sagacity. Science has accepted them with satisfaction, and, after employing them usefully, will preserve the memory of their author, taken away in the flower of his age, at the outset of a noble career as a physiologist and surgeon.

It seemed, to quote his words, as though in these experiments we were assisting at the formation of the odorous molecules. Those delicate atoms emitted from odorous substances and diffused through the atmosphere are, in fact, the very same that impinge on our pituitary membrane, and give us the sensation of odors. Moreover, facts long ago observed display this revealing action, so to call it, of water upon odors. At morning, when the verdure is moist and the flowers covered with sparkling pearls of dew, a fresher and balmier fragrance exhales from every plant. It is the same after a light shower. Vegetation gains heightened tints, at the same time that it diffuses more fragrant waves of perfume. We remark an effect of the same kind in the physiological phenomenon of taste. The saliva serves as an excellent vehicle for diffusing the odorous principles; then the movements of the tongue, spreading that fluid over the whole extent of the cavity of the mouth, and thus enlarging the evaporating surface, are clearly of a kind to aid the dispersion of the odorous principles, which, as we have seen, take a considerable part in the perception of tastes.

Now, in the phenomenon of smell, air acts in the place of water. It seizes the odorous particles and brings them into contact with the pituitary membrane. It is the vehicle, the solvent, of those extremely subtile atoms which, acting on the delicate fibres of the nerve, produce in it a special movement, which translates itself into the most varied sensations. Oxygen, and the existence in that gas of a certain proportion of odorous molecules, are the two essential conditions of this phenomenon.

Such is, at least, the result of earlier experiments, and of those performed of late years by Nicklès. A curious fact, well worthy of attention, is the remarkable diffusibility and degree of subdivision exhibited by some odorous substances. Ambergris just thrown up on the shore spreads a fragrance to a great distance, which guides the seekers after that precious substance. Springs of petroleum-oil are scented at a very considerable distance. Bartholin affirms that the odor of rosemary at sea renders the shores of Spain distinguishable long before they are in sight. So, too, every one knows that a single grain of musk perfumes a room for a whole year, without sensibly losing weight. Haller relates that he has kept papers for forty years perfumed by a grain of amber, and that they still retained the fragrance at the end of that time. He remarks that every inch of their surface had been impregnated by 12691064000 of one grain of amber, and that they had perfumed for 11,600 days a film of air at least a foot in thickness. Evidently the material quantity of the odorous principle contained in a given volume of such air is so minute as to elude imagination. We can readily conceive how philosophers cite such instances to give a notion of the divisibility of matter.

In fact, we are now considering matter emitted by odorous bodies. This shows that they do not act as centres of agitation, occasioning vibrations which pass in waves to our organs, to exert on them a purely dynamic influence. This giving off of odorous matter, with the necessary aid of oxygen in the atmosphere, proves, too, that odors are in no respect comparable to light or heat, which one may regard in an abstract way, in the immaterial and ethereal space which is the region of their motion, as proper forces, and acting from a distance. Odors, to be perceived, must be taken up by oxygen, and borne by it to the organ of smell. In a word, odor is the odoriferous particle itself, while light is not the light-giving body.

Does oxygen exert a chemical influence on those atoms of which it robs odorous substances? We do not know, neither do we know of what kind is the action which occurs on the contact of odor with the olfactory nerve, whether the phenomenon is a mere mechanical agitation, or whether some chemical decomposition takes place in the case. At any rate, it is allowable to reason from the observed facts that smell and taste are two senses peculiarly distinct from the others, as well with respect to the object of sensation as to the ideas which the mind derives from the sensation itself. Sight, touch, and hearing, in a manner physical senses, furnish us the ideas of external forms, harmonies, and motions. They introduce us to the conception of the beautiful, and are true fellow-laborers with the intellect. Taste and smell are rather chemic senses, as Nickles calls them. They come into action only upon contact, and awake in us only such sensations as life and mind gain no profit from. While the former are the spring of the highest functions, the latter are of use only for the performance of acts of nutrition.

The learned and capable author[1] of a book on odors, published within a few years, fancies, however, that he can establish a kind of aesthetics of odors, more or less resembling that of tones. He has investigated olfactory harmonies, hoping to find in them the elements of a sort of music. "Odors," he says, "seem to affect the olfactory nerves in certain definite degrees, as sounds act on the auditory nerves. There is, so to speak, an octave of smells, as there is an octave of tones; some perfumes accord, like the notes of an instrument. Thus almond, vanilla, heliotrope, and clematis, harmonize perfectly, each of them producing almost the same impression in a different degree. On the other hand, we have citron, lemon, orange peel, and verbena, forming a similarly associated octave of odors, in a higher key. The analogy is completed by those odors which we call half-scents, such as the rose, with rose-geranium for its semitone; 'petit-grain' and neroli, followed by orange-flower. With the aid of flowers already known, by mixing them in fixed proportions, we can obtain the perfume of almost all flowers." In accordance with these fancies, Piesse has formed gamuts of odors, parallel with musical gamuts, and exhibiting concords of scents at the same time with those that produce discords. As a painter blends his tints, the perfumer should blend his fragrances; and Piesse maintains he can only gain that object by following the laws of harmony and contrast in odors. This theory is certainly quite ingenious, and deserves attention, but it is open to serious objections. If the harmony of colors and of sounds exists, it is because optics and acoustics are exact sciences, and harmony in this case is reduced to numerical relations, determined in a positive way. These relations, as concerns odors, can have no other basis than a capricious and relative sensibility. They are thus incapable of being reduced to form, a fortiori of being translated into fixed precepts.

To complete these details, it remains to say something of the delusions of the sense of smell; for this sense, like the others, has its aberrations and hallucinations. The delusions of smell are hardly ever isolated; they accompany those of hearing, sight, taste, and touch, and are also less frequent than the latter. Insane people, who are affected by them, complain of being haunted by fetid emanations, or congratulate themselves on inhaling the most delicious perfumes. Lelut mentions the case of a woman, an inmate of la Salpêtrière, who fancied that she constantly perceived a frightful stench proceeding from the decay of bodies she imagined buried in the courts of that institution. Impressions of the kind are usually very annoying. Brierre de Boismont relates the account of a woman affected by disorder of all her senses. Whenever she saw a well-dressed lady passing, she smelt the odor of musk, which was intolerable to her. If it were a man, she was distressingly affected by the smell of tobacco, though she was quite aware that those scents existed only in her imagination. Capellini mentions that a woman, who declared that she could not bear the smell of a rose, was quite ill when one of her friends came in wearing one, though the unlucky flower was only artificial.

Such facts might be multiplied; but, as they are all alike, it is not worth while to mention more of them. The latest observations made in insane asylums, among others, those of M. Prévost, at la Salpêtrière, have shown also that these delusions and perversions of the sense of smell are more common than had hitherto been supposed among such invalids, and that if they usually pass unnoticed, it arises from the fact that nothing spontaneously denotes their existence.

The intensity and delicacy of the sense of smell vary in mankind among different individuals, and particularly among different races of men. While some persons are almost devoid of the sense of smell, others, whose history is related in the annals of science, have displayed a refinement and range in the distinction of odors truly wonderful. Woodward, for instance, mentions a woman who foretold storms several hours before their coming, by the help of the sulphurous odor, due probably to ozone, which she perceived in the atmosphere. The scientific journals of the day relate the account of a young American girl, a deaf-mute, who, by their odor alone, recognized the plants of the fields which she collected. Numerous instances, moreover, prove that in savage races this sense is very greatly more developed than among civilized men. It is a traveler's story, that some tribes of Indians can pursue their enemies and animals of the chase by mere scent.

But it is among the other mammals that we find the sense of smell displayed in its highest degree of power and perfection. Among ruminants, some pachyderms, and particularly among carnivorous mammals, the olfactory membrane attains the keenest sensitiveness. Buffon has described these animals with extreme exactness, in saying that they smell farther than they see, and that they possess in their scent an eye which sees objects not only where they are, but even wherever they have been. The peculiarity of scent in the dog is too well known to need more than an allusion.

If we can hardly give faith to those ancient historians who relate that vultures were attracted from Asia to the fields of Pharsalia by the smell of the corpses heaped together there after a famous battle, yet we must accept the assertions of naturalists so well qualified to observe as, for instance, Alexander von Humboldt. The latter relates that in Peru, and other countries of South America, when it is intended to take condors, a horse or cow is killed, and that in a short time the smell of the dead animal attracts a great number of these birds, though none had before that been seen in the country. Other more extraordinary facts are told by travelers. These must usually be received only with the greatest caution, because in most cases the sense of smell gains credit for what is due to the sense of sight, which, with these birds, is very keen and far-reaching. Yet, making allowance for exaggeration, it must be admitted that these animals have a very highly-developed sense of smell. Scarpa, who has made admirable researches on this subject, found that they refuse food which is saturated with odorous substances, and, as an odd instance, that a duck would not swallow perfumed bread till after it had washed it in a pond. The waders, which have the largest olfactory nerves, are also those birds that display the greatest keenness of scent. Reptiles have very large olfactory lobes, leading us to believe that they discern odors readily, but at present we know little of the impressions they are sensitive to in this respect. Fish also have an olfactory membrane. Fishermen have always remarked that they may be attracted or driven off by throwing certain odorous substances into the water. Sharks, and other voracious fish, collect in crowds and follow from very far about a body thrown into the sea. It is even said that, when blacks and whites are bathing together in latitudes where these fish abound, they particularly single out and pursue the more strongly odorous blacks. Nor are the crustacea indifferent to emanations which act on the olfactory nerve. The method used for attracting and taking crabs is familiar.

Regarding the lower animals we have only still more uncertain information, except as to insects. Entomologists maintain that scent is very delicate in most insects, and rely on plausible conjectures on this subject, but they do not as yet know what the seat of the sense of smell in insects is. When meat is exposed to the air, in a few moments flies make their appearance in a place where none had before been seen. If refuse matter or bodies of animals are left on the ground, insects flock to them at once, feeding on such substances, and depositing their eggs in them. Scent alone seems to guide them, exclusively of sight even, for, if the object of their desire is hidden, they easily manage to find it. A curious fact as to the scent of insects is furnished by those kinds that prefer decaying substances. A beautiful arum is found in our woods, the cuckoo-pintle, whose white flower diffuses a disgusting odor. Now, the inside of this flower is often filled with flies, snails, and plant-lice, seeking the putrid source of this fetid smell. We may see the little creatures, in quest of their food or of a fit place to lay their eggs, move about in all directions, and quit most unwillingly the flower whose scent has misled them.


Having thus learned what physiologists think of the sense of smell and the conditions of the perception of odors, let its see what naturalists and chemists have ascertained respecting the latter as viewed in themselves, what place they give to odorous bodies, and what character they attribute to them all. The three kingdoms possess odors. Among mineral substances, few solids, but quite a number of liquids and gases, are endowed with more or less powerful scents, in most cases not very pleasant ones, and usually characteristic. Those odors belong to simple substances, such as chlorine, bromine, and iodine; to acids, as hydrochloric and hydrocyanic acid; to carburets of hydrogen, as those of petroleum; to alkaline substances, ammonia, for instance, etc. The odors observable among minerals may almost all be referred either to hydrocarbonic or hydrosulphuric gases, or to various solid and liquid acids produced by the decomposition of fats, or to peculiar principles secreted by glands, such as musk, ambergris, civet, and the like. Vegetables present quite another variety of odors, from the faintest to the rankest, from the most delicious to the most disgusting. Absolutely scentless plants are very rare, and many, that seem to be so while they are fresh, gain, on drying, a very decided perfume.

The odor of plants is due to principles very unequally distributed throughout their different organs; some solid, as resins and balsams, others which are liquid, and known by the name of essences or essential oils. In most cases the essence is concentrated in the flower, as occurs with the rose and the violet. In other plants, as in bent-grass and Florence iris, only the root is fragrant. In cedar and sandal wood, it is the wood that is so; in mint and patchouli, the leaves; in the Tonquin bean, the seed; in cinnamon, the bark, which is the seat of the odorous principle. Some plants have several quite distinct fragrances. Thus the orange has three: that of the leaves and fruit, which gives the essence known by the name of "petit-grain;" that of the flowers, which furnishes neroli; and again the rind of the fruit, from which essence of Portugal is extracted. A great number of vegetable odors belong exclusively to tropical plants, but the flora of Europe furnishes a large proportion of them, and almost all the essences used in perfumery are of European origin. England cultivates lavender and peppermint largely. At Nîmes, gardeners are particularly attentive to rosemary, thyme, petit-grain, and lavender. Nice has the violet for its speciality. Cannes extracts all the essences of the rose, the tuberose, cassia (the yellow acacia), jasmine, and neroli. Sicily produces lemon and orange; Italy, bergamot and the iris.

What, now, is the chemical nature of the odorous principles in plants? The chemistry of to-day reduces almost all of them to three categories of well-ascertained substances: hydrocarburets, aldehydes, and ethers. We will endeavor to give a clear account of the constitution of these three kinds of substances, and to mark their place in the register of science. The hydrocarburets are simple combinations of carbon and hydrogen, as, for instance, the petroleum-oils. They represent the simple compounds of organic chemistry. As to aldehydes and ethers, their composition is rather more complex; besides carbon and hydrogen, they contain oxygen. Every one knows what chemists mean by an alcohol; it is a definite combination of hydrogen, carbon, and oxygen, neither acid nor alkaline, which may be regarded as the result of the union of a hydrocarburet with the elements of water. Common alcohol, or spirits of wine, is the type of the most important series of alcohols, that of the mono-atomic alcohols. Chemists represent it by the formula C2H6O, to indicate that a molecule of it arises from the union of two atoms of carbon with six atoms of hydrogen and one of oxygen. Independently of the alcohols, which are of great number and varying complexity, organic chemistry recognizes another class of bodies, of which vinegar is the type, and which receive the name of organic acids, to mark their resemblance to mineral acids, such as oil of vitriol or aqua-fortis. Now, every alcohol, on losing a certain amount of hydrogen, gives rise to a new body, which is called an aldehyde; and every alcohol, on combining with an acid, produces what is called an ether. These rapid details allow us to understand precisely the chemical character of the essences or essential oils which plants elaborate within their delicate tissue. Except a small number among them which contain sulphur, as the essences of the family of crucifers, they all present the same qualitative composition—carbon and hydrogen, with or without oxygen. Between one and another of them merely the proportion of these three composing elements varies, by regular gradations, but so as always to correspond either to a hydrocarburet, or to an aldehyde, or to an ether. In this case, as in almost the whole of organic chemistry, every thing is in the quantity of the composing elements. The quality is of so little importance to Nature, that, while following always the same laws, and constantly using the same materials, she can, by merely changing the ponderable relations of the latter, produce, by myriads of various combinations, myriads of substances which have no resemblance to each other. The strange powers of the elements and the mysterious forces concealed in matter make themselves known to us in a still more remarkable phenomenon, to which the name of isomery is given. Two bodies, thoroughly unlike as regards their properties, may present absolutely the same chemical composition with respect to quality and quantity of elements. "But in what do they differ?" it may be asked. They differ in the arrangement of their molecules. Coal and the diamond are identical in substance. Common phosphorus and amorphous phosphorus are one and the same in substance. Now, the odorous principles of plants offer some exceedingly curious cases of isomery. Thus the essence of turpentine, the essence of lemon, that of bergamot, of neroli, of juniper, of savin, of lavender, of cubebs, of pepper, and of gillyflower, are isomeric bodies, that is, they all have the same chemical composition. Subjected to analysis, all these products yield identical substances in identical proportions, that is, for each molecule of essence, ten atoms of carbon, and sixteen atoms of oxygen, as denoted by their common formula, C10O16. We see how these facts as to isomery prove that the qualities of bodies depend far more on the arrangement and the inner movements of their minute particles, never to be reached by our search, than on the nature of their matter itself; and they show, too, how far we still are from having penetrated to the first conditions of the action and forces of substances. Among odoriferous essences placed by chemists in the class of aldehydes may be named those of mint, rue, bitter almonds, anise, cummin, fennel, cinnamon, etc. The rest are ranged in the great series of ethers, which vary greatly in complexity, notwithstanding the simple uniformity of their primary elements.

Such is the chemical nature of most of the odorous principles of vegetable origin. But chemistry has not stopped short with ascertaining the inmost composition of these substances; it has succeeded in reproducing quite a number of them artificially, and the compounds thus manufactured, wholly from elements, in laboratories, are absolutely identical with the products extracted from plants. The speculations of theory on the arrangements of atoms, sometimes condemned as useless, do not merely aid in giving us a clearer comprehension of natural laws, which is something of itself, but they do more, as real instances prove; they often give us the key to brilliant and valuable inventions. An Italian chemist, who was then employed in Paris, Piria, in 1838, w r as the first who imitated by art a natural aromatic principle. By means of reactions suggested by theory, he prepared a salicilic aldehyde, which turned out to be the essence of meadow-sweet, so delicate and subtile in its odor. A few years later, in 1843, Cahours discovered methyl salicilic ether, and showed that it is identical with the essence of wintergreen. A year after, Wertheim composed essence of mustard, while believing himself to be making only allylsulphocyanic ether. These discoveries produced a sensation. Nowadays the chemist possesses the means of creating many other natural essences. Common camphor, essence of bitter-almonds, that of cummin and of cinnamon, which are aldehydes, as we have seen, may be prepared without camphor-leaves or almonds, without cummin or cinnamon. Besides these ethers and aldehydes whose identity with essences of vegetable origin has been proved, there exist, among the new bodies known to organic chemistry, a certain number of products formed by the union of common alcohol or amylic alcohol with different acids, that is to say, of ethers, which have aromatic odors more or less resembling those of some fruits, but as to which it cannot yet be affirmed that the odors are due to the same principles in both cases. However this may be, perfumers and confectioners, more industrious and wide-awake than chemists, have immediately made good use of these properties. Artificial aromatic oils made their first appearance at the World's Fair of London in 1851. There was there exhibited a pear-oil, diffusing a pleasant smell like that of a jargonel, and employed to give an aroma to bonbons. This product is nothing else than a solution of amylacetic ether in alcohol. Apple-oil was exhibited beside the pear-oil, having the fragrance of the best rennets, and produced by dissolving amylvaleric ether in alcohol. The commonest essence was that of pineapple, which is nothing else than ordinary butyric ether. There was observed, too, an essence of cognac, or grape-oil, used to impart to poor brandies the highly-prized aroma of cognac. The product which Was then, and still is, the most important article of manufacture, is the essence of "mirbane," which very closely resembles in its odor that of bitter almonds, and which commerce very often substitutes for the latter. Essence of mirbane is nothing else than nitrobenzine, which results from the action of nitric acid on benzine. Benzine, in turn, is met with among the products of distillation of tar, which also yield the substances used in preparing those beautiful colors called aniline. Besides the essences we have just mentioned, which are gaining an increasing importance in the manufacturing arts, artificial essences of quinces are also prepared, and essences of strawberries, of rum, etc. All these preparations serve, it must be admitted, to give an aroma to the cordials, confectioneries, and sweetmeats, which are so largely sold nowadays. In other words, the products of industry are constantly taking the place of those of Nature more and more. In all these cases, these instances of composition of odorous principles are among the finest triumphs of organic chemistry. The creative power of the chemist is ever widening its range. After the labors of Piria, Wertheim, and Cahours, came those of Berthelot, who has imitated the fatty matters of the animal economy. We are at this moment in progress toward the artificial manufacture of sugar. If we succeed in that, nothing more will remain but to effect the composition of albuminous substances, in order to give us the complete mastery of the processes which Nature follows in her elaboration of immediate principles. That gift of making its object a reality, which is the peculiar privilege of chemistry, is also one of the strongest arguments to bring in proof of the absoluteness of those laws which we ascertain respecting the system of forces external to us.

Linnæus, whose mind was remarkably analytical and classifying, has not only arranged vegetables and animals in order, but has also classified diseases, and even odors. He refers the latter to seven classes: aromatic odors, such as that of laurel-leaves; fragrant, like those of lilies and jasmine; ambrosial, such as amber, musk, etc.; garlicky, like that of garlic; fetid odors, like those of the goat, the orrage, and others; disgusting odors, as those of many plants of the solaneæ order; and, last of all, nauseous odors. The terms of Linnæus have generally become current in language, but we understand, of course, that their value is merely conventional. As we have said before, there is no standard for the comparison of odors. We can only describe them by making comparisons between them, according to the degrees of resemblance existing between the impressions with which they affect our olfactory membrane. They have no qualities capable of being rigorously defined. This is the reason why it is impossible to give them any natural classification.


The sensations produced by smells are perceived and judged of in a great variety of ways, though with less of difference than prevails as to tastes. "I have seen a man," says Montaigne, "fly from the smell of apples quicker than from a cannonade." The instance he alludes to in this passage is that of Quercet, Francis I.'s secretary, who rose from table and took flight whenever he saw apples upon it. History tells us that Louis XIV. could not bear perfumes. Grétry was greatly annoyed by the odor of roses; that of a hare caused Mdlle. Contat to faint. Odors which disgust us, like that of asafœtida and of the valerian-root, are on the contrary highly enjoyed by the Orientals, who use these substances for condiments. Among-other singular instances related by Cloquet on this subject, we will mention that of a young girl who took the greatest delight in inhaling the scent of old books, and that of a lawyer to whom the exhalations of a dunghill yielded the most agreeable sensations. So that it is out of our power to fix general rules with respect to the influence of odors on our organs, and the character of the sensations which they effect in us; still, from a purely physiological point of view, it is certain that some of them exercise a uniform influence. Chardin and other travelers mention that, when musk-hunters take from the animal the pouch containing musk, they must have the nose and mouth covered by a cloth doubled in several folds, if they would escape violent hæmorrhage.

The smell of the lily, the narcissus, the tuberose, the violet, the rose, the elder, etc., when it reaches a certain point of concentration, usually exerts an injurious influence on the system. It occasions more or less severe headaches, fainting-fits, and sometimes even more serious disorders. Some odors, which have an agreeable perfume in a state of considerable diffusion, gain when concentrated a noxious and sometimes dangerous smell. This is particularly true of civet, patchouli, and the essences of neroli and thyme. Scientific records mention several cases of death occasioned by the poisonous action of some odorous emanations. It has been remarked that plants of the family of labiates, such as sage, rosemary, etc., offer in this respect no sort of risk, and seem rather to enjoy wholesome properties. Yet it is of consequence at this point to distinguish between the action of the odor which is in a manner purely dynamic, the intoxication from the essence, and the effect of carbonic acid thrown off by plants. These three influences have often been confounded by authors who have recorded accidents occurring after the inhalation, more or less prolonged, of odoriferous air.

This variable action of odors on the nervous system, sometimes wholesome, sometimes noxious, explains the part they have always played in the various circumstances of life among mankind. It would need a volume to relate the religious, political, economic, and gallant history of odors and perfumes. We must be content here with noticing its chief lessons, as far as they are connected with the physiological theory which is the basis of this study. For there is unquestionably something instinctive at the bottom of these general and uniform customs which exhibit the affinity of man for odors. Doubtless we must recognize in this rather a refinement of sensuality than a natural craving; but the same result has occurred in this case as in the instance of beverages, of music, etc. Habit has become in some sort a second nature; the senses have acquired a taste for that especial intoxication which beguiles them and disguises painful realities for them.

It is in religion, in the first place, that we observe the use of perfumes. Nothing holy or lofty was conceived of in which their influence was not present. Perfumes won the gods to give ear to the vows addressed to them in temples where burning incense diffused its fragrant clouds. From the highest antiquity we find that the priests of different religions avail themselves of the use of odoriferous substances. Five times a day the disciples of Zoroaster laid perfumes upon the altar where the sacred flame glowed. Moses, in Exodus, recorded the composition of two perfumes used in the sacred rites. The Greeks assigned a leading place to odors in their ingenious fictions of theology. They believed that the gods always declare their presence by an ambrosial fragrance, as Virgil tells us, in speaking of Venus;[2] and Moschus, describing Jupiter transformed to a bull. The use of perfumes in religious ceremonies had for its purpose the excitement of a sort of intoxication in the priests and priestesses, and also to disguise the smell of blood and of decaying matters, the offal of the sacrifices. The Christian religion borrowed from paganism the use of perfumes in the rites of worship. There was even a period at which the Church of Rome owned estates in the East devoted exclusively to plantations of trees yielding balsamic resins.

Besides these uses, odors were, in old times, still oftener employed in private life. Nothing surprises us more, in reading the ancient authors, than their relations on this subject. Among the Jews, the use of perfumes was restrained within proper limits, by the regulations of the Mosaic law, which consecrated them to worship. But, with the Greeks, it reached an extraordinary height and refinement. They kept their robes in perfumed chests. They burned aromatic substances during their banquets; they scented their wines; they covered their heads with fragrant essences at their festivals. At Athens, the perfumers had shops which were places for public resort. Apollonius, a scholar of Theophilus, left a treatise on perfumes which proves that, even as regards the extraction of essences, the Greeks had attained astonishing perfection. Neither Solon's laws nor Socrates's rebukes could check the progress of that passion. The Romans inherited it from Greece, and enlarged the stock of Eastern perfumes by those of Italy and Gaul. They used them profusely to give fragrance to their baths, their rooms, their beds, and their drinks. They poured them on the heads of guests. The awning shielding the amphitheatre was saturated with scented water, which dripped, like a fragrant rain, on the spectators' heads. The very Roman eagles were anointed with the richest perfumes before battle. At the funeral of his wife Poppæa, Nero burned on the pyre more incense than Arabia yielded in a whole year. It is related, too, that Plancius Plancus, proscribed by the triumvirs, was betrayed by the perfumes he had used, and thus discovered to the soldiers sent to pursue him. Besides the odors extracted from mint, marjoram, and the violet, which were the most common, the ancients made much use of the roses of Paestum, and various aromatic substances, such as spikenard, megalium, cinnamon, opobalsamum, etc.

It is singular to notice that the use of perfumes, brought to Rome with Grecian manners, was in its turn conveyed to France and Northern Europe with Latin manners, and chiefly by the Romish religion. It is from religious rites, indeed, that it passed into ceremonies of state, and thence into private life. Among the presents sent by Haroun-el-Raschid to Charlemagne were many perfumes. In the middle ages, among princes and men of highest rank, they washed their hands with rose-water, before and after eating; some even had fountains from which aromatic waters flowed. At this period, too, it was the custom to carry the dead to their burial-place with uncovered face, and to place little pots full of perfumes in the coffins. The French monarchy always showed an unrestrained passion for enjoyments of this nature, which seemed created as a necessary attendant upon all others. Marshal Richelieu had so extravagantly indulged his passion for perfumes under every form, that he had lost the perception of them, and lived habitually in an atmosphere so loaded with scents that it made his visitors ill. Madame Tallien, coming from a bath of juice of strawberries and raspberries, used to be gently rubbed with sponges saturated with perfumed milk. Napoleon I. every morning poured eau-de-Cologne, with his own hands, over his head and shoulders.


Above all these questions which we have just skimmed, there rises another, of a graver and more mysterious kind, one which occurs at the end of all studies that treat of sensation, and with regard to which some reflections will not be out of place here. To what, outside of us, do those sensations which we experience within us correspond? What relation is there between the real world and that image of the world shadowed in our soul? In the special case we are concerned with, what is it in these substances which is the cause why they affect our sense of smell? It seems certain, in the first place, that odor in itself, so far as it is odor, is a mere figment of our mind. Contemporaneous physiology proves that excitement of the nerves of sensation is followed, in each one, by the sensation that corresponds with each. When we electrify the eye, we call up in it an appearance of light; when we electrify the tongue, we produce in it a sensation of taste; when we electrify the inside of the ear, we provoke in it the effect of a sound. So, too, a similar excitement, electric or otherwise, of the olfactory nerves, creates in our mind the sensation of smell, even though no odorous molecule takes part in the phenomenon. Sensation, therefore, seems to depend chiefly on the nature of the sensitive nerve. The external world seems to contribute to it only by setting in motion the nerve-fibres. Even this condition of an impulse impinging from without is not indispensable, since, in sleep and in madness, we experience sensations of smell which, by the testimony of our other senses, answer to no external agent. Still, we believe that we can distinguish cases of hallucination from cases of true perception; still, we maintain that there are, outside of ourselves, distinct causes of our distinct sensations. No skepticism has prevailed, nor will prevail, against this testimony of the most powerful evidence which exists in our inmost being. How can we account for this apparent contradiction In reality, there is no contradiction. Observe, indeed, that, even if the most indifferent causes can effect in us one and the same sensation, and thus delude us as to the outer world, our soul is never cheated. It knows perfectly well how to refer this one sensation to the dissimilar objective causes which have affected it; in other words, the causes which are alike, and are confused in one in the purely physiological act of sensation, divide and grow distinct in the psychological act by which the soul recognizes them, and conceives them as different. If we had, to give us knowledge, only the dull and ignorant passivity of our senses, there would be no reality for us; but the wise activity of the soul can not merely assert the reality of outward objects, for a reason similar to that which makes it assert its own existence—it can still further argue, from its various modes of affection, to a corresponding variety of external forces. It moves in harmony with the world, rather than in harmony with the senses. In presence of the latter, it is like a good prince, who would be nothing without his subjects, but who regulates and civilizes them, by giving them laws, and ruling their morals. Thus, and this is the conclusion at which we aim, it is in the soul, regarded as the focus of all those rays refracted through the senses, as the central light outshining all others, that we must set the power and the right to discern what the senses do not discern, and to pierce to a depth forever beyond their reach. We shall never know what relation there is between the outward world and those images of it which we perceive, but the soul can hold the unshaken belief that the various points of those images correspond to points in the outer world situated in a like order, and that the forces which affect it are, in their essence, of the same nature as those forces of which, in its inmost depths, it feels itself the lord.

  1. Piesse, on "Odors, Perfumes, and Cosmetics."
  2. "Then, as the goddess turned, a rosy glow
    Flushed all her neck, and from her head the locks
    Ambrosial breathed celestial fragrance round."