even been made to treat " colloids " as a separate branch. To make chemistry of avail some change of attitude is desirable. The prime need of our time appears to be that we should recog- nize the essential unity of chemical science, in order that we may teach the fundamental principles and the syntactical issues as a single discipline. The characteristic of organic chemistry has been the attention paid to the determination of molecular struc- ture and to that of function, both chemical and physical, as an outcome of structure; too little attention has been paid by the inorganic chemist to these issues. It is essential that the con- ception of structure and the methods followed in determining structure in the case of the simpler compounds of carbon should be brought before the student at an early stage.
Ceasing to draw the invidious distinction now made by classing carbon apart, mainly because this element has so numerous a progeny, we shall with advantage treat each of the great family groups of elements as a separate stock or tribe, but take into account the graded interrelationship of families and the effects of unions between their members.
No science can work alone. The chemist in future will be associated either with the physicist or with the biologist, if not with both. In conjunction with the former, he will extend his studies of structure and function into atomic regions: the quest is one that seems to need the mathematical habit of mind. He will cooperate with the latter by applying his knowledge of molecular structure and function to the explanation of the living mechanism and of its activities as functions of structure even including those of mind: in this field the mathematical habit of mind seems to be almost out of place.
We may anticipate, wrote Liebig, more than 70 years ago, that from organic chemistry the laws of life the science of physiology will be developed. It is in this sense that we need to raise up a science of organic chemistry in future the organic chemist must once more be the proclaimed student of vital phenomena, not merely of materials. The two outstanding exponents of the art thus defined have been Liebig himself and Pasteur, the one having rendered supreme service by his general prescience, the other by demonstrating the essential inter- dependence of chemical and vital phenomena.
The great lesson we have thus far learnt is that the activity of nature is of a circumscribed character, far more so, in fact, than is that of the chemist in the laboratory. At some time choice has been made of particular types of material and definite lines on which alone action may proceed have been laid down. Nature has learnt to wear only a "single glove: all living things are essentially composed of one-handed (asymmetric) materials. The controversy long waged over spontaneous generation must be regarded as futile, in face of this conclusion. Whether the lines of action in nature are innate in the primary materials used, time alone can show: the chemist is tempted to think that this may well be, as within his own field of operation he finds that the structural possibilities are most definite in character and relatively few in number. The underlying policy of nature would seem to be the repetition of units of a simple kind. Tennyson has summed up the situation in the line
So careful of the type she seems,
and Pasteur, in the more definite comprehensive phrase, La vie est dominie par des actions dissymetriques.
Apparently the destinies of life are determined by the element carbon, which is distinguished from all others not merely by the multiplicity of its compounds but by their relative stability a stability, however, which is accompanied by remarkable plas- ticity. If there be life elsewhere, it can scarcely be very different from ours carbon seems to be the only possible nucleus element, the only one which can give rise to combinations imbued with the necessary stability and also sufficiently reactive.
Next to carbon, water is the factor of primary importance. The operations of dehydration and of hydration play the deter- mining part in the constructive process; next to these come those of oxidation and reduction, which are but the separated activi- ties of those of hydration or its reverse.
The level of energy is raised by oxidation ; it is gradually lowered by successive " hydrations," as in the process of fermentation. Whilst the chemist is frequently forced to resort to high tem- peratures and high electromotive forces to produce his result, nature does most of her work at a low energy level. In only one operation is she helped by a transcendent, irresistible power that of solar radiations of short wave length: but this is the primal step in life and the energy taken in at this stage must suffice in all subsequent acts, as even that derived from oxygen is to be thought of as stored up in the same operation; the separa- tion of the oxygen from the natural system carbon dioxide plus water, now with the aid of chlorophyll but primarily through some simpler agency. Nothing is more wonderful than the silent, steady way in which the glucose, formed at the expense of the carbon dioxide present to the extent of only three ten-thousandths in the air surrounding the plant, is built up underground, in the dark and at atmospheric or a lower temperature, into starch as in the potato tuber, for example. In no way can the chemist imitate the act. Selective and directive influences are clearly at work: we have reason to believe that these are to be found in an enzymic mechanism.
The observations made, of late years, on the formation of minute amounts of formaldehyde and even of glucose on ex- posure of solutions of carbonic acid to rays of short wave length, are of little if any assistance in enabling us to follow the natural process. A complete mechanism is provided in the chlorophyll system but what this includes we do not know. The suggestion has been made that there is a factor at present unknown, as assimilation (measured by the amount of oxygen liberated) is less active in leaves brought into light when only a few days old than in leaves equally greened several days older.
Of chlorophyll itself much is now known. So long ago as 1864, the late Sir George Stokes came to the far-reaching conclusion that the chlorophyll of land plants is a mixture of four substances, two green and two yellow, which by proper treatment may each be obtained in a state of very approximate isolation. Most of the chemists who followed him succeeded only in isolating de- composition products, but Willstatter, who took up the inquiry in 1906, has shown that the inference of the great physicist was correct. He finds that all green plants contain Chlorophyll a, blue-black, in solution green-blue CssHyjOs^Mg Chlorophyll b, green-black, in solution pine green CssHyoOeNiMa
Carotene, orange-red crystals C 5
Xantophyll, yellow crystals C^HseOz
The brown algae contain a third yellow constituent, fucoxanthin C^HMOe though a very small proportion of b chlbrophyll. The pigment of the ripe tomato is an isomeride of carotene, lycopin. Egg-yellow is coloured by an isomeride of xanthophyll, lutein. Willstatter finds that there is less variation in the amount of chlorophyll in plants of different species than in leaves of any one plant of different age or subject to different conditions of exposure. The amount Varies from 0-6% to 1-2% of the dry weight and is usually about 0-8%, 0-6% being the a and 0-2% the b component. There is no noticeable variation during the day. The yellow pigment varies in amount between o-i and 0-2%, 0-07 to 0-12 being xanthophyll and 0-03 to 0-08 carotene. Expressed in molecular proportions, the a component is present in the ratio of 3 to that of i of the b variety ; the yellow pigments are present in the reversed ratio of i of carotene to 1-5-2 of the oxidized compound xanthophyll but the variation being greater between exposed and shaded leaves than in the chlorophylls. The ratio (a+b) of the chlorophylls to the yellow pigments (c+x) as a mean of all the determinations made is 3-56, the value for exposed leaves being 3-07 and for shaded 4-68. Only further inquiry can show whether the coloured components of the chloroplasts are all genetically connected and which have functional significance.
It is a striking fact that chlorophyll has the closest affinity with haemoglobin, the red colouring matter of blood, the central system of each being apparently a complex of four substituted pyrrole rings; the two compounds are so closely related, in fact, that they may be reduced to the same compound, athiopor-
