Page:The American Cyclopædia (1879) Volume XI.djvu/733

MOLECULE 715 tears the earth to pieces, and so does the chemist deal with the molecules; but to the astronomer the earth is a unit, and so is the molecule to the physicist. The word molecule, which means simply a small mass of matter, expresses our modern conception far better than the old word atom, which is derived from the Greek a privative and r^vw, and means therefore indivisible. In the paper just re- ferred to, Sir W. Thomson used the word atom in the sense of molecule, and this con- fusion of the two terms is still common. We shall give to the word atom an utterly differ- ent signification, which we must be careful not to confound with that of molecule. In our modern chemistry the two terms stand for wholly different ideas, and, as we shall see, the atom is the unit of the chemist in the same sense that the molecule is the unit of the phys- icist." (The writer of this article, in "The New Chemistry," page 35, New York, 1874.) The chemist studies the molecular theory from a point of view quite different from that of the physicist. To the physicist the mole- cules are the points of application of those forces which determine or modify the physical condition of bodies, and he defines molecules as the small particles of matter which under the influence of these forces act as units. To the chemist, on the other hand, the molecules determine those differences which distinguish substances. Sugar, for example, has the qual- ities which we associate with that name, be- cause it is an aggregate of molecules which have those qualities. Divide up a lump of sugar as small as you please : the smallest mass that you can recognize still has the qualities of sugar ; and so it must be if you continue the division down to the molecules ; and so is it with every substance. It is the molecules in which the qualities inhere. Hence the chem- ist's definition of a molecule: The smallest particles of a substance in which its qualities inhere. By no physical process, that is, by no process which leaves the qualities of a sub- stance essentially unchanged, is subdivision carried beyond the molecules. The molecules can however be divided, but by the division you destroy the substance; new substances result, and you have what is called a chemical process. The distinguishing feature of a chem- ical change is simply this : From one or more substances one or more new substances are produced, and according to the molecular the- ory these changes depend on the reciprocal action of different kinds of molecules on each other. The molecules become subdivided, and new molecules are formed by a new grouping of the fragments. And here chemistry comes in to substantiate the molecular theory with most important evidence. If in a chemical change the reaction takes place between mole- cules, then we should expect that the weights of the substances involved in the process would bear some simple relation to the weights of their molecules, as determined by physical methods. Now this is exactly what we find to be true. It is the great law of chemistry that in every chemical process definite proportions are preserved between the weights both of the factors and of the products of the change; and wherever observation is possible it has been found that these weights bear a simple relation to the specific gravities of the substances in the state of gas, which, as will be remembered, are the measures of the molecular weights of these substances. A few examples will illustrate this point. When hydrochloric acid gas com- bines with ammonia to form ammonic chloride (sal ammoniac), 3 6 -5 parts by weight of the former unite with 17 parts of the latter. Now the specific gravities of these two gases with reference to hydrogen are by observation 18-32 and 8-53 respectively, and the molecular weights 36-5 and 17 microcriths, very nearly. When water is decomposed, 9 parts by weight of water yield 8 parts of oxygen gas and 1 part of hydrogen. The specific gravities of the several substances in the state of gas are (by observation) 8'98, 15-95, and 1, and the weights of the molecules therefore 18, 32, and 2 microcriths, very nearly. Examples like these might be multiplied indefinitely. The relation is very simple. The chemical pro- portions are always very nearly either as the molecular weights, deduced from the gas densities, or as some simple whole multiples of these weights ; a fact which wonderfully confirms the molecular theory. But it may be asked, why is not the relation just de- scribed absolute ? First, because a certain amount of error of observation is inherent in all measurement, and the determinations of gas densities are peculiarly exposed to errors of this kind; and secondly, and chiefly, because the vapors, on which we mostly experiment, are not in the condition of perfect gases. They do not perfectly obey the law of Mariotte, and, as the molecular theory shows, it is only when they exactly obey this law that they contain the same number of molecules in the unit volume, and that their densities are the measures of their molecular weights. In the case of a few perfect gases the agreement be- tween the two classes of observations is very close ; but with most vapors, especially when near the point of condensation to liquids, we can only expect to find an approximation. It is evident moreover that the definite propor- tions of chemistry, which can be weighed with the greatest accuracy, are a far more exact measure of the molecular weights than the gas densities. Of course we cannot tell from these proportions alone whether they are the ratios between the weights of equal or of multiple numbers of molecules; but here the gas den- sities come to our aid, and fix approximately the values of the molecular weights, which wf need only correct by the chemical evidence. When, as is most frequently the case, the substances with which we are dealing are in- capable of existing in the state of gas, we