TUE _ATOMIC THEORY. Once we have made cer tain that we will not, by insufficient definition of our concepts, confound mixtures of substances with isolated chemical compomuls, we are ready to undertake the investigation of compounds, their physical and physiological properties, their composition. and their constitution and reactions —i.e. their chemical properties. The physical properties, such as color, crystalline form. solu bility in various solvents, the boiling or melting point, etc., serve the purpose of readily identify ing known compounds. A knowledge of the physiological properties of compounds is desir able, because compounds are often capable of therapeutic action, and may, therefore, be used in medicine. The theoretical chemist, however, is interested in all such properties only inas much as they are manifestations of the intimate nature of the compounds characterized by them, and his principal aim is to find a precise expres sion for the dependence of properties on chem ical composition and constitution.
The composition of a compound is revealed by chemical analysis, which shows (1) what, the constituent elements are (qualitative analysis) and (2) in what relative quantities those ele ments are contained in the compound (quantita tive analysis). A remarkable law that governs the quantitative composition of compounds be came known about -the beginning of the Nine teenth Century. This law, called `the law of multiple proportions,' may be enunciated in the following form: There is for every chemical ele ment a characteristic number that represents its combining weight : and the composition of any chemical compound may be represented either by the eombining weights of its elements or by simple multiples of those weights. Thus, using the combining weights as we know them at pres ent, we may state Ihe composition of a few compounds as follows: oxide is eom posed of 12 parts of carbon and 16 parts of oxygen; carbonic acid of 12 parts of carbon and 32 parts of oxygen: water of 2 parts of hydrogen and 16 parts of oxygen: marsh gas of 12 parts of carbon and 4 parts of hydrogen: (defiant gas k ethylene) of 2.1 parts of carbon and 4 parts of hydrogen. etc. The combining weights of the three elements mentioned are. approximately: carbon, 12; hydrogen. 1 oxygen. 16: and it is easy to see that in stating the composition of our compounds we have been able to use either these numbers or simple multiples of them. Dal
ton was led to the discovery of this law by the hypothesis according to whieh all matter is mode up of 'atoms'—i.e. of minute particles ineatiable of further subdivision. The atomic theory, based on this hypothesis. comprises the following as sumptions: The atoms of any given element arc identical: the atoms of different elements are different and have different weights; by the force of chemical affinity several atoms may be held in combination. forming a particle, or 'molecule; of a compound, and very large num bers of molecules are necessary to make up 01'11 the smallest amounts of compounds which we are actually capable of handling. In accord ance with these assumptions. let, M stand for the number of mol•cules making up a certain amount of some compound containing ele ments; and let the amount of the first element in it lie a, the amount of the second element b. Let. further. A stand for the weight of a single atom of one of the elements, and it stand for the number of such atoms contained in a single molecule of the compound. Then. evidently. the weight a of the first element in the compound equals .11 X n X .1. Similarly. if A' stand for die weight of a single atom of the second ele ment, and ?I' for the number of its atoms con tained in a single Illidecule of the compound. then. evidently, the total weight Ti of the second element in .11 molecules equals if X it' X A'. 'We therefore have a: b:: 11N'.1' or, a: it:: : n'.I' In ease a molecule of the compound should contain equal numbers of atoms of the two ele ments. n = n', and hence 0: b:: A : .1' But these proportions tell us that the weights (a and b) of the elements in a compound are proportional either to the weights of single atoms. (A and A'), or to multiples (tht and WA') of those weights. Thus the fundamental low of chemical composition follows as a direct consequence from. and is, heneeg completely ex plained by, the atomic hypothesis. without which it would be a mystery. The `combining weights' metititmed above in connection with our state ment of the law arc seen. in the light. of the hypothesis, to represent. the relative weights of the atoms themselves, and are therefore termed weights.