But. while the fundamental assumptions of the atomic theory thus establish a general relation be tween I he quantitative eomposition of substances and the relative weights of atoms. they do not furnish a sufficient basis for determining these relative weights in an unequivocal manner. When we apply the above proportions to some given substance say, watcr—flr the purpose of determining the weights of its ele ments. we find ourselves compelled to make some additional assumption. Indeed. chemical analy sis shows that NY:act. contains 11.1 per cent. of li•drogen and S5.9 per cent. of oxygen. We therefore have b = 11.1: S8.9 = (nearly) and hence : 1: ti, where .1 and .1' are, respectively, the weights of single atoms I if hydrogen and oxygen. while is and a' are, respectively, the numbers of atoms of these elements in a molecule of water. What are after i- the ratio .1: A'—i.e. the relative weights of single atoms: but this we evidently cannot find unless we assign some numerical altie to the ratio a: 0'. Dalton assumed that a molecule of water is made up of one atom of hydrogen and one of oxygen—i.e. a.= = 1, and therefore lie found : A': : 1: 8, i.e. an atom of oxygen is S times as heavy as an atom of hydrogen. (In reality. Dalton thus obtained, for the atomic weight of oxygen, the tignre 6: but this was due to his imperfect knowl edge of the proportion of hydrogen and oxygen in w a ter l.
Dalton's assumption was quite arbitrary. But in subsequent years. as the substances known grew numerous and complex, (demists began to Mel the want of some ovneral theoretical prin ciple which would arbitrary, and hence confusing, assumptions unnecessary. Then Ger hardt and Cannizzaro enriched Dalton's atomic theory by adding to it a principle which had been enunciated by Avogadro as early as t811, but which had remained unemployed as long as it was not urgently needed. According to Avo gadro. equal volumes of different gases contain
equal numbers of molecules it the temperatures and pressures of the gases are the same. This theoretical principle. and its use in determining the atomic weights of the elements, have been explained at some length under ATOMIC WEtunTs and AvocAnao's Brix (qq.v.). and require no further discussion here. Suffice it to state that it. forms part of the very foundation of the pres ent atomic and molecular theory, and that it is involved in the discussion of most, if not all, problems of modern chemistry.
A still further addition was made to the fun damental hypotheses of the theory before it attained its maximum of possibilities. This last addition. gradually incorporated during the second half of the Nineteenth Century, consists of certain assumptions coneerning the combin ing forces of atoms. the number of such forces peculiar to the atom of each element, and the directions in which those forces act. (See below, midst' Chrmical Formulas: and see the articles VALENCY; CARBON and STEREO CIIEMISTIIY.) These assumptions, fainting the so-called 'doctrine of valency.' were adopted mainly because of the necessity of explaining the isomerism of organic compounds—i.e. t:let that quite different compounds may have exactly the same And it. was mainly when by these assumptions that the atomic theory enabled us to know compounds before they have actually been found in nature or in a chemical laboratory.
"But," says Ostwald, "it seems as if the adapt ability of the atonic hypothesis is near exhaus tion. and it is well to realize that. according to the lesson repeatedly taught by the history of science. such an end is sooner or later inevit able." In the future, lie believes, chemists will speak a language free from the atomic or any other hypothesis, and based on nothing but ex perimental observations.