ACCORDANCE OF QUANTITATIVE THEORIES, <bc. 205 



subject, we have often a wide choice opened to us as 

 regards the methods of measurement, which may thence- 

 forth be made to give the most accurate results. If we 

 can only measure one fundamental quantity we may often 

 be able by correct theory to assign with accuracy a great 

 many other quantitative results. Thus, if we can once 

 determine satisfactorily the atomic weights of certain ele- 

 ments, we do not need to determine with equal accuracy 

 the composition and atomic weights of their several com- 

 pounds. When we have once learnt the relative atomic 

 weights of oxygen and sulphur we can calculate the 

 composition by weight of the several oxides of sulphur. 

 Chemists accordingly select with the greatest care that 

 compound of any two elements which seems to allow of the 

 most accurate analysis so as to give the ratio of their 

 atomic weights. It is obvious that we only need to have 

 the ratio of the atomic weight of each element to that of 

 some other common element, in order to calculate with 

 the greatest ease that of each to each. Moreover the 

 atomic weight stands in simple relation to other quanti- 

 tative facts. The weights of equal volumes of elementary 

 gases at equal temperature and pressure have the same 

 ratio as the atomic weights ; now as nitrogen weighs 1 4*06 

 times as much as hydrogen, under such circumstances 

 we may infer that the atomic weight of nitrogen is about 

 14*06 (probably 14*00) that of hydrogen being unity. 

 There is much evidence, again, to show that the specific 

 heats of elements, and even of compounds, are inversely 

 as their atomic weights, so that these two classes of quan- 

 titative data throw light mutually upon each other. In 

 fact the atomic weight, the atomic volume, and the atomic 

 heat of an element, are quantities so closely connected 

 that the determination of any one may lead to that of the 

 others. The chemist accordingly has to solve a most com- 

 plicated problem in deciding in the case of each of 60 or 



