A CENTURY OF CHEMISTRY. 85 



compound (chemical species) contains the same num- 

 ber of elementary atoms arranged in the same way. 

 If the same elements can unite to form different 

 compounds, the elementary atoms composing the 

 molecules of the latter are either present in differ- 

 ent numbers, or if their number be the same, they are 

 differently arranged" * 



DEVELOPMENT OF THE ATOMIC THEOEY. 



Dalton's atomic theory, though not final, was 

 fructifying. It prompted a long series of researches 

 which led, after some vicissitudes, to the establish- 

 ment of the atomic view of nature on a firmer and 

 broader basis. Among the steps of importance, we 

 may especially notice (1) the more accurate deter- 

 mination of atomic weights, (2) the conception of 

 molecules, (3) the kinetic theory of gases, and other 

 physical theories as to the different states of matter, 

 and (4) the development of organic chemistry. The 

 general problem was to form conceptions of material 

 architecture which would harmonise with the facts 

 of chemical change. 



Determination of Atomic Weights. It is well 

 known that each element is conventionally de- 

 noted by the first letter or letters of its Latin name, 

 and that with each element a certain number is 

 associated; e.g., 16 with oxygen, 14 with nitrogen, 

 12 with carbon. This number, or some multiple of it 

 by a whole number, expresses the relative quantity 

 of the given element which enters into compounds. 

 It is the combining mass (or weight, though weight 

 must vary with place), or on Dalton's theory, the 

 atomic mass or weight, 



*W. Ostwald, General Chemistry, trans. 1890. 



