82 PROGRESS OF SCIENCE IN THE CENTURY. 



ways expressible by a fixed number or by a whole 

 multiple of that number. When elements unite with 

 one another in several different proportions — e.g., 

 oxygen and nitrogen — these proportions are related 

 to one another in a simple way. In other words, " If 

 two substances, A and B, form several compounds, of 

 which the compositions are all calculated with re- 

 spect to the same quantity of A, then the quantities 

 of B combined with this stand to each other in a 

 simple ratio " * (Law of constant equivalents and 

 multiple proportions). 



" Thou knowest no man can split an atom " was 

 one of Dalton's sayings, but it should be noted that 

 he meant by an atom the smallest conceivable particle 

 which exhibits the essential properties of the sub- 

 stance in question. Thus he spoke of an atom of 

 water (a compound, Hg O), just as he spoke of an 

 atom of carbon. 



With a vision of the grained structure of matter 

 clearly before him, he supposed in his theory that 

 while every atom of a given simple substance is like 

 every other atom of that substance, the atoms of dif- 

 ferent substances have different weights; that in 

 chemical union of elements there is a grouping of 

 definite numbers of elemental atoms into more com- 

 plex atoms of compounds, and contrariwise in chemi- 

 cal decompositions ; and that the elements combine in 

 the proportions indicated by the relative weights of 

 their atoms or in multiples of these. This is the 

 atomic theory " which at once changed chemistry 

 from a qualitative to a quantitative science '^ (Ros- 

 coe). 



An examination of some of Dalton's manuscripts 

 has led Roscoe and Harden to the conclusion that 

 * Ladenburg, p. 55. 



