84 PROGRESS OF SCIENCE IN THE CENTURY. 



feet, to determine the relative weights of his hypo- 

 thetical ultimate particles, and drew up what would 

 now be called a table of atomic weights. 



To do this he required a unit of comparison, 

 and he chose hydrogen, the lightest kind of matter 

 known. The weight of an atom of hydrogen was 

 called one. Then, as 8 parts by weight of oxygen 

 combine with 1 part by weight of hydrogen to form 

 water (combining weights), Dalton argued that the 

 atom of oxygen weighed 8 times more than that of 

 hydrogen. And so on for other elements. 



It must be borne in mind that the atomic weights 

 were determined with reference to an arbitrary 

 standard, and that they had at first only approximate 

 accuracy. 



Summary. — Through the aid of many, but notably 

 through the pioneering genius of Dalton, the atomic 

 theory has won a place among the conceptual for- 

 mul£e of chemistry. It cannot be said to be proved ; 

 indeed, neither " proved '' nor " disproved " are ap- 

 propriate words to use in regard to these hypotheses. 

 The tests are convenience, comprehensiveness, and 

 consistency (at once with facts and with other con- 

 ceptions), and the atomic theory has stood these 

 tests. Forestalling the history a little, we may sum 

 up the general idea in Ostwald^s words : 



"^ All substances consist of discrete particles of 

 finite hut very small size — of atoms. Undecom- 

 posahle substances or elements contain atoms of the 

 same nature, form, and mass. If chemical combina- 

 tion takes place between several elements, the atoms 

 of these so arrange themselves that a definite and 

 usually small number of atoms of the combining 

 elements form a compound atom which we call a 

 molecule. Every molecule of a definite chemical 



