84 THE REALITIES OF MODERN SCIENCE 



assumption. He, therefore, announced that equal 

 volumes of different gases at equal pressures and tem- 

 peratures contained equal numbers of molecules. He 

 also postulated the modern distinction between atoms 

 and molecules and stated that the molecules of an 

 element might be compounds of the atoms of the 

 element. 



Avogadro's assumption as to numbers of molecules 

 offered a very convenient method for comparing the 

 masses of the molecules or atoms of different substances 

 provided they were in the gaseous form. 1 It is possible 

 to make comparisons only, so the mass of the molecule 

 or atom of some gas must be taken as a standard. 

 Hydrogen as the lightest gas was so chosen. Con- 

 fusion, however, resulted until Cannizzaro's work, 

 because some investigators compared with the atom 

 and others with the molecule. Berzelius, a chemist 

 of influence, who investigated relative atomic weights, 

 denied Avogadro's idea, which conflicted with his 

 own assumptions of an electrical attraction between 

 the particles which combine in a chemical reaction. 

 His theories along this line have since proved to be erro- 

 neous and need not be discussed. 



The atomic weight of hydrogen was chosen as the 

 standard and called unity. Upon this basis the molec- 

 ular weight of hydrogen gas is 2. Oxygen was found 

 to be approximately 16 times heavier than hydrogen, 

 so that its molecular and atomic weights are about 

 32 and 16, respectively. Later it was agreed to express 



1 The atomic weights of non-gaseous elements are obtained by 

 weighing the constituents and the products of various chemical 

 reactions. 



