82 THE REALITIES OF MODERN SCIENCE 



showed that it had to be either one or the other. Thus 

 if he supplied more than 72 parts of nitric oxide to 100 

 parts of air there was a corresponding residue of nitric 

 oxide as well as of nitrogen. Similarly if less than 72 

 parts were employed the residue contained oxygen, 

 indicating that the amount of nitric oxide available 

 was insufficient to form compound particles with all 

 the particles of the 21 parts of oxygen. 



By this and other similar experiments Dalton estab- 

 lished the granular composition of matter. He sum- 

 marized these phenomena in his law of "multiple 

 proportions," the value of which to-day is largely 

 historical. It was the first satisfactory evidence as 

 to the molecular and atomic structure of matter. If 

 we start our study by accepting the concept of atoms 

 and molecules we need not burden our minds with its 

 formal expression. Combinations of atoms into mole- 

 cules must always involve whole numbers of atoms. 

 Molecules which differ in the number of similar atoms 

 which they contain must differ in properties both 

 physical and chemical. The converse is, however, 

 not always true. It may happen, particularly in the 

 case of molecules involving many similar atoms, that 

 different substances may be formed by the same com- 

 bination of atoms, just as different words may be 

 formed by the same combination of letters. The same 

 combination of atoms as forms the molecule of alcohol 

 may also be formed into a molecule of " methyl ether," 

 an ether somewhat like the anaesthetic. 



After the molecular composition of matter had been 

 demonstrated by Dalton it was natural to assume that 

 equal volumes of dissimilar gases would contain, under 



