THE PRINCIPLES OP ORGANIC CHEMISTRY 37 



Dalton and to conceive of chemical substances as being made 

 up of aggregations of atoms which are known as molecules. 



Now in considering the various states of matter it is 

 evident that it is in the gaseous state that the molecules or 

 atoms are most widely separated ; thus, e.g., we know that a 

 comparatively small volume of water will give rise on boiling to 

 a considerable volume of steam. And it is from the study of 

 chemical substances in the gaseous state that our fundamental 

 conceptions of the properties of atoms and molecules and of 

 their relative weights have been chiefly derived. 



Before Dalton's time Boyle discovered that various gases, 

 though they might differ in composition, obeyed certain 

 simple laws. Thus Boyle found that if the pressure upon 

 a gas was doubled, its volume at the same temperature was 

 halved, and the statement that the volume of a gas varies 

 inversely with the pressure is known as Boyle's law. The 

 same generalisation was made by the Frenchman Mariotte. 

 It was further found by Gay Lussac that all gases expanded 

 equally for equal increments of temperature. 



Although later researches have shown that the laws of 

 Boyle and Mariotte and of Gay Lussac only hold strictly 

 within certain limits of temperature and pressure, yet they 

 afford clear evidence that gases possess essentially the same 

 general physical properties whatever be their composition. 



When it was further discovered by Gay Lussac that a 

 given volume of oxygen, say, when compared with a given 

 volume of hydrogen under the same conditions of tempera- 

 ture and pressure, was always sixteen times the weight of 

 the hydrogen, the conclusion was inevitable that a definite 

 relation existed between the volume of the gas and the 

 number of atoms in it. 



A satisfactory explanation of the properties of gases, and 

 of the relations which exist between the weights of equal 

 volumes of gases differing in composition, was afforded by the 

 Italian chemist, Avogadro, who enunciated the law that equal 



