xv THE MOLECULAR THEORY 325 



air was gradually compressed from 1 2 to 3 inches, and the 

 pressures (including the atmospheric pressure of 29 T \ 

 inches) were measured for twenty-five different volumes of 

 the air. These pressures were found to agree very closely 

 with " what that pressure should be according to the Hypo- 

 thesis, that supposes the pressures and expansions to be in 

 reciprocal proportion." 



In the second experiment, on the debilitated force of ex- 

 panded air, a slender glass tube was immersed in mercury 

 and closed at the top with sealing wax, in such a way as to 

 enclose a column of air an inch long. The tube was raised 

 gradually until the air had expanded to 32 inches and the 

 position of the mercury was read for a series of nineteen 

 different volumes. Once again the pressures were found to 

 agree closely with those calculated from the Hypothesis. 



FIG. 46 TUBE USED IN BOYLE'S EXPERIMENTS ON THE CONDENSATION 

 OF THE AIR. 



Boyle's two tables are reproduced on p. 326, where all 

 the numbers represent inches. 



The Hypothesis, now known as BOYLE'S LAW, states that 

 if the temperature is constant the volume of a gas is 

 inversely proportional to its pressure. Although applied 

 at first only to air, it was found to be approximately 

 true for all other gases and vapours. But in no case is 

 it accurately true ; thus in the case of both hydrogen 

 and nitrogen the volume under a pressure of 300 atmos- 

 pheres is about 25 per cent, greater than that calculated 

 from Boyle's law. 



Gay-Lussac (1802) measures the expansion of gases by 

 heat. The fact that air is expanded by heat was well known 

 in the time of Boyle (see, for instance, Boyle's Works, II. 



