HISTORY OF THE ATOMIC THEORY. 41 



importance to determine not only the whole expansion of 

 each gas from two distant points, such as the freezing and 

 boiling, but likewise whether that expansion be uniform in 

 every part of the scale, they instituted a set of experiments 

 expressly for those purposes. The result of which was; 

 that betwixt the temperatures of 32° and 212°, the whole ex- 

 pansion of one gas diifers much from that of another, it being 

 in one instance about 4-lOths of the original, and in others, 

 more than twelve times that expansion ; and that the expansion 

 is much more for a given number of degrees in the higher 

 than in the lower part of the scale. These conclusions were 

 so extremely discordant with and even contradictory to those 

 of others, that I could not but suspect some great fallacy in 

 them, and found it in reality to be the fact ; I have no doubt 

 it arose from the want of due care to keep the apparatus and 

 materials free from moisture." 



After giving his experiments on air, hydrogen, oxygen, 

 carbonic and nitrous gas, in which " the small differences 

 never exceeded six or eight parts, on the whole 345," he 

 adds, " Upon the whole, therefore, I see no sufficient 

 reason why we may not conclude that all elastic fluids under 

 the same pressure expand equally by heat, and that for any 

 given expansion of mercury, the corresponding expansion of 

 air is proportionally something less, the higher the tempe- 

 rature." 



'' This remarkable fact that all elastic fluids expand the 

 same quantity in the same circumstances, plainly shews that 

 the expansion depends solely upon heat; whereas the expan- 

 sion in solid and liquid bodies seems to depend on an adjust- 

 ment of the two opposite forces of heat and chemical affinity, 

 the one a constant force in the same temperature, the other 

 a variable one, according to the nature of the body ; hence the 

 unequal expansion of such bodies. It seems, therefore, that 

 general laws respecting the absolute quantity and the nature 

 of heat, are more likely to be derived from elastic fluids than 

 



