CHANGES OCCASIONED BY HEAT. 551 



proportional to the heat. De Luc was led, by his 

 experiments, to conclude "that the dilatations of 

 mercury follow an accelerated march for equal 

 augmentations of heat." Dalton conjectured that 

 water and mercury both expand as the square of 

 the real temperature from the point of greatest 

 contraction: the real temperature being measured 

 so as to lead to such a result. But none of the 

 rules thus laid down for the expansion of solids 

 and fluids appear to have led, as yet, to any 

 certain general laws. 



With regard to gases, thermotical enquirers have 

 been more successful. Gases expand by heat ; and 

 their expansion is governed by a law which applies 

 alike to all degrees of heat, and to all gaseous 

 fluids. The law is this: that for equal increments 

 of temperature they expand by the same fraction of 

 their own bulk ; which fraction is three-eighths in 

 proceeding from freezing to boiling water. This 

 law was discovered by Dalton and M. Gay-Lussac 

 independently of each other 1 ; and is usually called 

 by both their names, the lam of Dalton and Gay- 

 Lussac. The latter says 2 , " The experiments which 

 I have described, and which have been made with 

 great care, prove incontestibly that oxygen, hydro- 

 gen, azotic, nitrous acid, ammoniacal, muriatic acid, 

 sulphurous acid, carbonic acid, gases, expand equally 

 by equal increments of heat." " Therefore," he adds 

 with a proper inductive generalization, "the result 



1 Manch. Mem. vol. v. 1802 ; and Ann. Chim. xliii. p. 137- 



2 Ib. p. 272. 



