158 HISTORY OF THERMOTICtt. 



in proceeding from freezing to boiling water. This law was disco\ered 

 by JJalton and M. Gay-Lussac independently of each other ;' and is 

 usually called by both their names, the law of Dal fan and Gay-Lussac. 

 The latter says, 2 " The experiments which I have described, and which 

 have been made with great care, prove incontcstably that oxygen, 

 hydrogen, azotic acid, nitrous acid, ammoniacal acid, muriatic acid, sul 

 phurous acid, carbonic acid, gases, expand equally by equal increments 

 of heat." " Therefore," he adds with a proper inductive generalization, 

 " the result does not depend upon physical properties, and I collect 

 that all gases expand equally ly heat" He then extends this to vapors, 

 as ether. This must be one of the most important foundation-stones 

 of any sound theory of heat. 



[2nd Ed.] Yet MM. Magnus and Regnault conceive that they have 

 overthrown this law of Dalton and Gay-Lussac, and shown that the 

 different gases do not expand alike for the same increment of heat. 

 Magnus found the ratio to be for atmospheric air, 1-366 ; for hydrogen, 

 1-365 ; for carbonic acid, 1-369; for sulphurous-acid gas, 1-385. But 

 these differences are not Greater than the differences obtained for the 



O 



same substances by different observers ; and as this law is referred to 

 in Laplace's hypothesis, hereafter to be discussed, I do not treat the 

 law as disproved. 



Yet that the rate of expansion of gas in certain circumstances is 

 different for different substances, must be deemed very probable, after 

 Dr. Faraday's recent investigations On the Liquefaction and Solidifi- 

 cation of Bodies generally existing as Gases, 3 by which it appears that 

 the elasticity of vapors in contact with their fluids increases at different 

 rates in different substances. " That the force," he says, " of vapor 

 increases in a geometrical ratio for equal increments of heat is true for 

 all bodies, but the ratio is not the same for all. . . . For an increase 

 of pressure from two to six atmospheres, the following number of 

 degrees require to be added to the bodies named :. water 69, sulphure- 

 ous acid 63, cyanogen 64 0< 5, ammonia 60, arsen in retted hydrogen 

 54, sulphuretted hydrogen 56 0> 5, muriatic acid 43, carbonic acid 

 32-5, nitrous oxide 30."] 



We have already seen that the opinion that the air-thermometer is 

 a true measure of heat, is strongly countenanced by the symmetry 



O v v t/ tf 



which, by using it, we introduce into the laws of radiation. If we 



i. Mem. vol. v. 1S02 ; and Ann. Chim. xliii. p. 1ST. 

 " Ib. p. 272. ' Phil. Trans. 184o, Pt. 1. 



