NOTES TO BOOK X. 595 



large a mass of etymological innovations is in general to 

 be avoided in scientific works. 



M. Melloni's discovery of the extraordinary power of 

 rock-salt to transmit heat, and Professor Forbes's dis- 

 covery of the extraordinary power of mica to polarize 

 and depolarize heat, have supplied thermotical enquirers 

 with two new and most valuable instruments. 



For an account of many thermotical researches, which 

 I was obliged to pass unnoticed, see two Reports by Prof. 

 Powell on the present state of our knowledge respecting 

 Radiant Heat, in the Reports of the British Association for 

 1832 and 1840. 



(QA.) p. 552. I am aware that 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 sul- 

 phurous-acid gas, 1.385. These differences are not greater 

 than the differences obtained for the same substance by 

 different observers ; and as this law is referred to in La- 

 place's hypothesis, hereafter to be discussed, I have left 

 the text unaltered. 



Yet that the rate of expansion of dry gas is different 

 for different substances, must be deemed very probable, 

 after Dr. Faraday "3 recent investigations On the Liquefac- 

 tion and Solidification of Bodies generally existing as Gases, 

 {Phil. Trans. 1845, Pt. 1) ; by which it appears that the 

 elasticity of vapours in contact with their fluids increases 

 at different rates in different substances. " That the 

 force," he says, " of vapour increases in a geometrical 

 ratio for equal increments of heat is true for all bodies, 



