APPENDIX A. 



to see if steam is liquefied in the pipe; to see 

 whether it comes out cloudy or transparent.) 



(4) The elevation of temperature which takes 

 place at the time of the entrance of the air into the 

 vacuum, an elevation that cannot be attributed to 

 the compression of the air remaining (air which 

 may be replaced by steam), can therefore be at- 

 tributed only to the friction of the air against the 

 walls of the opening, or against the interior of the 

 receiver, or against itself. 



(5) M. Gay-Lussac showed (it is said) that if 

 two receivers were put in communication with 

 each other, the one a vacuum, the other full of air, 

 the temperature would rise in one as much as it 

 would fall in the other. If, then, both be com- 

 pressed one half, the first would return to its pre- 

 vious temperature and the second to a much higher 

 one. Mixing them, the whole mass would be 

 heated. 



When the air enters a vacuum, its passage 

 through one small opening and the motion im- 

 parted to it in the interior appear to produce ele- 

 vation of temperature. 



We may be allowed to express here an hypothe- 

 sis in regard to the nature of heat. 



At present, light is generally regarded as the 



