D ALTON'S AND GAY-LUSSAC'S LAW. 101 



2. Find (1) what temperature C. is the same as 60 R., 

 and (2) what temperature R. is the same as 45 C. 



Ans. (1) 75 C. ; (2) 36 R. 



52. Expansion of Mercury. The expansion of mer- 

 cury is very nearly uniform between and 300. Experi- 

 ments show that, for an increase of 1 Centigrade, the 

 expansion of mercury is -5-5^, or .0001815 of its volume ;* 

 hence, if o ( be the density at a temperature t, and a the 

 density at a temperature 0, we have 



<7 = a t (1 4- .000180180 ; 

 or, if we put .00018018 = 6, we have 



<T O =*,(! + Bt], (1) 



which, in (1) of Art. 43, gives 



TT = ga t PB = go, (1 - Of) PB, (2) 



by means of which the atmospheric pressure at any place 

 can be calculated. 



53. Dalton's and Gay-Lussac's Law of the Ex- 

 pansion of Gases by Heat The following experimental 

 law was discovered by Gay-Lussac f and Dalton, and more 

 recently corrected by Regnault. 



// the pressure remains constant, an increase of 

 temperature of 1 C. produces in a given mass of air 

 an expansion of . 003 665 of its volume. 



By means of this experimental law, combined with Boyle's 

 (Art. 48), the relation between the pressure, density, and 

 temperature of a given mass of air or gas may be expressed. 



Conceive that a mass of air at the temperature of C. is 

 inclosed in a cylinder by a piston to which a given force is 



* Enc. Brit., Vol. XVI., p. 33. t See Deechanel's Nat. Phil., p. 307. 



