RUDBERG ON THE EXPANSION OF DRY AIR. 511 



But 



U _ p 

 V ~~ p — f^ 

 Therefore 



p — q h ^ 



Let r' be the weight of the mercury expelled from the globe 

 when heated from 0° to T'° ; r the weight of the mercury ex- 

 pelled when heated from 0° to 100° ; 100 M the true expansion 

 of mercury from 0° to 100° ; b, b' the weights of a unit of volume 

 of mercury at 0° and 100° respectively. Then 



r _ r' 

 Too" T^' 



b' (1 + 100 M) = 6 ; 



the volume of the mercury at 100° = m(1 + 100M); the volume 

 of the globe at 100 = m (1 + 100 G) ; therefore the volume of 

 the mercury at 100° expelled = u . 100 (M — G) ; therefore 



its weight r = &' M . 100 (M - G) = ^ , ^iqqm ^^^ ^^ ~ ^^ 



= -?-* — =^5—''. Therefore the true expansion of glass from 



1 + 100 M ^ ^ 



0° to 100° 



100 G = lOOM - — (1 + lOOM). 

 p 



The value of the true expansion of mercury is here assumed to 

 be known. This may be done with confidence, inasmuch as it 

 has been determined, quite independently of the expansion of 

 glass, by the masterly experiments of Dulong and Petit. They 

 found 100 M = 0-0180180. Therefore 



100 G = 0-018018 - 1-018018—. 



P 



The following table exhibits the values of 100 M - 100 G for 

 the glass employed, which was potash glass, from the manufac- 

 tory at Reymyra. The first fifteen results were obtained from 

 globes used in experiments upon the melting-points of easily 

 fusible metals ; the remainder were obtained from the globes 

 used in determining the expansion of air. They show that the 

 same kind of glass, though made at different times, and there- 

 fore in different meltings, possess the same expansibility. 



