Mr. C. K. Akin on the Compressibility of Gases. 291 



being bottom-pressures, and the latter convertible with mean 

 densities) in gases, as deducible from the Boylean law. The 

 formula is the following, — 



wherein p and j^ denote two pressures corresponding to the two 

 volumes, or rather the two heights a and a 1} of the identical 

 cylindric or prismatic vessel in which the gas, invariable itself in 

 mass, is supposed to be contained in either case ; whilst D is the 

 density of the gas when of the normal pressure, the height of 

 which is designated by H, it being understood that both D and 

 H are expressed in units referring to the same substance (for 

 instance, mercury, whose density Q is to be assumed =1), as also 

 H to mean inches or centimetres, according to the nature of the 

 units in which the a's are expressed. According to this formula, 

 if the law of Boyle were accurate, we should have 



1— - TJ « 1 + &c. 

 p o a 0== 2H =k 



and not, as supposed by M. Regnau.lt, 



Po a o =1 

 p x a x 



Hence also, if it be ascertained that the above relation is not 

 always practically verified, 



and not 



Mq_ 1 = /3 



as stated by M. Regnault, will be the true measure of deviations 

 from the law of gaseous compressibility as displayed in theBoylean 

 formula; whilst 



will measure the error committed by substituting one expression 

 for the other from misapprehension. But since jj is a very small 

 magnitude, k may be written simply 



*=i + |§K-«,); (ii.) 



U2 



