452 Prof. Tyndall's Contributions to Molecular Physics. 



A moment's reflection will show that the comparison here in- 

 stituted is not a strict one. We have taken the liquids at a 

 common thickness, and the vapours at a common volume and 

 pressure. But if the layers of liquid employed were turned 

 bodily into vapour, the volumes obtained would not be the same. 

 Hence the quantities of matter traversed by the radiant heat are 

 neither equal nor proportional to each other in the two cases ; 

 and to render the comparison strict they ought to be propor- 

 tional. It is easy, of course, to make them so ; for the liquids 

 being examined at a constant volume, their specific gravities 

 give us the relative quantities of matter traversed by the radiant 

 heat, and from these and the vapour-densities we can immedi- 

 ately deduce the corresponding volumes of the vapour. Calling 

 the quantity of matter q, the vapour-density d> and the volume 

 V, we have 



or 



Yd=q, 



Dividing, therefore, the specific gravities of our liquids by the 

 densities of their vapours, we obtain a series of volumes propor- 

 tional to the masses of the liquids employed. The densities of 

 both liquids and vapours are given in the following Table : — 



Table of Densities. 





Vapour. 



Liquid 



Bisulphide of carbon 



. . 2-63 



1-27 



Chloroform 



. 4-13 



1-48 



Iodide of methyle 



. 4-90 



2-24 



Iodide of ethyle . 



. 5-39 



1-95 



Benzole .... 



. . 2-69 



0-85 



Amylene . . . 



. 2-42 



0-64 



Alcohol .... 



. 1-59 



0-79 



Sulphuric ether . 



. 2-56 



071 



Formic ether . . , 



. 2-56 



0-91 



Acetic ether 



. 3-04 



0-89 



Water 



. 0-63 



1-00 



Substituting for q the numbers of the second column, and for 

 d those of the first, we obtain the following series of vapour 

 volumes, whose weights are proportional to the masses of liquid 

 employed. 



