THE KINETIC THEORY OF GASES. 349 



the ideal coefficient of condensation. The actual coefficient of condensation, when 

 the gas is reduced to the liquid or even the solid form, and exposed to the 

 greatest degree of cold and pressure, is of course greater than c. 



Multiplying equations (7) and (8), we find 



s = 6*Jzd (9), 



where s is the diameter of a molecule, c the coefficient of condensation, and 

 I the mean path of a molecule. 



Of these quantities, we know I approximately already, but with respect 

 to e we only know its superior limit. It is only by ascertaining whether calcu- 

 lations of this kind, made with respect to different substances, lead to consistent 

 results, that we can obtain any confidence in our estimates of e. 



M. Lorenz Meyer* has compared the " molecular volumes " of different 

 substances, as estimated by Kopp from measurements of the density of these 

 substances and their compounds, with the values of s 3 as deduced from experi- 

 ments on the viscosity of gases, and has shewn that there is a considerable 

 degree of correspondence between the two sets of numbers. 



The "molecular volume" of a substance here spoken of is the volume in 

 cubic centimetres of as much of the substance in the liquid state as contains 

 as many molecules as one gramme of hydrogen. Hence if p a denote the density 

 of hydrogen, and b the molecular volume of a substance, the actual coefficient 

 of condensation is 



*' = pob (10). 



These "molecular volumes" of liquids are estimated at the boiling-points of 

 the liquids, a very arbitrary condition, for this depends on the pressure, and 

 there is no reason in the nature of things for fixing on 760 mm. B. as a 

 standard pressure merely because it roughly represents the ordinary pressure 

 of our atmosphere. What would be better, if it were not impossible to obtain 

 it, would be the volume at 273 C. and o>B. 



But the volume relations of potassium with its oxide and its hydrated 

 oxide as described by Faraday seem to indicate that we have a good deal yet 

 to learn about the volumes of atoms. 



* Annalen d. Chemie u. Pharmacie v. Supp. bd. 2, Heft (1867). 



