12 Mr. W. Sutherland on the Fundamental 



in the second row the latent heat per equivalent, taking the 

 family-number as the valency of the family. 



Table la. 



Familynumber 1. .. 2. 3.... 4. 5. 8. 



Latent heat per gramme-atom 8*7 23'6 26*0 32'0 40 60 



Latent heat per gramme- equivalent 8*7 . .11 '8 8*7 80- 8*0 7*5 



These numbers in the second row show that the latent heat 

 of vaporization per gramme-equivalent due to molecular force 

 is nearly the same in all the main families, except the 

 second, where it is half as large again as in the other main 

 families ; as regards this relation the sub-familv Zn and Cd 

 would appear to take the place of the main family, for the 

 value per equivalent is 9*2. As it has already been pointed 

 out that the values per gramme-atom of most members of the 

 sub-families are simple multiples or submultiples of those in 

 the main family, the general principle can be enunciated for 

 the metals : — The latent heat of vaporization per gramme- 

 equivalent due to molecular force is approximately a constant- 

 or a simple multiple or submultiple of the constant. 



The fact that we have been led to a general result such as 

 this shows that the method of calculation has probably yielded 

 correct relative values of the latent heat of vaporization due 

 to molecular force ; it remains to see whether these are of 

 about the right absolute magnitude. The only metal whose 

 latent heat of vaporization we can determine independently 

 by means of existing data is mercury; for all the terms in the 

 thermodynamical relation J\=(v 3 — v 2 )# dp/dO are known for 

 mercury, dp/d0 being the rate of variation of the saturation 

 pressure of the liquid with the temperature, v 2 and v$ the 

 volumes of a gramme of the substance as liquid and saturated 

 vapour under the vapour-pressure at 0. With Regnault's 

 data and the assumption that at the boiling-point of mercury 

 a gramme-atom or 200 grammes of mercury would occupy 

 the same volume as 2 grammes of hydrogen at the same 

 temperature and pressure, Berthelot has calculated the atomic 

 latent heat of mercury as 15*5 kilocalories. With the values 

 of dp/dd given by Ramsay and Young (Phil. Mag. 5th ser. 

 vol. xxi.) I have calculated that 14 kilocalories would be the 

 value, still on the above assumption as to the volume of 

 saturated mercury vapour, but this volume is obviously too 

 large ; and as it is a difficult enough matter to measure the 

 true saturation volume of an ordinary vapour, it is evident 

 that the experimental determinations hitherto made of the 

 density of mercury vapour cannot be used in place of the 



