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XXXIII. Latent Heat of Evaporation of Zinc and Cadmium, 

 lhj William Sutherland*. 



IN an article on the Fundamental Atomic Laws of Thermo- 

 chemistry (Phil. Mao-. [5] vol. xl. 1895), certain prin- 

 ciples as to molecular force were used to calculate the latent 

 heats of evaporation of metals and compounds of metals. At 

 the time of these calculations it escaped my notice that Bams, 

 in his article on the Pressure-variation of certain High- 

 pressure Boiling-points (Phil. Mag. [5] vol. xxix. 1890), had 

 supplied data whereby the latent heats of vaporization of zinc 

 and cadmium could be determined. It therefore seems desi- 

 rable to show briefly now how the theoretical agree with the 

 experimental determinations. 



The usual thermodynarnical equation for the latent heat of 

 evaporation of a gramme of liquid at absolute temperature 6 

 and of volume i\ into saturated vapour of volume v s at satu- 

 ration-pressure p, dp 



has been so thoroughly verified that it furnishes an experi- 

 mental method of determining X without the necessity of 

 direct calorimetric measurements. Let M be the molecular 

 mass of the substance, that of hydrogen being 2; then ne- 

 glecting v l beside v s , we have 



M\=0^MiyJ. 



Now for 2 grammes of hydrogen under standard conditions 

 Mv 3 is 22,400 c. c; so that for a vaporized metal at a pressure 

 of 1 atmo and at 6 the value of Mv 3 is 22400(9/273. 



For the relation between/? and 6 for Zn and Cd, Barus 

 gives formulae which represent his experimental results satis- 

 factorily, and enable one to calculate dp/dd at values of p 

 from 1 atmo down to a fraction thereof. Using the con- 

 stants of the formula at p. 152 of his paper, we find for 

 (dp/d0)0/p when jt? is 1 atmo the value 11*9 for Zn, and 14*2 

 for Cd, the corresponding values of 6 being 1200 for Zn and 

 1050 for Cd. Thus, then, taking 1 atmo as 1*014 x 10 6 

 dynes per cm. 2 , and J as 42 x 1 6 ergs, we obtain as 

 the values of MA, in kilocalories 28*3 for Zn and 29*6 for 

 Cd : these are the heats of vaporization of gramme-mole- 

 cules of these metals as liquids. Now Person's values (Ann. 

 de Ch. et de Phys. [3] xxiv.) for the latent heats of 

 fusion of Zn and Cd per gramme-molecule in kilocalories are 

 1*8 and 1*5; so that the total heats of volatilizing gramme- 

 molecules of solid Zn and Cd are 30'1 and 31*1. These 

 numbers include the heat used in expanding the evaporating 

 * Communicated by the Author. 



