On the Melting-points of the Elements. 29 



point is such a measure, it will follow that in the equation 



Ag 2 + Cl 2 = 2AgCl + heat evolved, 



the inverse of the melting-point of silver + the inverse of the 

 melting-point of chlorine — twice the inverse of the melting- 

 point of silver chloride will give a number proportional to the 

 heat evolved. Let x be this number ; then 



Ag 2 + Cl 2 =2AgCl + heat evolved/ 



(1) 



Therefore 



1 1 2 

 1273 198 721 +x ' 





i 



1 , 1 

 X ~ 1273 + 198 



2 



'724 



} 



By carrying out calculations in this way for all the solid 

 chlorides, bromides, and iodides of those solid elements (inclu- 

 ding K, ]N T a, Li, Ag, (V, Mg, Zn, Cd, Ba, Pb, Sn", Sn% Cu", 

 Al, Tl, and Sb) of which the necessary melting-points and 

 heats of formation have been determined, we obtain numbers 

 for the heats of formation which follow the same, or nearly 

 the same, order of size as the experimental heats of formation. 

 The numbers, however, though following the same order of 

 magnitude, are not proportional to those found by experiment. 

 This is due to the fact that this relation strictly applies only 

 when all the reacting and resulting bodies are solid at the ordi- 

 nary temperature, to which the experimental heat-determina- 

 tions are generally referred. For the iodides among them- 

 selves the relation is a quantitative one ; but for the chlorides 

 and bromides this is not the case, since chlorine is a gas and 

 bromine a liquid at the ordinary temperature, so that their 

 chemical energy in these physical conditions will not be repre- 

 sented by the inverse of their melting-points. They have, 

 however, been calculated as follows from the experimental 

 heats of formation of KI, AgBr, and AgCl, and the melting- 

 points of K, Ag, I, KI, AgBr, and AgCl, thus : — 



Let a= the value required for Cl 2 , 

 6= „ „ Br 2 . 



Then from equation (1) we have 



2 K + 1 2 - 2 KI heat of formation of KI 



Ag 2 -f Cl 2 — 2 AgCl heat of formation of AgCl' 



(2) 



