NEW LAW IN THERMOCHEMISTRY 3 1 



chloric, hydrobromic and hydriodic acids from their gaseous 

 elements are as follows : 



HCl, +21984 calories. 

 HBr, 4- 12244 calories. 

 HI, — 605 calories. 



and between these numbers there is no evident connection. 

 The equations, however, are of this type : 



2[HC1]-[HJ-[C1J=43 968. 

 whence, with the already determined values for Hj and CI2 the 

 absolute heat of formation of HCl can be evaluated. Apply- 

 ing the process to all three compounds we have, for their ab- 

 solute heats of formation from free atoms 



HCl = 37842. 



HBr ^ 34941.1 

 HI =35869. 



These qualities are of the same order, and are, moreover, ap- 

 proximately double the value assigned to hydrogen. The 

 apparent heat of formation of hydrobromic and hydriodic acids 

 were measured by very indirect processes, and therefore a close 

 agreement could hardly be expected, but probably the three 

 quantities are actually identical. A similar identity is shown 

 by Thomsen's measurements ^ of the heat of reaction between 

 the gaseous halogen acids and gaseous ammonia, the results be- 

 ing as follows : 



NIIj.HCl 41899 

 NHj.IIBr 45021 

 NHj.HI 43462 



The first of these values is not far from three henotherms, 

 41 331 calories ; although the agreement maj^ be only a coinci- 

 dence. The equivalency of chlorine, bromine and iodine in 

 their unions with other elements is, however, clear, and the fig- 

 ures given to their respective molecules seem to be well sup- 

 ported. 



The approximate equality in heat of formation between the 

 three halogen acids becomes very significant when it is consid- 



' If Berthelot's determination of the apparent heat of formation of HBr be 

 taken, 13500 calories, the absolute value becomes 36 197. 

 ^ Therm. Unt., vol. 2, p. 75. 



