342 Mr. J. Dewar on the Atomic Volume 



Looking at the atomic thermal number, there is a far greater 

 likelihood of condensation taking place in the bromide and iodide 

 of potassium in the combined state than in the case of the 

 chloride, seeing that it would be relatively far more difficult to 

 condense. But neither the chlorate, bromate, nor iodatecan be 

 produced through the direct addition of oxygen to the respective 

 halogen salt. And the chlorate, it is well known, evolves heat 

 on giving off its oxygen, and thus necessitates an absorption of 

 heat during combination. It is just possible that the heat pro- 

 duced during the decomposition is the result of the necessary ex- 

 pansion of volume in the chloride of potassium in combining 

 with oxygen, and its return to its normal volume on losing it. 

 It makes no change in volume to suppose that, in the one case, 

 the oxygen is added as a whole to the chloride of potassium, or, 

 in the other, that it is between the potassium and chlorine, each 

 occupying its individual volume unchanged; but it would alter 

 greatly the heat evolved in so doing. If oxygen combined with 

 chloride of potassium as a whole, without any condensation 

 taking place, the natural result would be an evolution of heat. 

 But if the addition of the oxygen diminishes the coefficient of 

 contraction, as compared with that of the free compound, then 

 we have a physical explanation of the evolution of heat on de- 

 composition. In this case the actual work performed by the 

 condensation of oxygen is retained in a potential form, and 

 therefore reappears as heat on its decomposition. If, now, we 

 examine the mode in which the oxygen is attached to the respec- 

 tive halogen compounds, we can trace, as a necessary consequence, 

 the retention of varying amounts of energy. Chlorate, bromate, 

 and iodate of potash are formed by a similar chemical reaction, 

 according to the following formula of exchange, given in equiva- 

 lents, the whole reaction supposed to take place in the presence 

 of water : — 



+ K CIO 6 

 -I- K BrO 6 

 + KIO 6 



We have appended the thermal equivalents attending the for- 

 mation of these bodies in a large volume of water. It will be 

 obvious on comparing the formation of chlorate of potash, 

 through the above reaction, that it may be the result of absorp- 

 tion of heat; whereas it is certain that the formation of iodate 

 of potash must be attended with an evolution of heat, or else 



6KO -f 6C1 



== 5KC1 



6(76288) 



5(97086) 



6KO + 6Br 



= 5KBr 



6(76238) 



5(85666) 



6KO + 61 



= SKI 



6(76238) 



5(72721) 



