194 KEPORT 1871. 



succeed better when I substituted hydrochloric acid for the hydriodic. A 

 few experiments were made on the action of magnesium on chloric acid, 

 with the view of ascertaining its thermal value from the oxidation of the 

 nascent hydrogen ; but, so far as my experiments extended, the results did 

 not agree satisfactorily. 



I had recourse then to the direct action of iodine on chloric acid, which 

 I found acted easily on a solution of tvnce the normal strength, at a tempe- 

 rature of SO'' C, although it did not act on a dilute aqueous solution in the 

 cold. The reaction only taking place readily at a temperature of 80° C, 

 complicates very much the mode of procedure, necessitating, as it does, a 

 very constant temperature. 



A series of observations gave as a mean 35,500 heat units evolved per 

 equivalent of iodine acting on excess of chloric acid. This number repre- 

 sents the heat evolved in the transformation of chloric acid into iodic acid ; 

 and by subtracting from it the thermal value of the latter acid, we obtain 

 the heat evolved from the decomposition of the chloric acid. The thermal 

 value of iodic acid is very readily obtained through the reaction of dilute 

 hydriodic acid, thus — 



IOg + 5HI=5HO + 6I, 



which takes place with extreme rapidity in dilute solution, evolving 10,000 

 units per equivalent of hydriodic acid dccom2:)Osed. 



Assuming, then, the thermal value of hydrogen to be 34,000 units, and that 

 of hydriodic acid to be 15,000, we obtain on calculation 15,000 units evolved 

 during the formation of a molecule of iodic acid in aqueous solution. This 

 number agrees vciy closely with that of A. Ditto's for the formation of iodic 

 acid as found through the oxidation of phosphorus. Subtracting the number 

 found for the formation of dilute iodic acid from the former number ex- 

 pressing the action of iodine on dilute chloric acid, we have the number 

 20,500 left for the thermal value of dilute chloric acid. Favre estimated 

 the thermal value of dilute chloric acid as high as —65,234 per equivalent — 

 this result being based^'ion the action of chlorine on concentrated caustic 

 potash, thus, 



6E:0 + 6€1=5KC1+KC103, 



and inserting in the equation the known values of oxide of potassium and 

 chloride of potassium, and further correcting for dilution. It is obvious, 

 however, where we have one atom of a compound formed for five atoms of 

 another whose thermal value is not very accurately known, we multiply 

 any error enormously. 



In looking over Favre's original paper, in the * Journal de Pharmacie ' for 

 1853, on this subject, I observed that he mentioned a very ciuious observa- 

 tion with reference to the heat evolved during the saturation of hypochlorous 

 acid with dilute oxide of potassium. He shows that an equivalent of 

 caustic potash, when neutralized with an equivalent of hypochlorous, gives 

 rise to an evolution of 10,768 heat imits ; but if two molecules of hypo- 

 chlorous acid were employed per equivalent of caustic potash, he found an 

 evolution of 22,114 heat units. The additional heat evolved is not due to the 

 formation of an acid salt, because, on adding another atom of caustic potash^ 

 we obtain the normal amount of heat due to the saturation of the acid. It 

 is reasonable to suppose, tlierefore, that the additional atom of hypo- 

 chlorous acid induces the following reaction : — 



-^o 



3K0 C10=2KC1+K0 C10„ 



