THERMAL EQUIVALENTS OF THE OXIDES OF CHLORINE, 193 



Preliminary Report on the Thermal Equivalents of the Oxides of 

 Chlorine. By James Dewar, F.R.S.E. 



During tlic course of the last Meeting of the British. Association, I took 

 occasion to lay before the Chemical Section two short notes bearing directly 

 on the subject of Thermal Equivalents ; they were respectively entitled 

 " Thermal Equivalents and Fermentation/' and " Observations on the Oxides 

 of Chlorine." In the first-mentioned communication it was proved that the 

 decomposition of sugar into carbonic acid and alcohol was a reaction taking- 

 place without any great evolution of heat, if we accepted the thermal equi- 

 valent of sugar as determined by Frankland, along with the similar value 

 of alcohol obtained from Eavre and Silbermann's researches ; and con- 

 sequently the heat of fermentation must be derived from some other source 

 than the sugar molecule itself, — the continued hydration of the alcohol pro- 

 duced, the secondary decompositions taking place, and the transformations 

 of the ferment itself being the three available sources of supply. 



The note on the oxides of chlorine had special reference to the heat 

 evolved during the decomposition of these oxides. The researches of Eavre 

 and Silbermann having shown that the formation of hypochlorous acid and 

 of chloric acid is attended with a large absorption of heat, it became in- 

 teresting to ascertain if in this series of oxides we had a regular increment 

 of absorption in passing from the lowest member of the series to the highest 

 member, just as Andrews had found a similar relation to hold for certain 

 oxides of the same metal, whoso successive formation was attended with an 

 evolution of heat. I suggested it would be interesting to make a complete 

 examination of the thermal relations of these bodies aloag with the similar 

 derivatives of bromine and iodine, and with this object in view I ac- 

 cepted a grant in order to prosecute these researches ; and although my 

 spare time has been variously occupied during the past year, I have found 

 opportunity to make a considerable number of prehminary observations in 

 connexion with this subject. 



Heat absoried during the Solution of Salts belonging to this Series per 



equivalent. 

 Units. Heat units. 



KCl . . . . : 4320 KCIO3 10,100 



KBr 4900 KBr03 9>680 



KI 4800 KIO3 5,300 



Comparing the solution-values of chloride of potassium and bromide of 

 potassium vdth the corresponding values obtained for the chlorate and 

 bromate, the latter salts are observed to have a very much higher solution 

 thermal equivalent ; whereas, comparing iodide of potassium with iodate, we 

 have only a slight increase in the latter salt. The highest absorption- 

 values are therefore connected with the acids whose formation is attended 

 with an absorption of heat. It will be interesting to find how these sub- 

 stances act with regard to the absorption of radiant heat, and if a similar 

 relation is maintained. 



The method I proposed to adopt in examining the thermal relation of the 

 oxides of clilorine was based on the easy and rapid decomposition of dilute h)^- 

 driodic acid, whoso thermal equivalent in aqueous solution has been carefully 

 determined. I soon found, however, chloric acid did not appreciably decom- 

 pose dilute hydriodic acid when the strength of the respective acids in 

 aqueous solution amounted to a half gramme equivalent per litre, nor did I 



1871. o 



