704 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Following the analogy of the reduction of sulphuric acid hy nascent 

 hydriodic and hydrobromic acids, the products of reaction (1) are 

 more or less changed according to 



(3) 2SO3 + 2HCI = SO2 + CI2 + S02(OH)2, 



while the products of reaction (2) become, to a greater or less extent, 



(4) SO2CI2 + S02(OH)2 = SO2 + CI2 + S02(OH)2 ; 

 hence the ultimate observed dissociation would be represented by 



(5) 2SO2OHCI = SO2 + CI2 + SO3 + H2O, 



the same result being reached in whichever direction the breakdown 

 primarily started. 



Under diminished pressure, at temperatures from 60° to 80°, we have 

 found reaction (1) to take place, and in proportion to the decrease of 

 pressure. The greater dissociation at the lower pressure is due, ac- 

 cording to the principle of Le Chatelier, to the fact that the vapor 

 increases in volume, and, indeed, the effect of a pressure as low as 1 to 

 4 mm. is to pump out the more volatile hydrochloric acid, hence that 

 portion of the distillate which is condensed by liquid air (Figure 2) 

 contains relatively much more hydrochloric acid than sulphur trioxide.^^ 

 As we have found also sulphuryl chloride, sulphur dioxide, and chlorine 

 among the products of distillation under diminished pressure, with a 

 residue of sulphuric acid, we consider that reaction (2) occurs to a 

 slight extent, and also reaction (3), though to a less degree than at 

 higher temperatures. 



At atmospheric pressure and at the boiling point, reaction (1) takes 

 place, as Williams ^^ and Ruff *^ have stated. But, while the primary 

 action is less than at low pressure, the secondary action (3) is greater, 

 and we find more sulphur dioxide and chlorine, with a larger residue 

 of sulphuric acid, than under lower conditions of temperature and 

 pressure. The presence of sulphuric acid, however, is partly accounted 

 for by reaction (2), for we find sulphuryl chloride among the products 

 of distillation at ordinary pressure, and, indeed, in greater quantity 

 than when the pressure is low. That sulphuryl chloride is formed 

 under these conditions was not recognized by Ruff, but he showed 

 clearly that the presence of a catalyser brings about reaction (2) at the 



^^ The presence of sulphur trioxide in chlorsulphonic acid distilled at this 

 pressure was indicated by the analysis and specific gravity of the product, and 

 was confirmed qualitatively and quantitatively on careful fractionation of the 

 product at ordinary pressure. 



