BY HYDROCHLORIC ACID AND CHLORINE. ."ilil 



III taking a general view of this class of decompositions, some cases present themselves 

 rather difficult of explanation. 



In the instances of the decomposition of sulphate of baryta by hydrochloric acid gas, 

 chlorine, and iodine, it is to be observed that the reverse action is known to take place 

 when anhydrous sulphuric acid is passed over the chloride or iodide of barium at a red 

 heat, chlorine and sulphurous acid being evolved, and sulphate of bars ta being formed. 

 In this respect the case resembles the well known reciprocal decomposition of vapour of 

 water at a red heat by metallic iron, and of the oxide of iron thus produced by hydrogen. 

 The chlorine and sulphuric acid having each a strong and possibly nearly equal affinity 

 for the base, we can imagine that the successive preponderance of the one or the other 

 might be determined by the presence of an excess or atmosphere of the decomposing gas. 



At the heat at which sulphate of baryta is decomposed by vapour of water, its b 

 forms a stable hydrate, and its affinity for water may be viewed as aiding the decomposi- 

 tion. So, also, at the other end of the scale, sulphate of magnesia is decomposed at a 

 heat which allows of the existence of sulphuric acid in combination with water. 



But in the case of the decomposition of the sulphate of lime by vapour of water, neither 

 of these affinities can assist the action. At the temperature employed no hydrate oflimi 

 can exist, and neither the sulphurous acid nor the oxygen, into which the sulphuric arid 

 resolves itself, shows any marked attraction for vapour of water. 



We have, therefore, an instance of a powerful affinity between two bodies being over- 

 come by the mere presence of a third, with which neither of them has any inclination to 

 combine. 



It is probably to the action of heat in weakening the intensity of all kinds of attract! 

 as well chemical as mechanical, that we arc to look for an explanation. The experimi m 

 of Grove in the decomposition of water, shows that the most powerful chemical affinity 

 can be overcome by a sufficiently intense heat. 



Co-operating with this, is the tendency of substances to diffuse themselves into a foreign 

 atmosphere, which Graham has shown, in the case of liquids, to be a force capable of pro- 

 ducing by itself chemical decomposition. 



