3G0 THE DECOMPOSITION OF ALKALINE SULPHATES 



The reactions which take place in this decomposition are very simple, being precisely 

 the reverse of those which occur in the production of hydrochloric acid, by the action of 

 sulphuric acid on a chloride. One atom of water is formed from the hydrogen of the 

 hydrochloric acid, and the oxygen of the baryta; their two remaining elements combine to 

 form chloride of barium, while the sulphuric acid is set free in the state of sulphurous acid 

 and oxygen. 



The sulphates of strontia, lime, and magnesia, are decomposed in the same way as sul- 

 phate of baryta by hydrochloric acid gas. 



The sulphates of potash and soda, when exposed in a platinum crucible at a high red heat 

 to a current of dry hydrochloric acid gas, after some time give dense fumes of the volati- 

 lizing chlorides, large portions of which are also found in the fused salts. 



Moist hydrochloric acid gas as evolved by boiling dilute hydrochloric acid acts in a 

 similar manner, but rather less quickly than the dry gas. Sulphuric acid is known to be 

 expelled at a high heat from its combinations, by several acids, such as phosphoric, arsenic, 

 and boracic, whose affinities at common temperatures are less powerful than its own; but 

 as all these acids are fixed at heats which volatilize sulphuric, it is to this property that 

 their action has generally been attributed. In the present case, however, both the hydro- 

 chloric acid and the chlorides produced, are more volatile than the sulphuric acid and the 

 sulphates decomposed. Rose has recently shown that the alkaline sulphates can be de- 

 composed into chlorides by ignition with a large excess of sal ammoniac; but, in this case, 

 the reducing agency of the hydrogen of the ammonia upon the sulphuric acid of the salt ? 

 must play an important part in the decomposition, and entirely distinguishes the action 

 from the one now in question. 



As in the above experiments both elements of the hydrochloric acid enter into combi- 

 nation with the constituents of the decomposed salt, and aid in producing the decomposi- 

 tion, it became a matter of interest to examine the action of the chlorine alone under the 

 same circumstances. 



A current of chlorine gas liberated from hydrochloric acid and peroxide of manganese, 

 was washed with water to remove any traces of hydrochloric acid, then dried by sulphuric 

 acid, and conveyed by a platinum tube through small pieces of sulphate of baryta heated to 

 high redness in a platinum crucible. After an operation of half an hour, the pieces of sul- 

 phate of baryta were found converted on their exposed surfaces into chloride of barium, 

 the action appearing to be nearly as powerful as in the case of hydrochloric acid gas. In 

 this experiment the chlorine must expel both the sulphuric acid of the salt and the oxygen 

 of the base, a decomposition which might be more simply expressed in the language of 

 Davy's Binary theory of salts, by saying that the simple radical chlorine replaces the 

 compound radical SO 1 , the sulphat-oxygcn of Graham, in its combination with barium 

 sulphurous acid and the two equivalents of oxygen, being the gaseous products of the 

 decomposition. 



Vapour of iodine passed over sulphate of baryta, under the same circumstances, forms 

 a considerable quantity of iodide of barium, but the action seems less energetic than that 

 of chlorine. . 



