280 On the Reciprocal Decomposition of Bodies. [Apkil, 



apply the law of which we have spoken by supposing that the 

 chlorides remain indifferently in the solutions in the state of 

 chlorides, or that of hydrochlorates ; which is not always the 

 case. Sulphuric acid at common temperatures partially sepa- 

 rates the boracic and arsenic acids from their combinations ; 

 but at a high temperature, it is, on the contrary, expelled by 

 them. 



Nitric and hydrochloric acids decompose the fluorides, and in 

 its turn hydrofluoric acid decomposes the nitrates and chlorides. 



Acetic acid decomposes several chlorides, and reciprocally 

 hydrochloric acid decomposes the acetates. Many other vege- 

 table acids, and particularly lactic acid, present analogous 

 phgenomena. 



Gases soluble in water, and which are separable from it by a 

 vacuum, are all expelled from that liquid by another gas passed 

 into it in excess. 



One might quote a multitude of similar facts, but we shall 

 confine ourselves to the mention of the decomposition of hydro- 

 sulphates by carbonic acid, and that of carbonates by hydrosul- 

 phuric acid (sulphuretted hydrogen), respecting which M.Henry, 

 the younger, has lately been engaged in a very long investigation, 

 to demonstrate what a little reflection would have easily con- 

 vinced him of, on the laws established by BerthoUet. 



Bicarbonate of potash, for instance, exposed in solution to the 

 contact of the air, loses a portion of its acid, and acquires the 

 property of precipitating sulphate of magnesia. If we pass a 

 current of hydrosulphuric gas into it, whose acid properties, as 

 is well known, are nearly the same as those of carbonic acid, a 

 por Jion of carbonic acid will necessarily be liberated ; and as it 

 will be gradually expelled by the current of hydrosulphuric gas, 

 the bicarbonate remaining in solution will always be under the 

 same circu'uistances as to its decomposition, which, therefore, by 

 degrees, wil' become complete. 



In like manner, if a current of carbonic acid be passed into a 

 solution of a hydrosulphate, the latter will be partially decom- 

 posed, and the hydrosulphuric acid set free, being carried off by 

 the current of carbonic acid, the decomposition of the hydrosul- 

 phate will continually go on till it is complete. 



We must observe that these decompositions require a much 

 larger quantity of acid than would be necessary to saturate the 

 base ; i'or the eliminated acid cannot escape from the solution, 

 but by the action of a great excess of the acid which takes its 

 place, according to the theory of vapours. 



Moreover if the carbonate and hydrosulphate be not in the 

 state of bisalts, neither the one nor the other will begin to let go 

 their acid till they have attained that state. M. Henry has 

 observed that the insoluble carbonates experience only a very 



