14 



M. Cahours has shown us a reagent (the pentachloride of phos- 

 phorus) which is capable of forming from a great number of mono- 

 basic acids the chlorides of the acid radicals. Whilst extending our 

 knowledge of the action of the body on monobasic and organic acids, 

 and preparing numerous compounds of their radicals with one atom 

 of chlorine, M. Gerhardt examined also the nature of its action upon 

 bibasic acids and their compounds; and states that it consists of two 

 successive phases, first, the liberation of the anhydrous acid, secondly, 

 the substitution of two atoms of chlorine for one of oxygen in that 

 anhydrous acid. These facts, if correct, would be unfavourable to 

 the above view of the constitution of sulphuric and the other bibasic 

 acids; and M. Gerhardt adopted accordingly the old formulae, repre- 

 senting in their composition an atom of water ready-formed, SO 3 H 2 O . 



Confining my remarks for the present to the case of sulphuric 

 acid, whose decomposition is doubtless typical of that of other bibasic 

 acids, I may state as the result of numerous experiments with the 

 most varied proportions of pentachloride and acid, performed on a 

 scale of considerable magnitude, that the first action of the penta- 

 chloride consists in removing one atom of hydrogen and one of 

 oxygen (empirically peroxide of hydrogen) from the acid, putting in 



H Q 



an atom of chlorine in their place and forming the compound SO 2 , 



Cl 



which is strictly intermediate between the hydrated acid and the 

 final product SO 2 C1 2 formed by a repetition of the same process of 

 substitution of chlorine for peroxide of hydrogen. The existence 

 and formation of this body, which we may call chloro-hydrated sul- 

 phuric acid, furnishes the most direct evidence of the truth of the 

 notion, that the bibasic character of sulphuric acid is owing to the 

 fact of one atom of its radical SCv replacing or (to use the customary 

 expression) being equivalent to two atoms of hydrogen. Had this 

 radical been divisible like an equivalent quantity of a monobasic 

 acid, we should have obtained a mixture, not a compound of the 

 chloride with the hydrate, or, at least, the products of decomposi- 

 tion of that mixture. 



Chloro-hydrated sulphuric acid boils at 145 Cent., distilling 

 without decomposition. The intensity of its action upon water 

 varies according to the manner in which the two bodies are brought 



