544 Proceedings of Royal Society of Edinburgh. 
simultaneous substitution of one atom of hydrogen by peroxide of 
nitrogen, the other by potassium. Sulphuric acid is formed from 
two atoms of water 
H 
0 
H 
H o 
H u 
; one of hydrogen from each is removed, 
and the two replaced by the indivisible radical S0 2 . The series 
Sulphuric acid Acid sulphate of potash Neutral sulphate of potash 
H 
SO 
H 
O 
2Q ’ 
H 
so 2 g. 
K U 
K 
SO 
K 
0 
2 0 
explains itself.” 
He then describes the action of pentachloride of phosphorus on 
sulphuric acid : — “ 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 pentachloride consists in removing one atom of 
hydrogen and one of oxygen (empirically peroxide of hydrogen) 
from the acid, putting in an atom of chlorine in their place, and 
0, which is strictly intermediate be- 
2 
Cl 
tween the hydrated acid and the final product S0 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-liydrated sulphuric 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 S0 2 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 decomposition of that 
mixture.” 
In another paper in the same volume of the Proceedings we find 
the following : — “ According to the results of recent researches in 
the constitution of salts and the method thence introduced of 
H 
forming the compound 
