OXALATES, NITRATES, PHOSPHATES, SULPHATES, AND CHLORIDES. 69 
at 212°, as we have described. The constitution of this salt of iron I would therefore 
represent by the formula 
Fe 2 NH 4 P + 2 IT. 
In the same paper Dr. Otto describes another extraordinary phosphate, under the 
name of paraphosphate of soda, ammonia, and oxide of manganese, which does not 
belong to any class of phosphates that I have examined, but may possibly be a com- 
bination of two bibasic phosphates. Its constituents are 2 P, 2 Mn, NH 4 and 6 H. 
It is prepared from bibasic phosphates, and would be said in the old language to con- 
tain pyrophosphoric acid. 
IV. Of Sulphates. 
In a former paper upon water as a constituent of sulphates *, I examined particu- 
larly the constitution of hydrated sulphuric acid and of the sulphates of the magne- 
sian class of oxides. All these salts contain one atom of constitutional water, which 
is displaced in the formation of the double sulphates by an atom of an alkaline sul- 
phate. This view is illustrated by the following formulae : 
Sulphate of water (acid of sp. gr. 178) H S IT 
Sulphate of magnesia Mg S H + 6 II 
Sulphate of magnesia and potash ....... Mg S (K S) + 6 H 
Sulphate of water and potash (bisulphate of potash) . II S (K S). 
It will be found upon experiment that the salts sulphate of magnesia and sulphate 
of zinc become anhydrous at much lower temperatures when mixed with sulphate of 
potash than by themselves, the sulphate of potash displacing the constitutional water 
of the other salt at a very moderate heat, although the salts are mixed in the state of 
dry powders. 
In that paper the opinion was supported, originally suggested 1 believe by M. Mi- 
tscherlich, that the bisulphate of potash is a double sulphate of water and potash, 
and therefore really neutral in composition. The only difficulty which stood in the 
way of generalizing this result, and maintaining that all the salts usually considered 
as bisalts are really neutral in composition, was the composition of the bichromate 
or red chromate of potash, a salt which unquestionably is anhydrous. Here, it 
might be said, is a true bisalt. But M. H. Rose has lately published some observa- 
tions in regard to anhydrous sulphuric acid, which, I think, afford a clue to the 
discovery of the true constitution of the red chromate of potash. It appears that the 
vapour of anhydrous sulphuric acid is absorbed by sulphate of potash and by chlo- 
ride of potassium, without decomposition, and definite compounds formed ; which, 
however, are destroyed by solution in water. Here we appear to have a class of 
* Edinburgh Transactions, vol. xiii. p. 297 ; or London and Edinburgh Philosophical Magazine, 3rd series, 
vol. vi. pp. 327, 417. 
