Ἔριν τιν 
Ὁ 
TRANSACTIONS OF THE SECTIONS. 63 
Owing to the fact that nitric acid forms no insoluble compound, its accurate deter- 
mination has been accompanied with considerable difficulty, and its amount was at 
no distant period only determined by very uncertain and difficult processes. The 
author having referred to the processes employed for this purpose by M. Crum, M. 
Pelouze, and Prof. Penny, proposes the following process :— 
The process is generally this :—nitric acid has the power of oxidizing arsenious acid, 
and it is from the amount of arsenic acid formed that I propose to calculate its amount. 
It was however necessary to determine how much arsenious acid a certain quantity 
of nitric acid could oxidize. In experiments for this purpose, I found that I obtained 
one equivalent of arsenic acid for every equivalent of nitric acid used, showing that 
two atoms of oxygen had been transferred from the nitric to the arsenious acid. 
After various trials, which I need not here describe, I fixed upon the following 
method as the best. 
The nitrate to be analysed is mixed with three times its weight of pure atsenious 
acid, and concentrated hydrochloric acid is poured over it, in a small flask or basin. 
It is then cautiously heated and evaporated slowly, almost todryness. When cool, water 
is added and the whole transferred to a beaker-glass. The fluid is now rendered al- 
kaline by ammonia, and a considerable amount of chloride of ammonium is added, and 
subsequently sulphate of magnesia. The precipitate is allowed to subside, and is then 
collected on a weighed filter. It is washed on the filter with water containing am- 
monia and chloride of ammonium, till all traces of arsenious acid are removed, after 
which ammoniated water only is used, till all the chloride of ammonium is washed 
out. After this it is dried at 212° Fahr. till it ceases to lose weight. 
I have also used a modification of the above process, as follows :—The dry nitrate 
was mixed with rather more than three times its weight of pure arsenious acid, and 
the mixture very cautiously heated in a covered porcelain crucible, till the nitrate 
began to melt, when copious red fumes were disengaged. The heat was continued 
till these had entirely ceased. Towards the end, the temperature was slightly ele- 
vated to ensure the thorough decomposition of the nitrate. This mode, however, 
required much caution, on account of the vapour of arsenious acid which was given 
off, and it could not of course be performed in the open laboratory, but under the 
hood of a chimney. 
In experiments to test the accuracy of the process, I obtained the following results : 
‘436 erm. of nitrate of potash yielded -813 grm. of ammonio-arseniate of magnesia, 
dried at 212° Fahr., and -5745 grm. gave 1.071 grm. of that salt. The formula of 
the ammonio-arseniate of magnesia, driedat 212° Fahr., is 2MgO NH, O, AsO,, HO, 
and these numbers consequently indicated respectively 52-995 and 52-985 per cent. of 
nitric acid in the saltpetre. This is slightly less than the theoretical amount. 
It is evident that this process cannot be applied in the presence of phosphates or 
lime, as these bodies would also be precipitated, and would therefore require to be 
previously removed. I hope, however, to be able to apply to it a method of Centi- 
grade testing which will get clear of this difficulty, and at the same time render it 
more expeditious. 
On the Chemical Composition of the Rocks of the Coal Formation. 
By Henry Taypor. 
On the Proportion of Phosphoric Acid in some Natural Waters. 
By Dr. A. Voretckenr. ‘ 
The object of this paper was to draw attention to a natural source from which many 
of our fields may be ceconomically supplied with phosphoric acid. Prof. Fownes has 
shown that traces of phosphoric acid are met with in many rocks of igneous origin; 
but also in stratified rocks, particularly in limestone rocks, the presence of phosphoric 
acid has been indicated by several chemists. The author found the proportion of 
phosphoric acid in great oolite from the neighbourhood of Cirencester amounting to 
0.124 per cent., equal to 0°260 of bone-earth, and in Stonesfield slate from the same 
locality amounting to 0-117, equal to 0-244 per cent. of bone-earth. As water charged 
with carbonic acid is capable of dissolving bone-earth, this important fertilizing sub- 
