ON DILUTED NITRO-HYDROCHLORIC ACID. 
581 
During the time that the acids were allowed to stand together before dilution, 
the stopper of the bottle was several times thrown out by the escaping gas ; 
but, by keeping the mouth closed by a watch-glass during the whole of the 
twenty-four hours, and taking the opportunity of a cold night, the loss by va¬ 
porization was reduced very considerably, though not altogether avoided. The 
least unfavourable result obtained was as follows :—Two acids, which, wheu 
mixed in Pharmacopoeial proportions, but diluted at once with the water, re¬ 
quired 904'315 grain-measures of soda to neutralize six fluid drachms, took 
for the same quantity, when made according to official directions, only 870-36 
grain- measures. 
It deserves to be noticed that these are results obtained in the winter season ; 
the disparity would be very greatly exaggerated in the summer at a tempera¬ 
ture from 30° to 40° higher. 
The saturating power as well as the density attributed to the nitro-hydro- 
chloric acid, described in the Pharmacopoeia, is in excess of that required by 
theory,—a circumstance that can be accounted for only by assuming that contrac¬ 
tion of volume occurs when the mixture is effected. For calculated from the 
data found in the Pharmacopoeia, and assuming that no diminution of satura¬ 
ting power at all occurs in the making, six fluid drachms should require 899 '7 
instead of 920 grain-measures, as stated. Actually, it is always a considerable 
but variable amount below this. The experiments, therefore, in which such 
numbers were obtained, must have been performed under some exceptionally 
favourable circumstances, such as in practice could not be at command. 
Then comes the question as to the relative advantages of the two modes of 
mixing. Every one is acquainted with the usual explanation of the change that 
occurs in producing what is known as aqua regia. The investigation of the 
constitution of this liquid was successfully carried out twenty years ago by Gay- 
Lussac. The nitric and hydrochloric acids, after standing for twenty-four hours, 
as the Pharmacopoeia describes, constitute a kind of aqua regia. It is a liquid 
which holds in solution hydrochloric and nitric acids, part of which have under¬ 
gone change, producing chlorine and a compound which may be called nitric 
chloroperoxide:— 
2 HFTO 3 + 6 HC1=: 2 NOCl 2 + Cl 2 + 4 HoO. 
The chloronitric compound, being very volatile, partly escapes. If this mix¬ 
ture be diluted with the proper quantity of water and immediately examined, 
it has these characters :—it bleaches litmus and indigo; it dissolves gold-leaf 
without the application of heat; it decomposes and decolorizes permanganate of 
potassium; mixed with iodide of potassium, it sets free a large quantity of 
iodine, simultaneously causing an effervescence due to the escape of nitric oxide. 
The free iodine and gas were measured once or twice, but, as might have been 
expected, the results were not constant. From six fluid drachms, in one case 
as much as 9£ cub. cent, of gas (about half the volume of the liquid) were col¬ 
lected. It was proved to be nitric oxide by reddening when mixed with air, 
and by being totally absorbed by ferrous sulphate. The same acid, examined a 
few days afterwards, was still capable of bleaching and dissolving gold, though 
less rapidly, but, when mixed with iodide of potassium, there was a much 
smaller amount of liberated iodine, and a bubble of gas no larger than a pea. 
In little more than a week, change had progressed so far that there was very 
slight decomposition of iodide of potassium, and the bleaching effect could be 
only imperfectly produced; moreover, permanganate was very little affected 
by it. It seems from this, that the action of the water upon aqua regia is to 
set up immediate decomposition of the chloronitric compound, effecting its reso¬ 
lution into hydrochloric acid, and the products of the decomposition of nitric 
peroxide, namely, nitric and nitrous acids :— 
