120 
PURIFICATION OF NITRIC ACID. 
rising, an acid was obtained of the density 1.20; on putting 
platinum wire into the retort, the point of ebullition fell to 
252°, but the acid collected had only a specific gravity 1.175, 
instead of following the general rule with regard to the in- 
creasing density. 
In distilling either a concentrated or a weak acid, the dis- 
tillation of the last third of it presents some remarkable 
phenomena. A period arrives when the ebullition ceases; 
at the same time the thermometer, which stood at 250° or 
252° Fahr., rises to 266°, or even 273°, and the acid passes 
into the receiver with great rapidity, without any appear- 
ance of bubbling or agitation; but it generally happens, 
that, in a few minutes there is suddenly disengaged an evo- 
lution of vapor, colored yellow by nitrous acid; the ther- 
mometer falls 14° or 18° Fahr., and in a few moments the 
ebullition recommences. 
M. Millon concludes from these experiments, in relation 
to the distillation of the concentrated and the weak nitric 
acid, that the latter arrives at a state of hydratation, at 
which it contains four and a half equivalents of water, and 
has a density not greater than 1.405, at 68° Fahr., while 
the former tends to stop at a point at which its densit}^ is 
1.419, and which contains four equivalents of water. 
We may, therefore, obtain, says M. Millon, by the 
processes which I have described, four hydrates of nitric 
acid. 
Az 5 HO 
Az 5 2 H 
Az 5 4 HO 
Az 5 4 H H0 
5 2 
Ibid, from Journal de Pharmacie. 
