460 DR LYON PLAYFAIR AND MR J. A. WANKLYN ON A MODE OF 
Nitric Acid. 
It can hardly be said that the state of condensation of the acids has been 
determined by direct experiment. Most of the acids are too little volatile, or 
suffer decomposition at too low temperatures, to admit of such experiments. 
And those acids whose vapour-densities have been actually observed are seldom 
in perfect accordance with theory. The vapour-density of sulphuric acid, for 
example, is abnormal: and though we are far from demurring to the explanation 
that that acid suffers decomposition into anhydride and water, it must be allowed 
that direct experiment has not yet established the molecular weight indicated by 
the formula 80 } Op. 
2 
4 
Again, the series of fatty acids, although normal at high temperatures, are 
abnormal at low temperatures: indeed, it is difficult to find an example of an 
acid which does not offer some anomaly of the kind. 
We have selected nitric acid forexperiment. As will be apparent by-and-by, 
this acid is regular, having for its molecular weight the number answering to the 
NO 
formula “jy ° On 
The acid employed was distilled from sulphuric acid, then from dried sulphate 
of copper, and, lastly, warmed in a stream of dry carbonic acid, so long as its 
colour became less yellow. The final product was found to be quite free from 
sulphates and chlorides. Its strength, as indicated by alkalimetry, was satisfac- 
tory. Its colour was a very pale yellow, which the transmission of carbonic acid 
was incapable of rendering fainter. 
From our observation, we are inclined to believe that the last yellow tinge can- 
not be removed from monohydrated nitric acid by warming for any length of time 
in a current of perfectly dry gas; but the presence of a minute trace of mois- 
ture in the transmitted gas at once decolorises the acid. The success of the 
employment of urea as a means of depriving nitric acid of colour, depends partly 
on the production of water. We need hardly remark that we avoided the use of — 
urea. We believe, moreover, that the quantity of peroxide of nitrogen requisite to 
give the yellow tinge is imponderable. 
As nitric acid attacks mercury, Gay-Lussac’s process is quite inapplicable 
to it; and since nitric acid undergoes some decomposition on boiling, Dumas’ 
process would not answer without some modification. 
The following determinations have been made by the method previously 
described. The gas used to dilute the nitric acid vapour was dried air. 

