76 PROVOSTAYE ON THE ACTION OF 
lution quite similar to that which I have mentioned above. It 
would appear that within certain limits water replaces the nitric 
oxide which is evolved, in nearly equal weights. 3°209 grammes 
of substance fusing at 217° cent. (422° Fahr.) were introduced 
into a small tube, in which the air circulated with difficulty. 
After seven or eight hours the weight had not sensibly varied, 
although nitric oxide was continually evolved. At the expira- 
tion of about twenty hours there was an increase of 5 or 6 milli- 
grammes; but this arose from the thin stratum of sulphuric 
acid absorbing a little water; for on heating until fusion oc- 
curred, which took place at 160° cent. (320° Fahr.), the weight 
became precisely as at first, that is, 3°209 grammes; doubtless 
because the water decomposed one portion of the substance, 
disengaging a quantity of nitric oxide exactly equal to its own 
weight. 
With indigo the sulphuric solution gives a series of magnifi- 
cent colours, provided it is sufficiently concentrated ; a green in 
the cold, which, on heating, changes to a rose-colour, and then 
through every possible shade to purple, more or less deep. 
The composition and properties of this substance once well 
established, it is very easy to comprehend the reaction which 
has given birth to it. This can, in fact, be represented by the 
following formula :— 
2(NO, + SO,) = $0: NO, 22 N Op 
It is therefore nitrous acid, which, on opening the tube, as- 
sumes the gaseous state with such violence as to shatter the 
vessel, if care is not taken to adopt the necessary precautions. 
It is also nitrous acid, combined with hyponitric acid in excess, 
which forms the green liquid of which we have previously spoken. 
It exhibits, in fact, all the properties which M. Dulong observed 
that acid to possess (Ann. de Ch. et de Ph., t. ii. p. 323). 
As to the rational formula, it may either be 
NO, + 2S 0,, or ape. 
It may be advanced against the last view,—1st, that if sulphur- 
ous acid be found partly in a state of combination with oxygen 
and partly with hyponitric acid, it is not apparent why, on pre- 
senting two liquids to each other, capable of combining without 
decomposition, there is formed anhydrous sulphuric acid. 
2nd. If a mixture of nitric oxide and oxygen gases, in proper 
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