33'l Observations on some Comlinatlons 



ought to admit tliat the compound of azote and oxygen which 

 is generally known by the name of introus acid gas, is not a gas 

 at the common temperature and pressure of the atmosphere, 

 but rather a liquid wliich I shall call in the meantime dry or 

 anhydrous nitrous acid, to avoid all misunderstanding. 



I found l'-45l for its specific gravity, and 28° for the tem- 

 perature at which it enters into ebullition, the barometer being 

 at 76". 



If we have hitherto mistaken the physical properties of this 

 combination, it is because the vapour which it forms maintains 

 a very strong tension at the conmion temj)erature ; and in the 

 greater number of cases in which it is |noduced, it is mixed 

 with permanent gases which prevent its condeusation. 



We may easily see from this, that the condensation of the 

 dry nitrous acid will be the more difficult, and that it will be 

 necessary to employ, in order to produce it, a temperature so 

 much the lower as the proportion of foreign gas is greater. This 

 accounts for the differences vvhich are observed in the products 

 of the distillation of the nitrates. When the base of the salt 

 has but a feeble affinity for the acid, and when it allows it to be 

 extricated at a low temperature, the nitric acid is decomposed 

 only into oxygen and nitrous acid ; and even if we shall suppose 

 that these two bodies are set free at the sa,me time, the vapour 

 of the nitrous acid forming at least the two-thirds of the gaseous 

 mixture, it might be partly condensed, even at the temperature 

 of 15'^: this is what hapjjens with the nitrate of lead. When 

 on the contrary the base forcibly retains the acid, and precipi- 

 tates, the having recourse to a very high temperature for the 

 decomposition of the salt, the greater part of the nitric acid 

 being then reduced into oxygen and azote, it will require a con- 

 siderable degree of cold to liquefy even in part the nitrous acid. 

 Thus by submitting the gases which are liberated during the 

 decomposition of the nitrate of barvtes to a culd of 20^ below 

 0', I have not obtained a single drop of liquid ; because, as 

 we know, the greater part of the nitric acid was then transformed 

 into a mixture of oxygen and azote. 



It is also for this reason, that when it is our object to make 

 directly with oxygen and nitrous gas dry nitrous acid, we must 

 so manage as to leave but a feeble excess of oxygen, as I have 

 indicated above ; otherwise we shall obtain but very little li- 

 quid. 



If we pass into the apparatus described a little higher a mix- 

 tvire of nitrous gas and oxygen gas, in which there is a little more 

 than four parts of the former to one of the latter, there is still 

 condensed a liquid in the cooled tube: but this liquid is of a 

 very deep green, and much more volatile than the foregoing. 1 



have 



