{the Hydrate of) Sulphuric Acid tvith Nitric Oxide. 89 



til sulphuric acid passed over, always obtaining pure sulphuric 

 acid as the residuum ; I had, however, especially with the ad- 

 dition of nitric acid, to continue the heating so long, that the 

 remaining sulphuric acid had a spec. gr. of 1*84<. Even when 

 concentrated sulphuric acid is mixed with nitric acid, and the 

 mixture exposed to a very gentle heat, sulphuric acid, almost 

 jiure, is obtained as the residue. If the concentrated acids be 

 mixed together without any change of temperature taking 

 place, and the mixture be left to stand for several weeks, no 

 decomposition appears to occur; if large quantities be quickly 

 mixed together, a trace of the sulphate of nitric oxide is formed, 

 which probably ai'ises from a disengagement of heat. But if 

 the mixture be quickly heated in a retort, a decomposition en- 

 sues, the neck of the retort is filled with red fumes, and at 

 first a sulphuric acid, containing nitric acid, distils over, then 

 pure sulphuric acid, and in the residue remains sulphuric acid, 

 containing the sulphate of nitric oxide dissolved. When the 

 sulphuric acid is coloured by organic substances, and it is 

 decolorated by warming and adding drop by drop nitric acid 

 to it, it becomes contaminated with sulphate of nitric oxide. 



According to this the sulphuric acid ought to occur in com- 

 merce free from every oxide of nitrogen, when it has a specific 

 gravity of r84', of which strength it however rarely occurs, and 

 only rendered impure by the sulphate of nitric oxide when it 

 has been discoloured by nitric acid. Of late the sulphuric 

 acid is concentrated in vessels of platina, which are so con- 

 structed that dilute sulphuric acid continually flows into the 

 more concentrated, and this is probably the cause that the 

 sulphate of nitric oxide is formed which then does not distil 

 over. 



Barruel* has proposed to digest the impure sulphuric acid 

 over sulphur at 200° cent, in order to destroy the acids of ni- 

 trogen, and then to distil. But if the acid be distilled, then 

 this is superfluous, for even did the sulphuric acid contain ni- 

 tric acid, a pure acid would be obtained by distillation, which 

 is even, as I have stated, obtained when one ounce of nitric 

 acid is added to a pound of sulphuric acid, a contamination 

 which occurs seldom if ever in commerce; it is merely requi- 

 site to change the receiver frequently. If the sulphuric acid 

 contains the sulphate ol' nitric oxide, then a pure acid passes 

 over immechately. 



To procure a pure sulphuric acid for the preparation of 

 hydrocldoric acid, it is only necessary to mix it, (it mat- 

 ters not whether it be contaminated by the sulphate of nitric 

 oxide or nitric acid, with two parts of water) and then to heat 

 * Ccntralblatt, 1836. p. 315. 



