76 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



tions of confining the nitrous acid vapour by animal oils, 

 it occurred to me, that, in lieu of this, it might not be wholly 

 without its use, if I could shut up this vapour in dry glass 

 phials, with ground stopples. And though, in this method 

 of procuring it, by the solution of bismuth, or other things 

 with which it unites most rapidly, there is necessarily a 

 mixture of nitrous air, it is inconsiderable in proportion to 

 the quantity of pure nitrous vapour itself. And although a 

 mixture of common air also would necessarily remain in the 

 phial, it could only serve to dilute the acid vapour, and 

 could not materially alter the properties of it. Also, if the 

 mouths of the phials were small, they might be opened, and 

 various substances admitted to the vapour, without much 

 loss of the acid ; especially as all acid vapours, I had reason 

 to think, were heavier than common air" (Experiments on 

 Air, 1777, III. 184). 



The most conspicuous quality of the nitrous fumes is 

 their brown colour, by which their presence is disclosed in 

 many operations in which nitric acid takes part. The 

 colour is permanent, and has the remarkable property of 

 becoming much intensified by heating. This observation 

 interested Priestley so much that he was in the habit of 

 carrying a bottle of the gas in his pocket in order to show 

 its curious behaviour to his friends. Priestley writes : 



" The change of colour given to this vapour by heat is 

 not a little remarkable, for it is altogether independent of 

 gravity or condensation. In order to make some experi- 

 ments of this kind to proper advantage, I procured a glass 

 tube, three feet long, and about an inch wide, closed at one 

 end, and fitted with a ground stopple at the other. This 

 tube I easily filled with red vapour, in consequence of its 

 being much heavier than common air ; and closing the 

 open end with the stopple, observed, that that part of the 

 tube which I held in my hand was manifestly of a deeper 

 colour than any other part of the tube. On this I held 

 one end of it to the fire, and found that that end grew 

 most intensely red, three or four times more so than the 

 rest of the tube. The direction in which the tube was 



