VOL. LXXIX.] PHILOSOPHICAL TRANSACTIONS. 55/ 



XL Experiments on the Phlogistication of Spirit of Nitre. By the Rev. J. 

 Priestley, LL.D., F.R.S. p. ISQ. 

 In my former experiments, vol. 4, p. 2, I found that the colourless acid be- 

 came smoking, or orange-coloured, and emitted orange-coloured vapours, on 

 being exposed to heat in long glass tubes, hermetically sealed ; and I then con- 

 cluded, that this effect was produced by the action of heat, evolving, as it were, 

 the phlogiston previously contained in the acid. Afterwards, having found that 

 it was not heat, but light only, that was capable of giving colour to spirit of 

 nitre, contained in phials with ground stoppers, in the course of several days; 

 and that in this case the effect was produced by the action of light on the vapour, 

 which gradually imparted its colour to the liquor on which it was incumbent (see 

 vol. 5, p. 342,) I was led to suspect, that as the glass tubes, in which I had for- 

 merly exposed this acid to the action of heat, were only held near to a fire, in the 

 day-light, or candle-light, it might have been this light, which in these circum- 

 stances had, at least in part, contributed to produce the effect. 



In order to ascertain whether the light had had any influence in this case, I now 

 put the colourless spirit of nitre into long glass tubes, like those which I had used 

 before, and also sealed them hermetically, as I had done the others; but, instead 

 of exposing them to heat in the open air, from which light could not be ex- 

 cluded, I now shut them up in gun barrels, closed with metal screws, so that it 

 was impossible for any particle of light to have access to them ; and I then placed 

 one end of the barrels so near a fire as was sufficient to make the liquor contained 

 in the tube to boil, which I could easily distinguish by the sound it yielded. The 

 consequence was, that in a short time the acid became as highly coloured as ever it 

 had been when exposed to heat without the gun barrel. It was evident therefore, 

 that it had been mere heat, and not light, which had been the means of giving 

 this colour to the acid, and which has been usually termed phlogisticating it. 



When I made the former experiments, I had no suspicion that the air con- 

 tained in the tube had any concern in the result of them ; and, in those which 

 I made in the phials in a moderate heat, I found that the acid received its colour 

 when the best vacuum that I could make with an air pump was over it. My 

 friend Mr. Kirwan, however, having always suspected, that the air was a princi- 

 pal agent in the business, I at this time gave particular attention to this circum- 

 stance; supposing that, if any part of the common air had been imbibed, it 

 must have been the phlogisticated, and that it was the phlogiston from this kind 

 of air which had phlogisticated the acid. The real result however was not so much 

 in favour of this supposition as I had expected; for the principal effect of the 

 process was the emission of dephlogisticated air, so that the acid seems to become 

 what we call phlogisticated, by parting with this ingredient in its composition. 



I put a small quantity of the colourless acid into a long glass tube, which, besides 

 the acid, would have contained 1.23 ounce measures of common air, but that 



