140 Dr. Priestley’s Experiments on the 
this effect was produced by the action of heat , evolving, as it 
were, the phlogifton previoufly contained in the acid. After- 
wards, having found that it was not heat, but light only, that 
was capable of giving colour to fpirit of nitre, contained in 
phials with ground (toppers, in the courfe of feveral days ; 
and that in this cafe the effect was produced by the aft ion of 
light upon the vapour , which gradually imparted its colour to 
the liquor on which it was incumbent (fee Vol. V. p. 342.), I 
was led to fufpeft, that as the glafs tubes, in which I had for- 
merly expofed this acid to the aftion of heat, were only held 
near to a fire, in the day-light, or candle-light, it might have 
been this light , which, in thefe circumftances, had, at lead in 
part, contributed to produce the effeft. 
In order to afcertain whether the light had had any influ- 
ence in this cafe, I now put the colourlefs fpirit of nitre into 
long glafs tubes, like thole which I had ufed before, and alfo 
fealed them hermetically, as I had done the others ; but, in- 
dead of expofing them to heat in the open air, from which 
light could not be excluded, I now (hut them up in gun bar- 
rels, clofed with metal fcrews, fo that it was impoffible for 
any particle of light to have accefs to them ; and I then placed 
one end of the barrels fo near to a fire as was fufficient to make 
the liquor contained in the tube to boil, which I could eafilv 
diftinguilh by the found which it yielded. The confequence 
was, that in a fhort time the acid became as highly coloured 
as ever it had been when expofed to heat without the gun 
barreh It was evident, therefore, that it had been mere beat* 
and not light, which had been the means of giving this colour 
to the acid, and which has been ufually termed phlogijlicating 
it. 
When 
