110 JOSEPH PRIESTLEY. [LEOT. 



had not occurred to him. 1 But in pursuing his 

 experiments on the evolution of air from various 

 bodies by means of heat, it happened that, on the 1st 

 of August 1 774, he threw the heat of the sun, by means 

 of a large burning glass which he had recently ob- 

 tained, upon a substance which was then called 

 mercurius calcinatus per se, and which is commonly 

 known as red precipitate. 



" I presently found that, by means of this lens, air was 

 expelled from it very readily. Having got about three or four 

 times as much as the bulk of my materials, I admitted water to 

 it, and found that it was not imbibed by it. But what surprised 

 me more than I can well express, was that a candle burned in 

 this air with a remarkably vigorous flame, very much like that 

 enlarged flame with which a candle burns in nitrous air, exposed 

 to iron or lime of sulphur ; but as I had got nothing like this 

 remarkable appearance from any kind of air besides this par- 

 ticular modification of nitrous air, and I knew no nitrous acid 

 was used in the preparation of mercurius cahinatiis, I was utterly 

 at a loss how to account for it. 



" In this case also, though I did not give sufficient attention 

 to the circumstance at that time, the flame of the candle, besides 

 being larger, burned with more splendour and heat than in that 

 species of nitrous air ; and a piece of red-hot wood sparkled in 

 it, exactly like paper dipped in a solution of nitre, and it con- 

 sumed very fast an experiment which I had never thought of 

 trying with nitrous air." 2 



Priestley obtained the same sort of air from red 

 lead, but, as he says himself, he remained in ignor- 

 ance of the properties of this new kind of air for 

 seven months, or until March 1775, when he found 



1 " Experiments and Observations on Different Kinds of Air," vol. 

 ii. p. 31. 2 Ibid. pp. 34, 35. 



