relative to Black, Watt, and Cavendish. 501 



common air in which candles have burned, or which has passed 

 through the Jire. Air which has passed through a charcoal 

 fire contains a great deal of fixed air which is generated from 

 the charcoal; but it consists principally of common air which 

 has suffered a change in its nature from the Jire. As I formerly 

 made an experiment on this subject which seems to contain 

 some new circumstances, I will here set it down*." 



This important communication Priestley scarcely turned to 

 better account than that which he afterwards received from 

 the same skilful friend, of the composition of water ; he quotes 

 it indeed explicitly, but most defectively, in his paper in the 

 Phil. Transactions of 1772. " Mr. Cavendish," he says, 

 "favoured me with an account of some experiments of his, in 

 which a quantity of common air was reduced from 180 to 

 162 oz. measures, by passing through a red-hot iron tube 

 filled with the dust of charcoal : this diminution he ascribed 

 to such a destruction of common air as Dr. Hales imagined to 

 be the consequence of burning: Mr. Cavendish also observed 

 that there had been a generation of fixed air in this process, 

 but that it was absorbed by soap-leys : this experiment I also 



* " I transferred some common air out of one receiver through burning 

 charcoal into a second receiver, by means of a bent pipe, the middle of 

 which was filled with powdered charcoal and heated red-hot, both receivers 

 being inverted into vessels of water, and the second receiver being full of 

 water, so that no air could get into it but what came out of the first re- 

 ceiver and passed through the charcoal. The quantity of air driven out of 

 the first receiver was 180 oz. measures, that driven into the second receiver 

 was 190 oz. measures. In order to see whether any of this was fixed air, 

 some soap-leys were mixed with the water in the basin into which the mouth 

 of this second receiver was immersed: it was thereby reduced to 166 oz. ; 

 so that 24 oz. measures were absorbed by the soap-leys, all of which we 

 may conclude to be fixed air produced from the charcoal ; therefore 14 oz. 

 of common air were absorbed by the fumes of the burning charcoal, agree- 

 able to what Dr. Hales and others have observed, that all burning bodies 

 absorb air. The 166 oz. of air remaining were passed back again in the 

 same manner as before, through fresh, burning charcoal into the other re- 

 ceiver: it then measured 167 oz. and was reduced by soap-leys to 162 oz.; 

 so that this time, only 5 oz. of fixed air were generated from the charcoal, 

 and only 4 oz. of common air absorbed. The reason of this was that since 

 the air was rendered almost unfit for making bodies burn by passing once 

 through the charcoal, not much charcoal could be consumed by it the se- 

 cond time; for charcoal will not burn without the assistance of fresh air, 

 and consequently not much fixed air could be generated, nor much common 

 air absorbed. The specific gravity of this air was found to differ very little 

 from that of common air, of the two it seemed rather lighter. It ex- 

 tinguished flame, and rendered common air unfit for making bodies burn, 

 in the same manner as fixed air, but in a less degree, as a candle which 

 burnt about 80" in pure common air mixed with ■£■% of fixed air, burnt 

 about 26" in common air, mixed with the same portion of this burnt air." 

 The gas thus obtained by Cavendish was nitrogen, with perhaps -^ of car- 

 bonic oxide. 



