64 



ous, and the Hon. Mr. Cavendish favoured me w^ith an account of some ex- 

 periments of his, in which a quantity of common air was reduced from 180 

 to 162 ounce measures, by passing through a red hot iron tube filled with the 

 dust of charcoal. This diminution he ascribed to such a destruction of com- 

 mon 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 

 repeated, with a small variation of circumstances, and with the same result." 

 The following paper, containing the first clear description of nitrogen as a 

 distinct gas, is the communication thus defectively described. Cavendish, 

 in fact, was the first to point out, as distinct /achVioi^s airs, besides hydrogen, 

 the carburetted gases, nitrous gas, muriatic acid gas, and nitrogen] : — 



Paper communicated to Dr. Priestley. 



The receiver used in the 9th experiment of my 2nd paper on factitious 

 air was a bolthead, from which I had cut off the greatest part of the neck, 

 & thereby consisted of a globular body about 9 inches in diameter, with a 

 neck about 2 or 3 inches in diameter & about 2 inches long. 



As the fixed air was let into the receiver first, & the common air after- 

 wards, I think they could hardly fail of being well mixed together by the 

 commotion made by letting in the common air. However, as Dr. Priestley 

 thinks they were not, & that it was owing to that that the candle went out 

 so soon, I made an experiment to see whether they were well mixed or not ; 

 this I did by seeing whether the candle would burn as long in a mixture con- 

 taining x'xV of fixed air, when held near the bottom of the receiver, as when 

 held near the top. For if the air was not perfectly mixed, the fixed air, as 

 being the heaviest, would have kept chiefly at the bottom of the receiver, & 

 consequently the candle would have burnt longer at the top than at the bottom 

 of it. The event was as follows : — When the candle was held at the bottom 

 of the receiver it burnt 21", when held near the top it burnt in three different 

 trials 17", 26", & 19". The same candle burnt, in the same receiver filled 

 with common air only, when held near the bottom, 82" & 69"; & when 

 held near the top it burnt 79" and 66" ; so that the 2 sorts of air seem to 

 have been perfectly mixed. The experiment was tried just in the same 

 manner & with the same receiver as that related in the Transactions ; & 

 the candle, in those trials where I have said it was held near the bottom of 

 the receiver, was held in the same situation as in that experiment. N.B. It 

 was by mistake that I made only 1 trial with the candle near the bottom & 

 3 with it near the top, as I intended to make two trials in each manner. 



There is no experiment related in that letter of Dr. Priestley's which you 

 read to me, that shows that mephitic air will mix with common air in time, 

 of itself, without any shaking, as he says that phials of common air, held in 

 vessels of mephitic air, became mephitic, which could only be owing to some 

 of the mephitic air mixing with the common air therein, which, according to 

 my experiment, wd render it unfit for candles to burn in, & in all pro- 

 bability wd render it unfit for breathing, which is what I suppose he means 

 by becoming mephitic. I am not certain what it is which Dr. P. means 

 by mephitic air, though from some circumstances I guess that what he 



