65 



speaks of in this letter was that to which Dr. Black has given the name of 

 fixed air. The natural meaning of mephitic air is any air which suffocates 

 animals (& this is what Dr. Priestley seems to mean by the word), but in 

 all probability there are many kinds of air which possess this property. I 

 am sure there are 2, namely, fixed air, & common air in which candles 

 have burnt, or which has passed thro' the fire. Air which has passed 

 thro' 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 suf- 

 fered a change in its nature from the fire. As I formerly made an experi- 

 ment on this subject, which seems to contain some new circumstances, I will 

 here set it down. 



I transferd some common air out of one receiver through burning char- 

 coal into a 2nd receiver by means of a bent pipe, the middle of which was 

 filled with powdered charcoal & heated red hot, both receivers being in- 

 verted into vessels of water, & the 2nd receiver being full of water, so that no 

 air could get into it but what came out of the first receiver & passed through the 

 charcoal. The quant, air driven out of the first receiver was 180 oz. measures, 

 that driven into the 2nd receiver was 190 oz. measures. In order to see whether 

 any of this was fixed air, some sope leys was mixed with the water in the bason, 

 into which the mouth of this 2nd receiver was immersed ; it was thereby re- 

 duced to 168 oz., so that 24 oz. meas. were absorbed by the sope 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, 

 agreeable 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 another 

 receiver; it then measured 16/ oz., & was reduced by sope leys to 162 oz., 

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

 4 oz. of common air absorbed. The reason of this is, 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 2nd time ; for char- 

 coal will not burn without the assistance of fresh air, & consequently not 

 much fixed air could be generated, nor much common air absorbed. The 

 specific gravity of this air was found to diff"er very httle from that of common 

 air; of the two it seemed rather lighter. It extinguished flame, & ren- 

 dered 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, 

 & which went out immediately in common air mixed ^vith /s- of fixed 

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

 burnt air. 



LETTER OF CAVENDISH TO MONGEZ. 



A Londres, ce 22 Fevrier, 1785. 



En lisant. Monsieur, la traduction de mon memoire sur I'air public dans le 



Journal de Physique, je fus frappe de le voir datte de Janvier 83, comme si la 



lecture en eut ete faite alors devant la Societe Royale. J'eus recours aux 



exemplaires detaches imprimes pour I'usage de mes amis sur I'un desquels 



