63 



ashes in the 2nd part of these experiments : it appeared to contain 62 grains. 

 As this experiment makes the quantity of fixed air produced from the caput 

 mortuum appear to be greater than the loss of weight which it suffered in de- 

 flagration, which is impossible, I took another method to find the quantity in 

 the remaining i of the alcaline solution ; namely, I mixed it with a sufficient 

 quantity of lime water, whereby all the fixed air therein was transferd into 

 the lime, which was thereby precipitated. 



I then found the quantity of fixed air in this precipitate ; it appeared to be 

 59 grains, which is only 3 grains less than it appeard to be the other way. By 

 a mean of these experiments, the quantity of fixed air separated from the 150 

 grains of caput mortuum should be 121 grains, which is 9 grains more than 

 the loss of weight which it suffered in deflagration. 



By a like experiment made with some more caput mortuum of the same 

 kind the quantity of fixed air seemd still greater. 



As it is impossible that the quantity of fixed air separated from the caput 

 mortuum should exceed the loss of weight which it suffers in deflagration, I 

 must either be mistaken in supposing that all the fixed air in the alcali pro- 

 ceeded from the caput mortuum, & not from the nitre ; or else some moisture 

 must have flown off along with the fixed air in saturating the alcali with the 

 acid : which would make the quantity of fixed air therein appear greater than 

 it really is. This last supposition seems much the most probable. 



PAPER ON MEPHITIC AIRS. 



[On this Paper Cavendish has written, " communicated to Dr. Priestley." 

 In the account given by Priestley of his Experiments and Observations made 

 in and before the year 1772 (Sect. 6. Ed. 1774. p. 109.), he says, " Ever 

 since I first read Dr. Hales's most excellent Statistical Essays, I was particu- 

 larly struck with that experiment of his, in which common air and air gene- 

 rated from the Walton pyrites by spirit of nitre, made a turbid red mixture, 

 and in which part of the common air was absorbed ; but I never expected to 

 have the satisfaction of seeing this remarkable appearance, supposing it to be 

 peculiar to that particular. Happening to mention this subject to Mr. 

 Cavendish, when I was in London in the spring of the year 1772, he said 

 that he did not imagine but that other kinds of pyrites, or the metals, might 

 answer as well, and that probably the red appearance of the mixture de- 

 pended upon the spirit of nitre only : this encouraged me to attend to the 

 subject." We have already seen the notice which Cavendish had taken of 

 nitrous gas and nitrous acid as early as 17G4. Section 9, p- 143, begins 

 thus : — " Being very much struck with the result of an experiment of the 

 Hon. Mr. Cavendish, related Phil. Trans, vol. Ivi. by which, though he 

 says he was not able to get any inflammable air from copper by means of 

 spirit of salt, he got a much more remarkable kind of air, one that lost its 

 elasticity by coming into contact with water, I was exceedingly d?sirous 

 of making myself acquainted with it." In Section 7 of the same account 

 of his Experiments in and before 1772 (.Ibid. p. 129), Priestley adds, "Air 

 infected with the fumes of burning charcoal is well known to be noxi- 



