50t» PHILOSOPHICAL TRANSACTIONS. [ANNO 17&4. 



which is immediately absorbed by the alkali, and renders it mild. In the second 

 case, no inflammable air is produced, the common air is not phlogisticated, and 

 consequently the alkali remains caustic* This experiment also proves that me- 

 tallic calces attract fixed air more strongly than alkalis attract it ; for the calces 

 of zinc arc known to contain fixed air, and yet alkalis digested with them remain 

 caustic ; and this accounts for the slight turbidity of lime-water when metals are 

 calcined over it ; for as soon as the phlogiston is disengaged from the metal, and 

 before it has absorbed the whole quantity of fire requisite to throw it into the 

 form of inflammable air, it meets with the dephlogisticated part of the common 

 air on the surface of the metal, and there forms fixed air, which is instantly 

 absorbed by the calx with which it is in contact ; so that it is not to be wondered 

 that it does not unite to the lime from which it is distant. 



Of the decomposition of nitrous air by mixture with common air. — As soon as 

 I had heard Mr. Cavendish's paper read, I set about trying whether lime would 

 be precipitated from lime-water during the process, an experiment I had never 

 made before with common air, taking it for granted that it was so, from the re- 

 peated experiments of Dr. Priestley, and indeed of all others who had treated 

 this subject :-f- and, in effect, when I made the experiment with nitrous air pre- 

 pared and confined by the water of my tub, I found lime-water admitted to it 

 instantly precipitated. But after I had read Mr. Cavendish's paper, which he 

 had the politeness to permit me, and had, according to his direction, received 

 the nitrous air over lime-water, I did not then perceive the least milkiness after 

 admitting common air. After 12 hours I indeed perceived a whitish dust, on 

 the bottom of the glass vessel in which the experiment was made, which I cannot 

 assure to be calcareous ; and, on breathing into the lime-water, an evident 

 milkiness ensued ; so that I little doubt but the precipitation I observed in the 

 first experiment arose from the decomposition of the aerial selenite contained in 

 the water of the tub. And it is very possible that the precipitation of lime, 

 which I perceived some years ago on mixing dephlogisticated air and nitrous air, 

 might have arisen from the same cause, or from fixed air pre-contained in the de- 

 phlogisticated, as this last had not been washed in lime-water. Yet I do not 

 think the failure of this experiment at all conclusive against the supposed pro- 

 duction of fixed air on this occasion, because the quantity of fixed air is so small, 

 that it may well be supposed to unite to the nitrous selenite formed in the lime 

 water. It is well known that a small quantity of fixed air is capable of uniting 

 to all neutral salts : thus Dr. Priestley has extracted it from tartar vitriolate and 

 alum ('2 Pr. 1 15, ll6,) and gypsum, ('2 Pr. 80;) and Dr. Macbride found it in 



* See Mr. Lassone's Experiments on zinc. Mem. Par. 1777, P- 7 and 8. — Orig. 



-)- See 1 Pr. 114, 18<). 2 Pr. 218. Font. Recherches Pliys. p. 77- 1 Cliy. Dij. 324.— Orig. 



