VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 507 



nitre and common salt, though in small quantity. But to try whether nitrous 

 selenite would attract any, I made a solution of chalk in nitrous acid, which, 

 when saturated, weighed 381.25 grs. ; but being exposed to the air for a few 

 hours, it weighed 382.25. I afterwards took a very dilute nitrous acid, in which 

 an arid taste was barely perceptible, and impregnated it with a very small propor- 

 tion of fixed air, and then let fall a few drops of it into lime-water ; not the 

 smallest cloud was perceived, and yet when I breathed into it afterwards it be- 

 came milky in a few seconds ; so that this experiment is perfectly analogous to 

 that in which nitrous and common air were mixed. 



But if nitrous air and common air be mixed over dry mercury, the result is 

 entirely adverse to the opinion of Mr. Cavendish, and favourable to mine ; for in 

 this case the common air is not at all diminished till water is admitted to it, and 

 the mixture agitated a few minutes, and then the diminution is nearly the same 

 as if the mixture were made over water. Thus, when I mixed 2 cubic inches of 

 common air with 1 of nitrous air, they occupied the space of 2-f inches, and the 

 surface of the mercury was immediately calcined ; which shows that the inch of 

 nitrous air was decomposed, and produced nitrous acid ; but the common air was 

 undiminished ; and the ± of an inch over and above the 2 inches of common air, 

 proceeded from an addition of new nitrous air, formed by the corrosion of the 

 surface of the mercury. That the common air should remain undiminished, is 

 easily explained in my system, because fixed air is formed, which, on this occa- 

 sion, must remain unabsorbed, at least for a long time, as there is nothing at 

 hand that can immediately receive it ; and hence, if water be admitted soon after 

 the mixture of both airs, the diminution will be nearly the same as if the mix- 

 ture had been originallv made over water, though not exactly the same ; because 

 the nitrous air, produced by the union of the newly formed nitrous acid with the 

 mercury, is not entirely absorbable by water. But, in Mr. Cavendish's hypothe- 

 sis, the common air should be diminished just as much as if the mixture were 

 made over water ; for, according to him, this diminution arises from the conver- 

 sion of the dephlogisticated part of the common air into water, which water 

 should immediately unite to the nitrous salt of mercury, and leave the common 

 air lessened in its bulk by a portion commensurate to that converted into water; 

 or, if he will not allow the water to have immediately united to the mercurial 

 salt, at least by the difference of the bulk of the water produced, and that of 

 an equal weight of the common air converted into it : but neither happens ; for 

 the common air is not at all diminished ; nor can he explain, consistently with 

 his system, why the admission of water should immediately produce a diminution 

 in the common air, as, according to him, it contains nothing that can be ab- 

 sorbed. Dr. Priestley has remarked, that if a mixture of both airs be suffered 

 to stand several hours, even the admission of water will produce no diminution. 



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