On€alcarim's and Gfpjious Earths, 45 



^''i^nis part of the theory vinll therefore hardly admit of a 

 doubt. The-exiftence of an alkali in the air I think is proved 

 from the extreme volatility of fome alkalies ; from the poffi- 

 bility by heat alone of rendering them permanently daltic. 

 From 'their being continiiklly exhaled fix)m burning and putri- 

 fying fubftances.— ^Dn Blac-k imputes the ruft of metals . to the 

 a6lion of an alkali exiiting in t-he air, and fays alkaline falts 

 are often collefted from the corks of bottles containing 

 acids — the atmofphere itfelf is a compound, which differs fo 

 efientialy from pure vi'tal air, as to contain only 28 parts in 

 100 of it ; the reft is fixed and inflam'maWe ah", 'fire, earth, 

 water, and an infinite variety- of-ibtlret fnbftances, befrdes va- 

 pours that are not permanently elafiic. The pureft earths, 

 the hardeft metals, may be converted into air, anti float in 

 the atmofphere, which may itfelf be changed into water, and 

 water into folid earth. 



V\''hat ftrikes me as a farther evidence of the exiftenceofan 

 alkali in the air, is, that fuch parts of. flint or limeftone, as are 

 expofedtothe a6lion.of the air and .water, are always white 

 and foft ; flint I believe is not foluble i;i acicls, but will diffblve 

 in alkalies ; nor can the decompofition of limeftone, be attri^ 

 buted to the aftion of acids,, fmce, as this is very gradually 

 effefted, it is to be prefumed, that if the acid in the air was 

 ftrong enough to expel the fixed air, it would unite ^'ith the 

 Calearious e-artb, and ftill form a fait v/ith an earthybafis of a 



