vi CARBONIC ACID AND THE CARBONATES 109 



vegetable fixed alkali to the air, in a broad and shallow 

 vessel, for the space of two months ; after which I found a 

 number of solid crystals, which resembled a neutral salt so 

 much as to retain their form .pretty well in the air, and to 

 produce a considerable degree of cold when dissolved in 

 water. Their taste was much milder than that of ordinary 

 salt of tartar ; and yet they seemed to be composed only of 

 the alkali, and of a larger quantity of air than is usually con- 

 tained in that salt, and which had been attracted from the 

 atmosphere : for they still joined very readily with any acid, 

 but with a more violent effervescence than ordinary ; and 

 they could not be mixed with the smallest portion of 

 vinegar. . . . without emitting a sensible quantity of air " 

 (A.C.R. XVII. 42). 



The same salt was prepared by Cavendish and by Berg- 

 man by the direct action of fixed air on a solution of the 

 alkali. Cavendish, who examined it in 1766, found that in 

 comparison with ordinary potash it contained twice as much 

 fixed air relatively to its power of neutralising acids. 

 Bergman also examined this salt, and made a rough 

 analysis, which showed that it contained both water and 

 fixed air in combination with the mild alkali (JSssays, p. 18). 



Such salts, which contain a double portion of fixed air, 

 were described by Bergman as "aerated vegetable alkali," 

 " aerated magnesia," and so forth, but are now known as 

 BICARBONATES. It is remarkable (as Bergman noticed) that 

 whilst the excess of fixed air converts the mild alkalis into 

 less soluble neutral salts, the solubility of chalk and magnesia 

 in water depends on their conversion into more soluble bi- 

 carbonates. Similar soluble bicarbonates are probably 

 formed when iron and zinc are dissolved by carbonic 

 acid, since the carbonates of these metals are insoluble in 

 water. 



SUMMARY AND SUPPLEMENT. 



Priestley (1/72) showed that fixed air is produced when 

 charcoal is burnt in air confined over water, or over mercury ; 



