58 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



discovery of the acids led to the preparation of a number of 

 new salts. It may now be pointed out how the alkalis were 

 used from a very early period to prepare earths, or bases, from 

 the salts in which they were combined. Thus it was found 

 that gold, which could not be converted into a calx by 

 heating, was transformed into an earth by dissolving it in 

 aqua regia and precipitating again with an alkali. This 

 earth was decomposed when heated, leaving behind a 

 residue of " revivified " metal. By precisely similar methods, 

 silver (which likewise resisted calcination) was converted 

 into an earth from which the metal could be recovered by 

 the action of heat. At a much later date these unstable 

 earths were used by Scheele for the preparation of oxygen. 

 The earth prepared by precipitating gold from its solutions 

 by means of the volatile alkali, exploded violently when 

 heated, and was called AURUM FULMINANS or FULMINATING 

 GOLD ; its composition is discussed in Chapter XII. 



These actions were studied by Mayow, who suggested 

 that the metallic earth was precipitated because the acid 

 had left it in order to combine with the stronger alkali, just 

 as in the action of oil of vitriol on nitre the alkali of the 

 nitre left it in order to combine with the stronger acid of 

 the vitriol. 



Preparation of magnesia from Epsom salts. The paper 

 "On Magnesia Alba," in which Black's experiments on 

 chalk and lime are described, derived its name from a white 

 earth, prepared by the action of alkalis on a variety of saline 

 liquids, and used as a mild aperient. It was first made from 

 MOTHER OF NITRE, the mother liquor left after crystallising 

 out saltpetre ; it was here present in combination with nitric 

 acid, which could be removed either " by the addition of an 

 alkali which attracted the acid to itself" or " by exposing 

 the compound to a strong fire in which the acid was 

 dissipated" (A.C.R. I. 6). Black prepared it from "the 

 bitter saline liquor called BITTERN, which remains in the 



