60 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The work upon " magnesia " was of special importance 

 because it provided Black with the knowledge which 

 enabled him to solve the more difficult problem of the 

 relationship between chalk and lime. 



Preparation of an earth from alum. Black also made 

 use of the alkalis in order to separate from alum an 

 earth to which the name of ALUMINA was afterwards given. 

 This earth is present in the alum as a vitriol, or sulphate. It 

 does not combine with fixed air, and this gas is therefore set 

 free when the alum is precipitated by means of a mild alkali. 



Preparation of earths from salts by heat A second 

 method of preparing earths from salts, which was practised 

 from the earliest period of alchemy, depended on dissipating 

 the acid of the salt by heat. When salts were thrown into a 

 charcoal fire, or were distilled from a retort, it was often 

 found that an acid vapour escaped, leaving behind an earthy 

 residue to which the name of CAPUT MORTUUM was given. 

 Amongst the salts distilled in this way were green vitriol and 

 alum, both used in the preparation of oil of vitriol ; the 

 residue of " colcothar " from the green vitriol was a red-brown 

 earth somewhat resembling rust; the alum left behind a 

 whitish residue of alumina. The nitrates were also 

 frequently decomposed, as for instance, in the preparation of 

 " red precipitate " by heating nitrate of mercury. Black 

 prepared magnesia in this way from mother of nitre ; and 

 Boyle in 1680 (Works, 1725, III, 372) prepared an alkali 

 from sea-salt by converting it into a nitrate and igniting this 

 with charcoal. As a further illustration, it will be shown in a 

 later chapter that fixed air may be regarded as an acid, and 

 the decomposition of chalk by heat as that of a salt into 

 acid and base. 



The method of decomposing salts by heat, although very 

 simple, was not always available. Some salts were not 

 changed in the fire ; others (such as sal-ammoniac and salt) 

 were dissipated without leaving any residue : in the case of 



