Chemical Manufactures. 109 



of it ; and therefore we shall select for consideration only alum 

 from the former, and madder, with its products, and the aniline 

 colouring matters, from the latter. 



Alum was well known to the ancients ; it is mentioned by 

 Pliny in his Natural History, but there is reason to believe that 

 he did not restrict the word to the sense in which we understand 

 it : — in his time sulphate of alumina, combined with more or 

 less sulphate of iron, was also termed alum. From a very early 

 period alum was used as a mordant, and also to render wood 

 and cloth incombustible. It was obtained most abundantly, 

 and of the best quality, from Egypt, and its name is probably 

 an Egyptian word. Until the fifteenth century it was imported 

 from the East, and was not made in England until the reign of 

 Elizabeth. Our alum is essentially a double salt, one of its 

 constituents being sulphate of alumina, and the other the sul- 

 phate of an alkali ; and as there are three alkalis there are 

 three corresponding alums. These are not, however, the only 

 salts to which the name is given ; every double salt consti- 

 tuted like an alum being considered such. Thus, in an alum, the 

 sesquioxide of alumina may have been replaced by the sesqui- 

 oxide of some other metal — as that of chromium, for example, 

 or of iron ; and in this way a great variety of alums may be 

 produced ; but each of them contains twenty-four atoms water. 

 The alum in which alumina is replaced by oxide of iron, is used 

 in Germany and other places as a mordant for logwood, galls, 

 sumach, etc. Alum is found native, but almost all of the vast 

 quantity which is used in dyeing, and for other industrial pur- 

 poses, is artificial, being produced from alum stone or slate, 

 schist or clay. Alum stone contains all the constituents of alum, 

 but, in addition, certain impurities, which must be removed; 

 schist contains only the alumina and sulphur ; the latter must 

 be oxidized, and an alkali must be added; clay requires the 

 addition both of sulphur and an alkali. Alum stone is not 

 abundant, but it yields alum by a comparatively simple pro- 

 cess, and with a facility dependent on the proportions of its 

 constituents. To obtain its alum, merely moistening it with 

 water would suffice, but this is not the method adopted : in 

 practice, sorted pieces of it are calcined, to deprive the free 

 alumina present of its water, and thus destroy its affinity for 

 the alum, with which it is in union. After which, they are ex- 

 posed to the atmosphere, which causes them to fall into powder, 

 and the alum is then dissolved out with water. The degree of 

 heat to which the stone is raised, during calcination, is a 

 matter of great importance ; if this is too high, alum will not 

 be formed ; if too low, the result will not fall into powder. 



Alum slate and shale are the most abundant sources of 

 alum. Some of the alum shales, on being moistened with 



