Chemical Manufactures. Ill 



thus formed, weigh about three tons ; one of them (No. 605) is 

 exhibited. 



Alum is used in a variety of ways, but chiefly as a mordant, 

 on account of the affinity of its alumina for both colouring 1 mat- 

 ters and vegetable fibre, between which it forms a bond of 

 union. When applied to calico-printing, it is very important 

 that it should be free from iron ; and the chief use of the alka- 

 line sulphate which it contains is to facilitate the separation of 

 that metal, by rendering the aluminous compound so much 

 more soluble as to allow the sulphate of iron to crystallize first, 

 and be removed. When heated, ammonia alum loses first the 

 whole of its alkali, and then the sulphuric acid passes off, pure 

 alumina being left. The well-known superiority of Roman 

 alum consists in its containing a considerable amount of cubic 

 alum, which has a larger quantity of base than the octahedral ; 

 the extra alumina being held very feebly by the sulphuric acid, 

 it is more easily detached as a mordant. 



The use of alum has been in a great degree superseded by 

 the employment of sulphate of alumina — incorrectly termed 

 " concentrated alum." It is made from clay, contains but 

 little alum, and is quite free from iron, that metal being easily 

 and completely removed by ferrocyanide of potassium. 



Madder is one of the most important substances used in 

 dyeing, whether we consider the beauty, the variety, or the 

 permanence of the colours it imparts ; and it is specially de- 

 serving attention on account of the improvements which have 

 been introduced into the mode of applying it. The plant from 

 which it is derived is the JRubia tinctorum of Linnaeus ; and 

 there is little doubt that its properties were as well known to 

 the ancients as to ourselves. Other plants also, of the same 

 genus, have been used for the same purpose. Thus, the H. 

 Manjista has been employed in India from the earliest times 

 for the production of the colour called Turkey red : and in the 

 eastern' parts of Europe the JR. Peregrina, or Alizari, has long* 

 been used to obtain the same tint. Western Europe was, for 

 many centuries, supplied with madder from Holland ; but, on 

 its being discovered that Dutch madder was incapable of pro- 

 ducing all the colours which madder will give, it was imported 

 from other places, and particularly from the south of France. 

 Madder may be grown in England, but not economically. The 

 rubia tinctorum is not an ornamental plant; its stem dies every 

 year; but the root, which is the important part, is pe- 

 rennial. The Levant madder is the finest we receive; that of 

 Avignon is next in quality. Cochineal imparts a richer tint 

 than madder, but it is far more expensive. According to the 

 mordants employed, madder produces every shade, from a pink 

 to a deep red, and from a lilac to a black ; and affords also va- 



