Soap- Suds. 377 



and both are dyed with cochineal, a cochineal-gray is ob- 

 tained, much lighter with cow-dung than without it. 



In pink, yellow,'lilac, and in general all light colours, the 

 employment of cow-dung should be avoided. In these 

 cases bran should be used. 



It has been already stated, that bosopric acid and bosopri?ie, 

 when mixed with the aluminous and iron mordants, preci- 

 pitate these, and form, with the alumina and iron, com- 

 pounds insoluble in water ; the same occurs with an 

 aqueous extract of cow-dung. This property explains the 

 use of cow-dung in cleansing white grounds. 



When calico is printed with a mordant consisting of 

 acetate of alumina thickened with starch, more mordant is 

 deposited on the cotton than can be retained by it in the 

 subsequent washing. This excess dissolves in the water, 

 which is converted into a weak solution of acetate of 

 alumina ; this acetate of alumina combines only with the 

 unprinted parts which should remain white, and mordants 

 them, so that they are likewise dyed. 



If cow-dung on the other hand is employed, no acetate of 

 alumina will be dissolved in the water; but as soon as it 

 separates from the calico, it will be seized on by the bosopric 

 acid, rendered insoluble, and thus inactive. 



With the acetate of iron mordant, the action is exactly 

 the same. 



When bosopric acid is boiled with chalk, bosoprate of 

 lime is formed, which is insoluble in water, and acts like 

 cow-dung. 



This property enables the dyer to add chalk to the 

 cow-dung bath, which is very necessary in many cases. 

 When many printed pieces are passed through the cow- 

 dung bath, they become very acid, from the acetic acid 

 which was contained in the alumina or iron mordant. 

 This would prove injurious to the pattern, but is prevented 

 by the addition of chalk, which combines with the acetic 

 acid and neutralizes it. 



By the addition of lime the same object is obtained, but 

 the colours are less clear and lively. 



Soap-suds produce a favourable change on a number of 

 colours, which is called clearing * A portion of the alkaline 



* See Records of General Science, i. 169. — Edit. 



