Alum. 279 



removed. This is done by digestion in dilute sulphuric 

 acid. Lime decomposes sulphate of iron, and dissolves 

 indigo with its assistance. Upon this depends the exhibition 

 of the cold vat. 



If we dissolve yellow chromate of potash in water, boil 

 it and introduce iuto it a portion of cotton impregnated 

 with the sugar of lead mordant, the cotton immediately 

 assumes a clear yellow colour. If the same experiment is 

 repeated, only using lime-water instead of common water, 

 the colour produced is not yellow but orange. Upon this 

 depends the production of chrome-orange.* 



Also other colours, viz., wood colours (Fernambuc, log- 

 wood, yellow wood) are made darker by the lime-water 

 which makes the red blueish. It is not, however, to be 

 recommended as it makes the colours dull and coarse. 



Lime-water is also used to moisten the cotton which is to 

 be dyed blue in the cold vat. 



Chalk is a combination of lime with carbonic acid. It 

 is quite insoluble in water, and, is therefore, without action 

 upon cotton. 



When chalk comes in contact with acids it combines with 

 them and destroys their action. This property renders the 

 cbalk a useful assistant in washing some calicoes which are 

 printed with acid dischargers. An excess of lime is, there- 

 fore, mixed with water, and the cotton passed through this 

 chalk-water before it is rinsed. As the chalk here im- 

 mediately combines with the acid and separates it from the 

 cotton, no injury can follow. Hence, these patterns pre- 

 serve their regular edge. It is well in most cases to have 

 the chalk-water, luke warm. 



Alum is employed for the exhibition of the aluminous 

 mordant. To obtain by means of it a pure yellow or red 

 colour, it is absolutely necessary that it should be free 

 from iron. As all the alum which occurs in commerce is 

 not free from iron, and, as the iron contained in alum cannot 

 be seen, it is necessary to have recourse to a test for it. 



This consists in placing a piece of the alum to be tested 

 in a solution of 1 loth (-469 oz.) Prussiate of potash in 200 

 loths (93-8 oz.) of water. If the colour of the surface of 

 the alum remains unchanged, tbe alum is free from iron. 



* See a Specimen of this colour. — Records, vol. i. p. 18. — Edit. 



