Sulphate of Iron. 281 



prove injurious to cotton fibre. If cotton impregnated with 

 a solution of alum is allowed to hang up for some time, the 

 cotton is brittle. Alum saturated with chalk or soda acts 

 less injuriously. 



Sutyhate of copper or blue vitriol is of little use to the 

 cotton dyer. As a mordant it cannot be employed by itself, 

 as it does not combine with the cotton fibre. If cotton is 

 saturated with a solution of sulphate of copper and dried, 

 the whole of the sulphate of copper is removed by washing. 

 Liquid ammonia dissolves sulphate of copper, forming 

 an azure blue colour, and affording a useful copper mordant 

 for many purposes. 



With decoction of logwood, sulphate of copper affords a 

 very remarkable combination which can be employed in 

 block-printing. 



Sulphate of copper almost always contains iron. This is 

 determined by liquid ammonia into which a solution of 

 sulphate of copper is poured, so that after being stirred, a 

 smell of ammonia is given off. If yellow flocks remain un- 

 dissolved, they are peroxide of iron. Very small quantities 

 of iron separate in the course of 24 hours. 



Sulphate of iron or green vitriol is an essential ingre- 

 dient in the cold vat. But every kind of sulphate of iron 

 is not equally good. Such must be chosen as appears yel- 

 lowish green, but not yellow. It must also contain no 

 copper. This is ascertained by means of a piece of polished 

 iron. If the latter does not change its colour after being 

 immersed for some hours in a solution of sulphate of iron, 

 then the vitriol contains no copper. If it acquires a red 

 coating the presence of copper is indicated. Such vitriol 

 may be purified from copper by boiling it with iron nails. 



In contact with air and cotton the sulphate of iron under- 

 goes a change which, the manufacturer should be acquainted 

 with, as it explains the numerous hybrid colours obtained 

 with the iron mordant. 



If we place \ an ell (1*09 feet), of calico in a flask, and 

 moisten it with a solution of 4 loths (1*876 oz.) sulphate 

 of iron in 16 loths (7J oz.) water, pour off all the pure 

 solution, and place the flask inverted in water, so that its 

 mouth shall penetrate about an inch below the surface, it 

 will be found that the water ascends. Hence, it is obvious, 



