Sulphuric Acid. 211 



the water, which combines with the lead of the chrome- 

 yellow, and makes it black. 



The water in dyeing plays a very peculiar part, as it must 

 first dissolve the colouring matter before the latter can be 

 taken up by the mordanted cotton ; so there exists an oppo- 

 sition between the water which holds the colouring matter 

 in solution, and between the cotton which will cause the 

 water to abandon the colouring matter, and render it in- 

 capable of dissolving it. Much water, therefore, increases 

 the influence of this opposing property in the mordanted 

 cotton; a small quantity of water, on the other hand, 

 diminishes it. When we wish, therefore, that a solution 

 shall exhaust as much colouring matter as possible, a small 

 quantity of water must be employed. With different dyes, 

 and different mordants the result varies. Thus, with 1 loth 

 (•469 Troy ounce) logwood-blue and 10,000 loths (4690 oz.) 

 of water, cotton impregnated with the acetate of alumina 

 mordant completely exhausts the colour at once ; while a 

 solution of berry-yellow, containing the same quantity of 

 water can be deprived of the whole of its yellow colour, 

 only by repeated boiling with fresh cotton. 



Clay is a substance, which, in this respect, assists the 

 water. Its striking action with madder colours shews this 

 in a very distinct manner. 



Substances which form a thick slime with water as tra- 

 gacanth and salap cannot be diluted anew with propriety 

 with much water, as the mixture will not be equable. They 

 must first merely be moistened with water, that they may 

 swell up ; then the necessary quantity of water should be 

 gradually added. The same proceeding is necessary with 

 isinglass. 



Alumina, which is used for reserves, must, in the same 

 way, first be moistened with water, and then diluted, other- 

 wise it loses its combining power. 



Sulphuric acid. — This acid is also termed Oil of Vitriol, 

 although this name is applied with more propriety to the 

 sulphuric acid which fumes or gives off a white smoke 

 when exposed to the atmosphere. Such acid is to be pre- 

 ferred for dissolving indigo. 



If a drop of sulphuric acid falls upon a piece of cotton, 

 the spot becomes black, and a hole is formed; consequently, 

 sulphuric acid corrodes cotton. 



p 2 



