162 Dr. Thomas Thomson on [March. 



which has the power of rendering alumina soluble in water. 

 The cleansing processes to which all cloths impregnated with 

 mordants are subjected before dyeing, remove that portion 

 of the alumina which has been rendered soluble, and leave 

 portions of the cloth in the shape of flowers, crosses, &c, 

 without any material capable of fixing the dye-stuff. When 

 the cloth is dyed in the way already described these portions 

 remain white, or at least become white after the requisite 

 washing. 



The substance which has been found to answer best for 

 the removal of alumina and peroxide of iron is citric acid. 

 Some of the advantages of such an acid are obvious. It 

 does not corrode the cloth, though subjected to a consider- 

 able degree of heat. It is a fixed acid, with little tendency 

 to swell or travel to other portions of the mordant than 

 those with which it is intended to be combined ; and it has 

 the advantage over other vegetable acids of dissolving away 

 very completely all the alumina or oxide of iron, so that no 

 portion of these mordants is retained by the cloth. When 

 we consider the ease with which this acid is abstracted by 

 water, from the insoluble citrates, we would, apriore, infer 

 that it is very little adapted for this purpose of the calico- 

 printer, which, in fact, it is found to answer better than 

 any other. But the probability is that water has no such 

 tendency to abstract it from the soluble citrates, as citrate 

 of alumina, and citrated peroxide of iron. 



The citric acid is often printed before as well as after the 

 application of the mordant. In the latter case it is generally 

 assisted by bisulphate of potash, or even sulphuric acid, by 

 which the more expensive acid is economized. 



14. Madder and Logwood. 



