Madder Dyeiiuj. 137 



culty; and even, when the solution of alum in sulphuric 

 acid is again used, there is still the expense of the indis- 

 pensable treatment with spirit of wine. 



The method of Robiquet of separating the colouring 

 matter of madder, by sublimation, from the sulphuric acid 

 and madder charcoal, would be the most preferable, as, by 

 carbonizing the madder, the sulphuric acid employed may 

 be again used in manufacturing salmiac, and in the forma- 

 tion of vitriol, and is thus restored. But, by this method, 

 the dyes are not separated ; for, by heating the madder- 

 charcoal, both madder-purple and madder-red are sublimed 

 together. Their separation must be effected by means of 

 alum solution ; and thus, a new difficulty arises. And, it 

 is not to be overlooked, that the sublimation is attended 

 with loss, as well as the subsequent treatment with alum 

 and spirit of wine. 



But, although this desirable object has not been attained, 

 it cannot be doubted, that now, when the properties of the 

 three dyes have been ascertained, a convenient method of 

 separating them will soon be found out. 



TESTING MADDER. 



The goodness of any species of madder has hitherto been 

 improperly attempted to be determined, by separating the 

 colouring matter of madder when in solution ; but the 

 proper mode is, by its power of dyeing. This is shewn by 

 the author in a table, where there are four sorts of madder 

 differing in price from each other. They should all be em- 

 ployed in dyeing with completely pure water in the same 

 manner. To show that this method gives a proper valua- 

 tion of the goodness of a variety of madder, and that a 

 quantitative estimate can be formed by means of the mor- 

 danted cotton, the varieties of madder are represented in 

 four different degrees in relation to dyeing, from which it 

 appears, that the darkness of the dye is in direct propor- 

 tion to the quantity of colouring materials. Munjeet is the 

 most powerful dyeing species of madder, next to it is the 

 Spiers madder, then the Dutch, and lastly, the Avignon 

 madder. 



] . Preparation of the mordanted cotton. — In order to form 

 a scale of colours, it is necessary to proportion the madder 



