employed in the Art of Dyeing. 185 



method would be more costly than the ordinary processes, 

 it is best to employ the solution of this salt in muriaiio 

 acid. But before recommending this mordant to he used 

 in the dye houses, we intend to make trial of it in liie large 

 way, so as to determine precisely the exp'-i;se of it, and 

 what advantages will be obtained by its employment. 



Art. 4. — Experiments vpon the Colour of Scmlet and 

 Oxides of Tin. 



Scarlet, as we have already seen, is ohtriied by trealinor 

 wool with determined proportions of cochineal, a-idulated 

 tartrlte of potass, and a highly oxidized solution of tin. 

 The operation of dyeing is divided into two parts : the first 

 taking up art hour and a half, the lalier half an hour; this 

 division is necessary to produce a good colour, which 

 would be weaker and more yellow if ail the substances were 

 n)ixed in the first operation, and applied to the wool for 

 two hours. This circumstance is owing to the very acid 

 state of the bath, which holds in solution a great part of the 

 mordant, and of the colouring matter. VVe obtain the 

 contrary elfect v. hen the mordants only are employed in the 

 first operation, and the cochineal reserved for the second. 



Pieces of very beautiful scarlet cloth, macerated in di- 

 stilled water, at a boding heat, gave out to the water a jwr- 

 lion of their colour, and when the operation was finished, 

 appeared only of a light flesh colour. The washings col- 

 lected and evaporated were very acid, and contained, be- 

 sides the colouring substance and animal maiier, tartarous 

 ^cid, muriatic acid, and oxide of tin. Scarlet, therefore, 

 as we have already shown, is a combination in some mea- 

 sure soluble, which in parting with a small quantity of acid 

 changes its shade, and may, by repeated wasiiings at ele- 

 vated temperatures, and with a lar^e bulk ot fluid, be ren- 

 dered completely colourlcrjs. 



It results from the experiments related in this memoir: 

 1st. That in ahnningall vegetable and animal substances, 

 it is not the alumina which combines with them, l)ut the 

 entire aUim ; and that when these niatters are not purified, 

 the lime which they contain, occasions a decomposition of 

 a part of this mordant. 



2. That all the alkaline and earthy bases, mixed with a 

 solution of alum, decompose it, and convert it into acid 

 sulphate of potass, and into an insoluble salt, less acid than 

 klum, which niav, by repeated washings, be converted into 

 pure alumina, sulphate of potass, and alum. 



3. 'J'hat itic acetate of alumina combines also in lis ch- 



lire 



