1 78 Memoir upon the Mordants 



the properly of taking the colour in the dye-bath. The 

 akuii-balh aud washings, when evaporated, afiorded us all 

 the alum we had en)ployed. We separated this alum from 

 the vegetable matter which it had dissolved in its different 

 crystallizations. To do this did not require such a num- 

 ber of washings as were employed for the wool or the silk, 

 because the combination of alum with vegetable substances 

 is so weak, that soaking the alumed cotton in boiling wa- 

 ter for a few minutes is suflicient to carry off the greatest 

 part of the mordant. Cotton, therefore, ought to be dyed 

 at a low temperature, since it is only after the colouring 

 matter has rendered the combination insoluble, that it can 

 support a great heat without being attacked. Thread 

 treated in the same manner as cotton, afforded us the same 

 results. 



Art. 4. — Analysis of the ImpregTiatioii of common IVbol. 



The analyses we have already related, most decidedly 

 demonstrate that in the aluniing of all animal and vegetable 

 substances, the alum combines with them without under- 

 going any decomposition ; but we thought it was neces- 

 sary to repeat the same exj)erimenis upon these substances 

 in the state in which they are commonly met with in com- 

 merce, as we had done in their" purified state. Wool, when 

 impregnated with alum alone, always renders the bath tur- 

 bid, which, upon cooling, throws down an abundant white 

 precipitate, as has been observed by several chemists. 

 Several analyses of this sediment, after being weM washed, 

 have constantly afforded us soihe sulphate of lime, satu- 

 rated sulphate of alumina, and sometimes a little alumina. 

 The bath contained a rchiarkable quantity of alum, ot" aci- 

 dulated sulphate of piilass, combined with a small propor- 

 tion of animal matter, l/pon the wool we found alum, 

 and a very minute proportion of the precipitate. These 

 c'xperimenis upon the sediment, formed in the alum bath, 

 do not differ from those made by M. Berthollet; but this 

 learned chemist not having examined the mother-waters, 

 nor the alumed wool, has not given, as he himself says, a 

 clear and precise explanation of the efi'ects produceii by 

 alum and tartar in the operation of dyeing. These pre- 

 cipitates, obtained by treating common wool wiih alum, 

 never lake place w'ith purified wool ; and as these only dif- 

 fer from each other by the former containing some car- 

 bonate of lime, it was natural in this case to aitribute to this 

 substance the decomposition of a part ot the alum. 



We satisfied ourselves of this^ by niising in glass vessels 



soluliouij 



