1819.] the late Mr. Thomas Henry. 169 



stance to be dyed, prevents it from exerting its attraction for 

 colouring matter. The ultimate object of these preliminary 

 steps, he states to be the obtaining a white ground that may 

 enable the colours to display the full brilliancy of their several 

 tints. To explain the preparation of cotton for the Turkey-red 

 dve, he endeavours to prove that cotton requires for this purpose 

 to be approximated in composition to the nature of an animal 

 substance. He next offers a classification of the Materia Tinc- 

 toria, and some general speculations on the nature of colouring 

 matter. 



In the second part of the essay, Mr. Henry investigates the 

 mode of action of those substances, which, though themselves 

 destitute of colour, are important agents in the processes of 

 dyeing. Substances of this kind had received, from the French 

 dyers, the name of mordants, because it was imagined that they 

 corroded and removed something which mechanically opposed 

 the entrance of the colouring matter into the pores of the mate- 

 rial to be dyed. To destroy this erroneous association, Mr. 

 Henry proposes that the word basis should be substituted, as a 

 general term, to denote every substance which, having an 

 affinity both for the colouring matter and for the material to be 

 dyed, is capable of serving as an intermedium between the two ; 

 and that a specific epithet should be added, to distinguish each 

 particular variety. In this essay, Mr. Henry, for the first time, 

 explained the true nature of the liquor which is employed for 

 affording the aluminous basis, prepared by mixing the solutions 

 of alum and of sugar of lead. This liquor he showed to be 

 essentially a compound of pure clay or alumine with acetic acid ; 

 and its superiority over a solution of common alum, for yielding 

 the earthy base in dyeing, he ascribes partly to the less affinity 

 of the acetic acid than of the sulphuric for alumine, and partly 

 to the greater volatility of the acetic acid, when exposed to a 

 moderate increase of temperature. The remainder of the paper 

 is chiefly occupied with the details of the operations then prac- 

 tised for dyeing Turkey-red ; with a theory of the process ; and 

 with a general view of the mode of action of the individual mor- 

 dants or bases. The method of dyeing Turkey-red has been 

 since much improved and simplified, though its theory is, even 

 yet, far from being well understood. But the opinions, first 

 inculcated by Mr. Henry, respecting the action of mordants, 

 evince a remarkable superiority to the prejudices, with which 

 he found the subject encumbered, and are, indeed, those which 

 are still held by the latest and best writers on the principles and 

 practice of dyeing. 



In the year 17N:}, an institution arose out of this Society, which 

 had great merit, not only in its plan and objects, but in the 

 ability exerted by the several persons who were concerned in 

 their fulfilment. It was destined to occupy in a rational and 

 instructive manner the evening leisure of young men, whose 

 time during the day was devoted to commercial employments. 



