494 PHILOSOPHICAL TRANSACTIONS, [aNNO 1774. 



vegetables, for medical purposes, Mr. C. was desirous of knowing if it would be 

 equally useful in the art of dying black : to this end he made the following expe- 

 riments. 



Exper. 1 . Four pennyweights of each of the following astringents, viz. galls, 

 sumach, oak bark, bistort root, and logwood, were boiled during 10 minutes, 

 in half a pint of pure river water i on mixing the decoctions with a saturated 

 solution of martial vitriol, in the proportion of -^ of the solution to f of the 

 decoction, they struck colours differently inclining to blackness, in the following 

 order: viz. oak bark, bistort root, sumach, galls. He then boiled the same 

 weight of all the astringents, in the same quantity of lime water, and on mixing 

 them as above, the colours they produced were inferior to those with plain water, 

 the astringency of the logwood, or whatever gives it the property of striking 

 black with green vitriol, was entirely destroyed; it produced not the least black- 

 ness with any quantity of vitriol. 



Exper. 2. Four pennyweights of each of the astringents above-mentioned, 

 were tritured in plain water, and 4 others in lime water; the measures of water 

 used were equal to those left, after boiling, in the last experiment; and, on be- 

 ing mixed with martial vitriol, as in the last experiment, the colours produced, 

 by this means, were superior to those produced by boiling. Those tritured in 

 lime-water were judged to be the deepest, which agrees with Mr. Henry's expe- 

 riments; but we must again except the logwood, which gave no colour by tritu- 

 ration, more than by boiling in lime-water. 



Exper. 3. All the above mixtures having been written with as inks, and ex- 

 posed 6 months to the air; those boiled in lime-water had failed much; those 

 tritured in lime-water, and in plain water, had faded a little; those boiled in 

 plain water evidently preserved their colour best. On slightly rubbing the faded 

 writings, with a fresh astringent liquor, they recovered their original blackness; 

 by which it appears, that it was the astringent parts of those inks which had 



failed. 



Does it not appear, by these experiments, that, though lime water tends to 

 deepen the colour produced by some astringents and martial vitriol, it by no 

 means adds to the duration of those colours; and as lime-water, either by tritu- 

 ration or coction, entirely destroys the property, in logwood, of striking black 

 with martial vitriol, it can by no means be of service, in the black dye, where 

 logwood is a material ingredient. Does it not also appear, that a slight boiling 

 is preferable to trituration, for the purposes of dying, when a durable colour is 

 wanted ? 



Having observed a solution of iron, in a vegetable acid, struck a deeper black, 

 on mixture with an astringent, and produced its effects much more expeditiously, 

 than a strong solution of martial vitriol ; it occurred, that the iron, being more 

 slightly combined with the vegetable acid than with the vitriolic, made it more 



