The shade of colour is also not to be
disregarded: a black, approaching to blue, is more agreeable to the eye
than a browner ink; and a degree of lustre, or glossiness, if compatible
with the due consistence of the fluid, tends to render the characters
more legible and beautiful. With respect to the chemical constitution
of ink, I may remark, that although, as usually prepared, it is a
combination of the metallic salt or oxide, with all the four vegetable
principles mentioned above; yet I am inclined to believe that the last
three of them, so far from being essential, are the principal cause
of the difficulty which we meet with in the formation of a perfect
and durable ink. I endeavoured to prove this point by a series of
experiments, of which the following is a brief abstract:--Having
prepared a cold infusion of galls, I allowed a portion of it to remain
exposed to the atmosphere, in a shallow capsule, until it was covered
with a thick stratum of mould; the mould was removed by filtration,
and the proper proportion of sulphate of iron being added to the clear
fluid, a compound was formed of a deep black colour, which showed no
farther tendency to mould, and which remained for a long time without
experiencing any alteration.
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