46 On the Composition of Writing Ink* 
if it be not that of ink, at least exhibits a shade which 
becomes perceptible enough for ascertaining the letters 
and words which had been traced on the paper before it 
'‘was touched by the acids. 
The gallic acid is, according to the author, one of those 
agents, which in this case succeeds very well. 
The liquid prussiate of lime also produces a good ef- 
fect. 
It is the same case with the alkaline hydrogenated 
sulphurets. But it is very certain that we never ob- 
tain any success from the employment of these agents, 
when we have left any acid long in contact with the wri- 
ting, and particularly if we have washed the paper after- 
wards. 
In short, we may easily conceive, that in this case the 
constituent parts of the ink which were combined with the 
acid, and had formed with it compounds soluble in water, 
having been taken up by this fluid, ought not to leave any 
trace of their existence longer; and consequently it is im- 
possible that the agents employed for discovering them 
can render them visible. 
It is also for this reason that the gallic acid, the liquid 
prussiate of lime, the alkaline hydrogenated sulphurets, 
and *so many other reagents which have been so much 
praised, can no longer be regarded as infallible methods 
for reviving writing. 
ARTICLE III. 
Improvement of Common Ink , 
Most of the inks now in use are of a bad quality. Some 
are spontaneously destroyed ; others imperceptibly lose 
their black colour, and assume a yellow one ; several, af* 
ter a length of time, enter into the paper, and spoil its 
