ON MARKING INKS. 
161 
scruple of indigo. I will not say that this makes a very good 
ink, because I have seen better, but it is one which has been 
much sold, and is, I believe, much better than a great deal 
which is to be met with. It is necessary to observe that, add 
What quantity of ammonia I would, there would always re- 
main an insoluble grayish precipitate, which led me to think 
I had been making fulminating silver, and which insoluble 
powder I considered to be the detonating substance. In sub- 
sequently making a quantity of the ink therefore, I carefully 
decanted the clear liquid and rejected that portion which I 
looked upon as dangerous, treating the solution as before, with 
gum and indigo. 
About, or soon after this time, Mr. Smee's work on Elec- 
tro-metallurgy came into my hands, and I there learn, on his 
authority, that not only is the gray insoluble precipitate to 
be regarded as so much to be avoided, but that the salt itself, 
contained in the solution (the triple salt of silver,) is equally 
fraught with danger. Mr. Smee says (page 77,) " The am- 
monio-nitrate and ammonio-chloride of silver are very solu- 
ble, but do not offer any particular advantages for the pur- 
poses of metallic precipitation. Great care is required in the 
use of these salts; for if the solution, by being kept for some 
time, be allowed to evaporate, so as to leave dried portions ad- 
hering to the sides of the vessel, it can no longer be even 
touched with safety, for a fulminating salt is thus formed^ 
which, if merely touched with the finger in order to remove 
it from the sides of the vessel, will explode with mischievous 
and awful violence." He adds, "I take particular notice of 
this fact, as I nearly lost my right eye in learning it." 
The correctness of these observations is confirmed by Mr. 
Charles Walker, secretary of the Electrical Society, who 
states in his work on Electro-Plating and Gilding, that fo r 
those purposes the ammonio-carbonates and the cyanurets of 
the respective metals are best adapted. " With regard to 
the first," he says, however, "there is one great objection to 
be urged against them — namely, that they are highly fulmi- 
