_Sympathetic Inks._
These kinds of inks are very curious, and may serve for a great number
of physical recreations, very surprising to such as are not acquainted
with the manner of preparing them.
One kind, very easy, is made by taking an ounce of common aqua fortis,
which you are to mix with three ounces of common water; you will use
this mixture to write on paper that is strong and very stiff: this
writing becomes totally invisible in drying; and in order to make it
reappear, you need only wet the paper; and when it dries the writing
disappears again. This effect may be repeated two or three times.
This process is the easiest to be done, as the necessary ingredients
are almost always at hand.
Many other things furnish the means of making sympathetic ink, such as
cobalt, bismuth, lime, &c. &c. but they require chemical and difficult
preparations to be efficient.
The easiest to be obtained are mentioned before; as the mixture of aqua
fortis and common water; and those that may be formed by dissolutions
of salt and acids, such as lemon or onion juice: in order to render
them visible, you need only approach them to the fire: the cold air
produces on them the contrary effect.