Sympathetic Inks. 375 
force of habit they are also enabled to endure the excessive 
heat incidental to places where little air can be introduced, 
and, where the warmth of their own bodies occasions a 
constant steam. 
(To be continued.) 
SVMEATHE TIC INKS. 
HERE are four kinds of sympathetic inks. 1. When 
the writing becomes visible by simply applying heat 
or atmospheric moisture or dryness. 2. When peculiar 
gases or vapours make it visible. 3. When solutions of 
chemical or other compounds accomplish the same thing. 
4. When the simple action of light will make the writing or 
drawing visible (Photographic preparations). Of these we 
give the first one, not on account of any merit or new dis- 
covery, but simply of their interest in popular science. 
FIRST CLASS—No. 1. Red Sympathetic Ink.—Nitrate of 
the deutoxide of copper. A weak solution gives an in- 
visible writing, which becomes red by heating. 
No. 2. Yellow Sympathetic Ink—Chloride of copper. 
A very dilute solution is used, invisible till heated. To 
make it, dissolve equal parts of blue vitriol and sal ammo- 
niac in water. 
No. 3. Yellow and Green Ink.—Nitrate of nickel and 
chloride of nickel. A weak solution forms an invisible ink 
which becomes green by heating when the salt contains 
traces of cobalt, which usually is the case; when pure, it 
becomes yellow. 
No. 4. Green and Red Ink.—Chloride of cobalt. A 
properly diluted solution will produce a pink writing, which 
will disappear when thoroughly dry, become green when 
heated, disappear when cold, and pink again when damp. 
When often or strongly heated it will at last become brown 
red. 
No. 5. Blue Ink.—Acetate of the protoxide of cobalt. 
When the solution of this salt contains nickel or iron, the 
writing made by it will become green when heated; when 
it is pure and free of these metals it becomes blue. 
No. 6. Light Brown Ink.—Bromide of copper. Perfectly 
invisible writing, which appears very promptly by a slight 
