}
Water + Iron = Oxide
of Iron + Hydrogen.
So all that the chemical combination in the above means is that the iron has taken the place of the hydrogen in the water used for the experiment. If weighed it would be found as always, that the water and the iron weighed precisely the same as the oxide of iron and the hydrogen.
It is to this same principle of chemical affinity that the curious experiments of magic writing with sympathetic inks are possible.
Sympathetic Inks.
By means of these may be carried on a correspondence which is beyond the discovery of all not in the secret. With one class of these inks the writing becomes visible only when moistened with a particular solution. Thus, if we write to you with a solution of sulphate of iron the letters are invisible. On the receipt of our letter, you rub over the sheet a feather or sponge, wet with a solution of nut-galls, and the letters burst forth into sensible being at once, and are permanent.
2. If we write with a solution of sugar of lead and you moisten with a sponge or pencil dipped in water impregnated with sulphuretted hydrogen, the letters will appear with metallic brilliancy.
3. If we write with a weak solution of sulphate of copper, and you apply ammonia, the letters assume a beautiful blue. When the ammonia evaporates as it does on exposure to the sun or fire, the writing disappears, but may be revived again as before.