Writing On Glass By The Rays Of The Sun
Dissolve chalk in aqua fortis, to the consistence of milk, and add to
that a strong solution of silver. Keep this liquor in a glass decanter
well stopped. Then cut out from a paper the letters you would have
appear, and paste the paper on the decanter, which you are to place in
the sun, in such a manner that its rays may pass through the spaces
cut out of the paper, and fall on the surface of the liquor. The part
of the glass through which the rays pass will turn black, and that
under the paper will remain white. You must observe not to move the
bottle during the time of the operation.