Then expose over mercury, as usual.

I have produced singularly interesting results by this process.
To Purify Water .-- Filter the water well, and then add about three drops of nitric acid to the pint.

This can be used as absolutely pure water, but I would recommend the use of distilled water as preferable.
Cleaning Mercury .-- Make a small bag of chamois skin, pour in the mercury, and squeeze it through the leather.

Repeat this several times, and filter by means of a funnel made of paper, with a very small aperture, through which it will escape and leave the particles of dust, or other substances, in the paper.

A paper with a pinhole through it will answer as well, and it is less difficult to make.
Adhesive Paper .-- Take gum arabic, four ounces, put it in a wide-mouthed bottle and pour on water about one-third above the gum.