CHING POINTS OR NEEDLES, for scratching the lines into the ground.
Rat-tail files of good quality, costing about twenty cents each at the
hardware-stores, are excellent for the purpose. Two are all you need for
your experiment, and even one will be sufficient. Still cheaper points
can be made of sewing, knitting, or any other kind of needles, mounted
in sticks of wood like the lead of a lead-pencil. Use glue or
sealing-wax to fasten them in the wood.

13. AN OIL-STONE for grinding the points.

14. AN ETCHING-TRAY to hold the acid during the operation of biting.
Trays are made of glass, porcelain, or india-rubber, and can generally
be had at the photographer's supply-stores. A small india-rubber tray,
large enough for your experiment, measuring four by five inches, costs
fifty-five cents. But you can make an excellent tray yourself of paper.
Make a box, of the required size and about one and a half inches high,
of pasteboard, covered over by several layers of strong paper, well
glued on. If you can manage to make a lip or spout in one of the
corners, so much the better. After the glue has well dried pour
stopping-out varnish into the box, and float it all over the bottom and
the sides; pour the residue of the varnish back into your bottle, and
allow the varnish in the box to dry; then paint the outside of the box
with the same varnish. Repeat this process three or four times. Such a
tray, with an occasional fresh coating of varnish, will last forever.
For your experiment, however, any small porcelain (_not_ earthenware) or
glass dish will do, if it is only large enough to hold your plate, and
allow the acid to stand over it to the height of about half an inch.

15. A PLATE-LIFTER, to lift your plate into and out of the bath without
soiling your fingers. It consists of two pieces of string, each say
twelve to fifteen inches long, tied to two cross-pieces of wood, each
about six inches long, thus [Illustration]. It is well to keep the
fingers out of the acid, as it causes yellow spots on the skin, which
remain till they wear off.

16. NITRIC ACID for biting in the lines. Any nitric acid sold by
druggists will do, but the best is the so-called chemically pure nitric
acid made by Messrs. Powers & Weightman, of Philadelphia. It comes put
up in glass-stoppered bottles, the smallest of which hold one pound, and
sell for about sixty cents.

17. WATER for mixing with the acid and for washing the plate.

18. BLOTTING-PAPER,