ge to
make the plates rough by hammering against them a coarse file. (See
explanations and experiments with this form of cell in text-book.)

APPARATUS 11.

[Illustration: Fig. 10]

_17. Porous Cups for Two-fluid Cells._ Fig. 10. Very good porous cups
can be made from ordinary blotting-papers, the average ones measuring
9-1/2 x 4 in. White ones should be used, so that you will not be
bothered with the color coming out. Soak the edge along one end of the
blotter in paraffine (Index) for about 1/4 in. When this is cold, roll
the blotter into the form of a cylinder that is a little over 1 in.
inside diameter, and have the paraffined end on the outside. This will
make 2 thicknesses of paper all around, and a little to spare. Rub a hot
nail over the paraffine to melt it, and stick the end to the cylinder.
By putting on a little more paraffine along the edge where the end laps
over, a good solid cylinder can be made. The cylinder should be
strengthened still more by dipping each end into melted paraffine for
about 1/8 in. The dark stripes around the ends and down the front of the
cylinder (Fig. 10) are to represent the paraffine. Cut out a bottom
about 1/4 in. larger all around than the cylinder. This may be
paraffined to make it stiff. It should be fastened to the cylinder with
paraffine. Paraffine is not acted upon or softened by water or acid, as
is the case with glue.

APPARATUS 12.

_18. Porous Cups for Two-fluid Cells._ Instead of the blotters of App.
11, you can use short lengths of mailing-tubes, which are used to
protect pictures, etc., when sent by mail. If you find that the
particular tube tends to unwind when soaked, you can use a little
paraffine along the edges of the spiral, as suggested in App. 11.
Bottoms can be made for the cups as before.

APPARATUS 13.

_19. Porous Cups for Two-fluid Cells._ Ordinary unglazed earthen
flower-pots make good cups. The hole in the bottom should be closed with
a cork, or by fastening a piece of pasteboard over the hole with
paraffine. The pasteboard may be fastened to the under side of the
bottom more easily than to the upper side.

20. Note. It is a good idea to soak the top edge of porous cups for
about 1/4 in. in paraffine to keep the solutions from crawling up by
capillary attraction. If the solutions constantly evaporate from the
soaked tops of the cups, they not only waste but they get the whole
thing covered with crystals.

CHAPTER II.

BATTERY