and one part of sugar. Mix them, and let
them soak for twenty-four hours; then melt at a gentle heat, stir
until thoroughly mixed, and pour the composition into a tray. Any
chemist will supply an aniline dye for the ink.

[Footnote 1: Both these sections were numbered 2486 in the original
text. txt Ed.]

[EVERY PEA HELPS TO FILL THE PECK.]

2487. Cements.

The term "cement" includes all those substances employed for the
purpose of causing the adhesion of two or more bodies, whether
originally separate, or divided by an accidental fracture. As the
various substances that may require cementing differ very much in
texture, &c., a number of cements possessed of very different
properties are required, because a cement that answers admirably under
one set of circumstances may be perfectly useless in others. The
general principles upon which the success or failure of cementing
usually depends are:

The different parts of a solid are held together by an attraction
between their several particles, which is termed the attraction of
cohesion. This attraction acts only when the particles are in the
closest possible contact: even air must not be between them. If, after
breaking any substance, we could bring the particles into as close a
contact as before, and perfectly exclude the air, they would re-unite,
and be as strongly connected as ever. But in general this is
impossible: small particles of grit and dust get between them; the
film of interposed air cannot be removed; and thus, however firmly we
press the edges of a broken cup together, it remains cracked china
still.

The cohesion between the particles of the cement is very much less
than the adhesion of the cement to other bodies; and if torn apart,
the connected joint gives way, not by the loosening of the adhesion,
but by the layer of cement splitting down the centre. Hence the
important rule that the _less_ cement in a joint the stronger it is.
To unite broken substances with a thick cement is disadvantageous, the
object being to bring the surfaces as closely together as possible.
The general principles that ought always to be borne in mind having
been mentioned, the manufacture and uses of some of the more useful
cements may be described.

2488. Mouth Glue.

The very useful preparation sold under this title is merely a thin
cake of soluble glue,