t is the chief obstacle to successful
soldering, as the solder refuses to unite with anything but pure metal.
Sal ammoniac dissolved in water is good to cleanse off the oxide; better
still is muriatic acid, with a little zinc and sal ammoniac added. This
is known as the soldering mixture.

One of the most convenient materials for use is common tin, which can be
obtained almost everywhere. A tin box can be melted apart, and cut into
any desired shape. Pipes to convey liquids, steam, or gas can be made by
cutting strips of the tin, and rolling them upon an iron rod. To make a
pipe, say, a quarter of an inch in diameter, get an iron rod of that
size, cut a strip of the tin about one inch wide, roll it upon the rod,
allowing the edges to lap a little. If the tin be not bright, make it so
by applying sal ammoniac with a small brush along the seam. Put on a
little powdered resin, and then solder neatly by drawing the heated
iron, with the solder clinging to it, over the joint. In this way a pipe
strong and tight is obtained; and such pipes can be joined to one
another indefinitely, in a straight line or at any angle. To unite them
in a straight line, pass the end of one into the end of the other before
soldering, or else wind an additional piece of tin over the two ends. To
make a turn, or elbow, file the ends on a bevel, or slant, bring them
together, and apply considerable solder for strength. If the solder be
rightly put on, it will hold surprisingly.

[Illustration]

A pretty device to illustrate the force of steam is shown in the
accompanying picture. The boiler is a simple tin can, which need not be
more than six inches high and four in diameter. To make the wheel, cut a
circle of tin two inches in diameter, and pieces for the buckets, shaped
as in the diagram. Bend each piece at right angles along the dotted
line, and solder them one after another on the circumference of the
wheel, which will then appear as in the picture. Bore a hole through the
centre, insert a piece of wire for a shaft, and solder it fast at right
angles to the wheel. File shoulders on the ends of the shaft, and mount
it in uprights fastened to the top of the boiler. Make a small opening
through the top of the boiler, and place over it a little spout in such
a position as to send a current of steam directly into the buckets of
the wheel. Make also a larger opening in or near the top of the boiler,
and surround it with a neck to receive a cork. Thr