d
may be removed by drawing the thumb-nail across the wheel at any point
on its flat rim, thus breaking the threads, and allowing the "hank" to
open.

_Brushes for Use with Strong Acids._--Glass wool, if of fine enough
texture to be highly flexible, can be used to make acid-resisting
brushes. A convenient method for mounting the spun glass is to melt the
ends of the threads together into a bead, and then to fuse the bead on
to a rod; thus giving a brush. If a pointed brush is necessary, the
point may be ground on an ordinary grindstone or carborundum wheel by
pressing the loose end of the spun glass against the grinding wheel with
a thin piece of cardboard.

When using brushes of this description, it is well to bear in mind the
fact that there is always a liability of a few threads of glass breaking
off during use.

CHAPTER IV

Glass, Its Composition and Characteristics. Annealing.
Drilling, Grinding, and Shaping Glass by methods other than
Fusion. Stopcocks. Marking Glass. Calibration and Graduation
of Apparatus. Thermometers. Exhaustion of Apparatus. Joining
Glass and Metal. Silvering Glass.

There are three kinds of glass rod and tubing which are easily
obtainable; these are soda-glass, which is that usually supplied by
chemical apparatus dealers when no particular glass is specified;
combustion-glass, which is supplied for work requiring a glass that does
not so easily soften or fuse as soda-glass; and lead-glass, which is
less common. There are also resistance-glass, made for use where very
slight solubility in water or other solutions is desirable, and a number
of other special glasses; but of these soda-glass, combustion-glass,
lead-glass, and resistance-glass are the most important to the
glass-blower whose work is connected with laboratory needs.

_Soda-Glass._--Consists chiefly of sodium silicate, but contains
smaller quantities of aluminum silicate, and often of calcium silicate;
there may also be traces of several other compounds.

The ordinary soda-glass tubing melts easily in the blowpipe flame, it
has not a long intermediate or viscous stage during fusion, but becomes
highly fluid rather suddenly; it does not blacken in the reducing flame.
Bad soda-glass or that which has been kept for many years, tends to
devitrify when worked. That is to say the glass becomes more or less
crystalline and infusible while it is in the flame; and in this case it
is often impossibl