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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
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