A very satisfactory arrangement may be made from an old bicycle; the back wheel having the tyre removed and a flat rim of tin fastened on in its place. The chain drive should be retained, but one of the cranks removed and a handle substituted for the remaining pedal. The whole device is shown by Fig. 13. [Illustration: Fig. 13] The procedure in "spinning" glass is as follows:--First melt the end of a glass rod and obtain a large mass of thoroughly softened glass, now spin the wheel at such a speed that its own momentum will keep it spinning for several seconds. Touch the end of the melted rod with another piece of glass and, without withdrawing the original rod from the blowpipe flame, draw out a thread of molten glass and twist it round the spinning wheel. If this is done properly, the thread of glass will grip on the flat rim, and by continuing to turn the wheel by hand it is possible to draw out a continuous thread from the melted rod, which must be advanced in the blowpipe flame as it is drawn on the wheel. If the rod is not advanced sufficiently the thread will melt off, if it is advanced too much, so as to heat the thick part and allow the glass to become too cool at the point of drawing out, then the thread will become too thick, but it is easy after a little practice to obtain the right conditions. Practice is necessary also in order to find the right speed for the wheel. When sufficient glass has been "spun," the whole "hank" of thin thread 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.