(3) When a heated tube or bulb of silica is to be expanded by blowing, it is best not to remove it from the flame, for if that is done it will lose its plasticity quickly unless it be large. The better plan is to move it slightly from the “hot spot” into the surrounding parts of the flame at the moment of blowing.
It is best to blow the bulb through an india-rubber tube attached to the open end of the silica tube. At first one frequently bursts the bulbs when doing this, but holes are easily repaired by stopping them with plastic silica applied by the softened end of a fine rod of silica and expanding the lump, after re-heating it, by blowing. After a few hours’ practice these mishaps gradually become rare.
I find it a good plan to interpose a glass tube packed with granulated potash between the mouth and the silica tube. This prevents the interior of the tube from being soiled. The purifying material must not be packed so closely in the tube as to prevent air from passing freely through it under a very low pressure.
It may be mentioned here that a finished tube usually contains a little moisture, and a recognisable quantity of nitric peroxide. These may be removed by heating the tube and drawing filtered air through it, but not by washing, as it is difficult to obtain water which leaves no residue on the silica.
Making larger tubes and other apparatus of Silica.—In order to convert a small bulb of silica into a larger one or into a large tube, proceed as follows:—Heat one end of a fine rod of silica and apply it to the bulb so as to form a ring as shown in the figure. Then heat the ring and the end of the bulb till it softens, and expand the end by blowing. If this process is repeated, the bulb first becomes ovate and then forms a short tube which can be lengthened at will, but the most convenient way to obtain a very long tube is to make several shorter tubes of the required diameter, and say 200 to 250 mm. in length, and to join these end to end. It does not answer to add lumps of silica to the end of the bulb, for the sides of the tube made in this way become too thin, and blow-holes are constantly formed during the making of them. These can be mended, it is true, but they spoil the appearance of the work.
Tubes made in the manner described above are thickened by adding rings of silica and blowing them when hot to spread the silica. If a combination of several jets is employed, very large tubes can be constructed in this way. One of Messrs. Baird and Tatlock’s workmen lately blew a bulb about 5 cm. in diameter, and it was clear that he could have converted it into a long cylindrical tube of equal diameter had it been necessary to do so.
Very thin tubes of 1·5 cm. diameter, and tubes of considerable thickness and of equal size, are easily made after some practice, and fine capilliaries and millimetre tube can be made with about equal readiness.
If a very fine tube of even bore is required, it may be drawn from a small thick cylinder after a little practice.
When a tube becomes so large that it cannot be heated uniformly on all sides by rotating it in the flame, it is convenient to place a sheet of silica in front of the flame a little beyond the object to be heated, in order that the former may throw back the flame on those parts of the tube which are most remote from the jet. A suitable plate may be made by sticking together small lumps of silica rendered plastic by heat.