The silica tubes thus made can be cut and broken like glass, they can be joined together before the flame, and they can also be drawn into smaller tubes when softened by heat.
In order to make a side connection as in a T piece, a ring of silica should be applied to the tube in the position fixed upon for the joint. This ring must then be slightly expanded, a new ring added, and so on, till a short side tube is formed. To this it is easy to seal a longer tube of the required dimensions. It is thus possible to produce Geissler tubes, small distilling flasks, etc. Solid rods of silica are easily made by pressing together the softened ends of the fine rods or threads previously mentioned. Such rods and small masses can be ground and polished without annealing them.
Quartz Fibres.—These were introduced into physical work by Mr. Boys in 1889. They may be made by attaching a fine rod of vitrified quartz to the tail of a small straw arrow provided with a needle-point; placing the arrow in position on a cross-bow, heating the rod of silica till it is thoroughly softened and then letting the arrow fly from the bow, when it will carry with it an extremely fine thread of silica. A little practice is necessary to ensure success, but a good operator can produce threads of great tenacity and great uniformity. Fuller accounts of the process and of the various properties and uses of quartz fibres will be found in Mr. Boys’ lectures (Roy. Inst. Proc. 1889, and Proc. Brit. Assn. 1890), and in Mr. Threlfall’s Laboratory Arts.
[22] A brief summary of the history of this subject will be found in Nature, Vol. 62, and in the Proceedings of the Royal Institution, 1901.
[23] In a recent communication Professor Callendar tells me that the devitrification commences at the outside and is hastened by particles of foreign matter.
[24] The silica blocks used were prepared by fusion in an electric furnace; it is therefore probable that they were not quite pure.
[25] Messrs. Baird and Tatlock.
[26] This is to avoid bubbles in the finished glass.
[27] The rock crystal exhibits a yellow flame when first heated in the oxy-gas flame, and most samples contain spectroscopic quantities of lithium.