Glass tubes are usually sold by weight, and therefore the weight of tube of each size that is wished for should be indicated, and also whether it is to be of lead or soda glass.
CHAPTER VII.
VITREOUS SILICA.
Introductory.—Vitreous Silica was made in fine threads by M. Gaudin in 1839,[22] and small tubes of it were made in 1869 by M. A. Gautier, but its remarkable qualities were not really recognised till 1889, when Professor C. V. Boys rediscovered the process of making small pieces of apparatus of this substance, and used the torsion of “quartz fibres” for measuring small forces. More recently the author of this book has devised a process for preventing the “splintering” of quartz which gave so much trouble to the earlier workers, and jointly with Mr. H. G. Lacell, has produced a variety of apparatus of much larger dimensions than had been attempted previously. At the time of writing we can produce by the processes described in the following pages tubes 1 to 1·5 cm. in diameter and about 750 cm. in length, globes or flasks capable of containing about 50 c.c., masses of vitreous silica weighing 100 grams or more, and a variety of other apparatus.
Properties of Vitreous Silica.—For the convenience of those who are not familiar with the literature of this subject, I may commence this chapter with a brief account of the properties and applications of vitreous silica, as far as they are at present ascertained. Vitreous silica is less hard than chalcedony, but harder than felspar. Tubes and rods of it can be cut with a file or with a piece of sharpened and hardened steel, and can afterwards be broken like similar articles of glass. Its conducting power is low, and Mr. Boys has shown that fine fibres of silica insulate remarkably well, even in an atmosphere saturated with moisture. The insulating qualities of tubes or rods of large cross sections have not yet been fully tested; one would expect them to give good results provided that they are kept scrupulously clean. A silica rod which had been much handled would probably insulate no better than one of glass in a similar condition. The density of vitreous silica is very near to that of ordinary amorphous silica. In the case of a small rod not absolutely free from minute bubbles it was found to be 2·21.
Vitreous silica is optically inactive, when homogeneous, and is highly transparent to ultraviolet radiations.