The shortest way of selecting glass is to go to a good firm, and let it be understood that if the glass proves to be badly annealed it will be returned. Though it was stated above that the glass should not be distinctly conical, of course allowance must be made for the length of the pieces, and, on the other hand, a few highly conical tubes will be of immense service in special cases, and a small supply of such should be included.
The glass, as it is obtained, should be placed in a rack, and covered by a cloth to reduce the quantity of dust finding its way into the tubes. It has been stated by Professor Ostwald that tubes when reared up on end tend to bend permanently. I have not noticed this with lead glass well supported. Each different supply should be kept by itself and carefully described on a label pasted on to the rack, and tubes from different lots should not be used for critical welds. This remark is more important in the case of soda than of lead glass.
In the case of very fine thermometer tubes it will be advisable to cover the ends with a little melted shellac, or, in special cases, to obtain the tubes sealed from the works. Soda glass can generally be got in rather longer lengths than lead glass; the longer the lengths are the better, for the waste is less.
It is useful to be able to distinguish the different kinds of glass by the colour. This is best observed by looking towards a bright surface along the whole length of the tube and through the glass. Lead glass is yellow, soda glass is green, and hard glass purple in the samples in my laboratory, and I expect this is practically true of most samples. [Footnote: Some new lead glass I have is also almost purple in hue. If any doubt exists as to the kind of glass, it may be tested at once in the blow-pipe flame, or by a mixture of oils of different refractive indices, as will be explained later.]
§ 12. The question of the solubility of glass in reagents is one of great importance in accurate work, though it does not always meet with the attention it deserves. It is impossible here to go into the matter with sufficient detail, and the reader is therefore referred to the Abstracts of the Chemical Society, particularly for the years 1889 and 1892. The memoir by F. Kohlrausch, Wied. Ann. xliv., should be consulted in the original. The following points may be noted. A method of testing the quality of glass is given by Mylius (C. S. J. Abstracts, 1889, p. 549), and it is stated that the resistance of glass to the action of water can generally be much increased by leaving it in contact with cold water for several days, and then heating it to 300° to 400° C. This improvement seems to be due to the formation of a layer of moist silica on the surface, and its subsequent condensation into a resisting layer by the heating. Mylius (C. S. J. Abstracts, 1892, p. 411), and Weber, and Sauer (C. S. J. Abstracts, 1892, p. 410) have also shown that the best glass for general chemical purposes consists of
Silica, 7 to 8 parts
Lime, 1 part
Alkali, 1.5 to 1.1 parts.
This is practically "Bohemian" tube glass.
The exact results are given in the Berichte of the German Chemical Society, vol. xxv. An excellent account of the properties of glass will be found in Grove's edition of Miller's Elements of Chemistry.