To make tine glass thread, the glass is brought to a state of fusion, a glass rod is dipped in it, and thus a thread is pulled out, which solidifies first in its thinnest parts, and so causes a uniform thickness to be obtained. If this thread is placed over a hot metallic cylinder, and the latter is revolved, any length of thread may be obtained, and finer by revolving the cylinder more rapidly. The heating of the cylinder has the effect of annealing the - glass to some extent, while the rapid cooling, if wound on a cold cylinder, would make it more brittle. A bundle of such threads looks like a bunch of silk, and it has therefore been called glass silk. It is largely used for filtering liquids in laboratories. In the microscope, the threads are as fine as those of silk or fibrillae of cotton; they break more easily than the latter, but are excessively supple. From the unalterability of the substance, it is very well suited for filtering acid or alkaline solutions, even concentrated, and various other substances, such as nitrate of silver, albumen, collodion, Fehling's liquor, etc. It affords great rapidity of flow, with good filtration; it does not, like filters of paper or tissue, communicate organic matters to the liquids, altering and perhaps giving them a disagreeable taste.
It is much preferable to asbestos, which, from the arrangement of its parallel fibres, cannot be formed into a flexible ball, and which lets fragments pass that float in the liquid. For analysis it is very advantageous, allowing of a ready determination of insoluble matters deposited; also by calcination and fusion of the glass may be found the volatile principles fixed in the passage of the liquid, unmixed with empyreu - matic products. Notwithstanding the price of glass silk is still high, it is no great expense to use it, as its excessive lightness admits of a considerable number of fitrations being made with a small weight of it; besides, it may serve an indefinite time, if after each operation, it is thoroughly washed with water and dried in the air.