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As regards the use of silica in experiments with gases, it must be remarked that vitreous silica, like platinum, is slightly permeable to hydrogen when strongly heated.

The skeleton is formed by small crystals and long threads of vitreous silica, cemented together, during life, by protoplasm.

The method of making Silica Tubes.—Before commencing to make a tube a supply of vitreous silica in rods about one or two millimetres in diameter must be prepared.

The rate of expansion of vitreous silica has been studied by H. le Chatelier, and more recently by Callendar.

The density of vitreous silica is very near to that of ordinary amorphous silica.