Powdered glass is frequently used instead of paper, cloth, cotton or sand for filtering varnishes, acids, etc. It is not soluble or corro-dible. Sand, if purely silicious, would be better, but such sand is difficult to get; it too often contains matters which are easily corroded or dissolved. Powdered glass when glued to paper is also used for polishing wood and other materials. It cuts rapidly and cleanly, and is better than sand for most purposes. Glass'is easily pulverised after being heated red hot and plunged into cold water. It cracks in every direction, becomes hard and brittle, and breaks with keenly cutting edges. After being pounded in a mortar it may be divided into powders of different degrees of fineness by being sifted through lawn sieves.
Very few stoppers properly fit the bottles for which they are intended. The stoppers and bottles are ground with copper cones, fed with sand and made to revolve rapidly in a lathe, and the common stock are not specially fitted. To fit a stopper to a bottle that has not been ground, use emery or coarse sand kept constantly wet with water, and replaced with fresh as fast as it is reduced to powder. When all the surface has become equally rough, it is considered a sign that the glass has been ground to the proper shape, as until that time the projecting parts only show traces of erosion. This is the longest and hardest part of the work, as after that the glass simply needs finishing and polishing. For that purpose emery only can be used, owing to the fact that the material can be obtained of any degree of fineness, in this respect differing from sand. Otherwise the operation is the same as before, the emery being always kept moistened, and replaced when worn out. The grinding is continued until both the neck of the bottle and the stopper acquire a uniform finish, of a moderate degree of smoothness, and until the stopper fits so accurately that no shake can be felt in it, even though it be not twisted in tightly.
To seal tubes hermetically ' after gases have been admitted under pressure, the following arrangement was employed with complete success: - The experimental tube A is joined to a T-piece B, the lateral limb of which is constructed, as shown in Fig. 283; a glass plug D is ground into the tube at E, and serves the purpose of a valve opening inward. When gas under pressure is allowed to enter the tube at 6, the valve opens, but on removing the pressure from without it at once closes; the escape of gas from A is thus prevented, and the tube may be sealed before the blowpipe at H*
Sealing glass tubes.
When the tube contains a liquid, the plug should be moistened with it; this will prevent the escape of gas while the tube is being sealed, even though the plug does not fit very accurately. In the absence of any liquid, greater care in grinding the plug is required. The tube F serves for the admission of liquid into ,the experimental tube in the first instance; it is then closed, and at the end of the experiment it is opened and the contents of the tube are removed. The rest of the apparatus is thus kept intact, and may be used repeatedly, especially if the tube at H is fairly long. (A. Richardson, in Chem. News,)