articles made of soda glass. Some glass-blowers content themselves with permitting the glass to cool gradually in a smoky flame till it is covered with carbon, and then leave it to cool upon the table. But under this treatment many joints made of soda glass which are not quite uniform in substance, but otherwise serviceable, will break down. In glass-works the annealing is done in ovens so arranged that the glass enters at the hottest end of the oven where it is uniformly heated to a temperature not much below that at which it becomes viscous, and slowly passed through the cooler parts of the chamber so that it emerges cold at the other end. This method of annealing is not practicable in a small laboratory. But fortunately very good results can be obtained by the following simple device, viz.:—

By wrapping the hot apparatus that is to be annealed closely in cotton wool, and leaving it there till quite cold. The glass should be wrapped up immediately after it is blown into its final shape, as soon as it is no longer soft enough to give way under slight pressure. And it should be heated as uniformly as possible, not only at the joint, but also about the parts adjacent to the joint, at the moment of surrounding it with the cotton. Lead glass appears to cool more regularly than soda glass, and these precautions may be more safely neglected with apparatus made of lead glass; but not always. At the date of writing I have had several well-blown joints of thick-walled capillary tube to No. 16 (see diagram, p. 82), break during cooling, in consequence of circumstances making it dangerous to heat the neighbourhood of the joint so much as was necessary.

The black carbonaceous coat formed on hot glass when it is placed in cotton wool may be removed by wiping with methylated spirit, or, if it be very closely adherent, by gently rubbing with fine emery, moistened with the spirit.

Cotton wool is rather dangerously inflammable; it should therefore be kept out of reach of the blow-pipe flame, and care should be taken that the glass is not placed in contact with it at a sufficiently high temperature to cause its ignition.

Another method of annealing is to cover the hot glass with hot sand, and allow it to cool therein.

As in the case of lead glass, so with soda glass. A thorough acquaintance with the effect of the various parts of the flame upon it should be gained before further work is entered upon, for which purpose an hour or more spent in observing its behaviour in the flame will be fully repaid by increased success subsequently.

The Use of Combustion Tube.—It is often necessary to construct apparatus of what is known as hard glass or combustion tube. It is almost as easy to work combustion tube as to deal with lead and soda glass if the oxy-hydrogen flame be employed.

It is not necessary to set up a special apparatus for this purpose; many of the ordinary blow-pipes can be used with oxygen instead of with air. It is only necessary to connect the air-tube of the blow-pipe with a bottle of compressed oxygen instead of with the bellows. The connecting tube should not be too wide nor too long, in order to avoid the accumulation in it, by accident, of large quantities of explosive mixtures.

Two precautions are necessary in manipulating hard glass in the oxy-hydrogen flame. The glass must not be overheated. At first one is very apt to go wrong in this direction. The supply of oxygen must not be too great; a small hissing flame is not what is wanted. If either of these precautions are neglected most glass will devitrify badly. With a little care and experience, devitrification can be absolutely avoided. Ordinary combustion tube can be used, but I find that the glass tube (Verbrennungsröhr) made by Schott & Co. of Jena, which can be obtained through any firm of dealers in apparatus, is far better than the ordinary tube.

By following these instructions, any one who has learned how to work with lead or soda glass will find it easy to manipulate hard glass.

[2] For details of the composition of the various glasses, some work on glass-making may be consulted.

[3] The presence of silicates of calcium and aluminum are considered to promote a tendency to devitrification in glass; and glasses of complex composition are more apt to devitrify than the simpler varieties. See Glass-making, by Powell, Chance, and Harris, Chap. IV.

[4] See Principles of Glass-making, p. 31.

[5] Nevertheless the supply of air must not be so excessive as to reduce the temperature of the flame sufficiently to prevent the thorough softening of the glass, which will occur if the bellows is worked with too much zeal.