order to avoid heating delicate apparatus which has become damp and needs drying, the water may be washed out with a few drops of spirit, which is readily removed at a low temperature.

Before using a glass tube for an operation in which it will be necessary to blow into it, one end of it must be contracted, unless it is already of such a size that it can be held between the lips with perfect ease; in any case, its edges must be rounded. For descriptions of these operations, see page 35. The other end must be closed. This may be done by means of a cork.

Presenting Glass to the Flame.—Glass tubes must never be brought suddenly into the flame in which they are to be heated. All glass is very likely to crack if so treated. It should in all cases be held for a little while in front of the flame, rotated constantly in the hot air and moved about, in order that it may be warmed over a considerable area. When it has become pretty hot by this treatment, it may be gradually brought nearer to the flame, and, finally, into contact with it, still with constant rotation and movement, so as to warm a considerable part of the tube. When the glass has been brought fairly into contact with the flame, it will be safe to apply the heat at the required part only. Care must be taken in these preliminary operations to avoid heating the more fusible glasses sufficiently to soften them.

Methods of working with Lead and soft Soda Glass respectively.—When lead glass is heated in the brush flame of the ordinary Herapath blow-pipe, or within the point of the pointed flame, it becomes blackened on its surface, in consequence of a portion of the lead becoming reduced to the metallic state by the reducing gases in the flame. The same thing will happen in bending a lead glass tube if it is made too hot in a luminous flame. A practical acquaintance with this phenomenon may be acquired by the following experiment:—

Take a piece of lead glass tube, bring it gradually from the point of a pointed flame to a position well within the flame, and observe what happens. When the glass reaches the point A (Fig. 3), or thereabouts, a dark red spot will develop on the glass, the area of the spot will increase as the glass is brought further in the direction A to B. If the glass be then removed from the flame and examined, it will be found that a dark metallic stain covers the area of the dark red spot previously observed. Repeat the experiment, but at the first appearance of the dark spot slowly move the glass in the direction A to C. The spot will disappear, and, if the operation be properly performed, in its place there will be a characteristically greenish-yellow luminous spot of highly heated glass. In this proceeding the reduced lead of the dark spot has been re-oxidised on passing into the hot gases, rich in oxygen, which abound at the point of the flame. If one end of the tube has been previously closed by a piece of cork, and if air be forced into the tube with the mouth from the open end before the luminous spot has become cool, the glass will expand. If the experiment be repeated several times, with pointed flames of various sizes, the operator will quickly learn how to apply the pointed flame to lead glass so that it may be heated without becoming stained with reduced lead.

If the spot of reduced metal produced in the first experiment be next brought into the oxidising flame, it also may gradually be removed. On occasion, therefore, apparatus which has become stained with lead during its production, may be rendered presentable by suitable treatment in the oxidising flame. The process of re-oxidising a considerable surface in this way after it has cooled down is apt to be very tedious, however, and, especially in the case of thin tubes or bulbs, often is not practicable. In working with lead glass, therefore, any reduction that occurs should be removed by transferring the glass to the oxidising flame at once.

Small tubes, and small areas on larger tubes of English glass, may be softened without reduction by means of the pointed oxidising flame; but it is not easy to heat any considerable area of glass sufficiently with a pointed flame. And though it is possible, with care, to employ the hot space immediately in front of the visible end of an ordinary brush flame, which is rich in air, yet, in practice, it will not be found convenient to heat large masses of lead glass nor tubes of large size, to a sufficiently high temperature to get the glass into good condition for blowing, by presenting them to the common brush flame.

It may seem that as glass which has become stained with reduced lead can be subsequently re-oxidised by heating it with the tip of the pointed flame, the difficulty might be overcome by heating it for working in the brush flame, and subsequently oxidising the reduced lead. It is, however, difficult, as previously stated, to re-oxidise a large surface of glass which has been seriously reduced by the action of the reducing gases of the flame, after it has cooled. Moreover, there is this very serious objection, that if, as may be necessary, the action of the reducing flame be prolonged, the extensive reduction that takes place diminishes the tendency of the glass to acquire the proper degree of viscosity for working it, the glass becomes difficult to expand by blowing, seriously roughened on its surface, and often assumes a very brittle or rotten condition.

When it is only required to bend or draw out tubes of lead glass, they may be softened sufficiently by a smoky flame, which, probably owing to its having a comparatively low temperature, does not so readily reduce the lead as flames of higher temperature. But for making joints, collecting masses of glass for making bulbs, and in all cases where it is required that the glass shall be thoroughly softened, the smoky flame does not give good results.

In the glass-works, where large quantities of ornamental and other glass goods are made of lead or flint glass, the pots in which the glass is melted are so constructed that the gases of the furnace do not come into contact with the glass;[4] and as the intensely-heated sides of the melting-pot maintain a very high temperature within it by radiation, the workman has a very convenient source of heat to his hand,—he has, in fact, only to introduce the object, or that part of it which is to be softened, into the mouth of the melting-pot, and it is quickly heated sufficiently for his purpose, not only without contact of reducing gases, but in air. He can therefore easily work upon very large masses of glass. In a special case, such a source of heat might be devised by the amateur. Usually, however, the difficulty may be overcome without special apparatus. It is, in fact, only necessary to carry out the instructions given below to obtain a considerable brush flame rich in air, in which the lead glass can be worked, not only without discoloration, but with the greatest facility.

To Produce an Oxidising Brush Flame.—The blower used must be powerful, the air-tube of the blow-pipe must be about half as great in diameter as the outer tube which supplies the gas. The operator must work his bellows so as to supply a strong and steady blast of air, and the supply of gas