operations which will be used and amplified later. They should in themselves be things which are of importance and commonly used in glass-work, and they should be so arranged that the fundamental points, such as the rotation of glass, the proper temperature, blowing and shrinking the glass may be learned with a minimum expenditure of time, glass and gas. It is therefore recommended that the beginner take them up in the order given, at least as far as No. 7, and that each be mastered before attempting the next. The beginner should not leave the first exercise, for example, until he can join together two pieces of tubing so that they form one piece of substantially uniform inner and outer diameter, and without thick or thin spots. From two to four practice periods of two hours each should suffice for this. This chapter and the following one should also be frequently read over, as many of the points discussed will not be understood at first and many of the manipulations described will not be necessary in the simpler exercises.

CHAPTER II

General Operations

Cutting the Glass.—For this purpose a "glass-knife" is preferred to a file, if the glass is cold: if it is hot a file must always be used, and its edge slightly moistened to prevent drawing the temper. The glass-knife is simply a flat piece of hard steel, with the edges ground sharp on an emery wheel. The bevel of the edge should be from 30 to 60 degrees. An old flat file can easily be ground into a suitable knife. The glass-knife makes a narrower scratch than the file but appears more likely to start the minute crack which is to cause the tube to break at that point, and the break is more likely to give a good square end. The scratch should be made by passing part of the knife or file once across the glass, never by "sawing" the tool back and forth. This latter procedure dulls the tool very quickly.

In breaking a piece of glass tubing, many persons forget that it is necessary to pull the ends apart, as well as to bend the tube very slightly in such a direction as to open up the minute crack started in the scratch. Care in breaking the tube is essential, as it is impossible to do as good work with uneven ends as with square ones.

When tubing of large diameter or thin wall is to be cut, it is often better not to attempt to break it in the usual way, but to heat a very small globule of glass (¹⁄16 to ¹⁄8 inch diameter) to red heat, and touch it to the scratch. This will usually start the crack around the tube; if it has not proceeded far enough, or has not gone in the desired direction, it may be led along with a hot point of glass. This is put a little beyond the end of the crack, and as the latter grows out toward it, moved along the path where the crack is desired. This point of glass is also very useful in breaking off very short ends of tubes, where there is not room to get a firm enough hold and sufficient leverage to break the tube in the ordinary way, and for breaking tubes attached to large or heavy objects, which would be likely to make trouble if treated in the ordinary way.

Another way of cutting large tubing, especially if it has rather thick walls, is to make a scratch in the usual way, and then turn on the smallest and sharpest possible flame of the blast lamp. The tube is next taken in both hands and held horizontally above the flame so that the scratch is exactly over it. The tubing is now rotated rapidly about its axis, and lowered so that the flame is just tangent to its lower side. After about ten seconds of heating, it is removed from the flame and the hot portion quickly breathed upon, when it will generally crack apart very nicely. Care must be taken to hold the tube at right angles to the flame during the heating, and to rotate it so that only a narrow strip of the circumference is heated, and the scratch should be in the center of this heated strip. By this means tubing as large as two inches in diameter is readily broken.

Griffin's glass cutter, which contains a hardened steel wheel, like that on any ordinary window-glass cutter, and a device by which this can be made to make a true cut clear around the tube, is a very handy article, especially for large tubing, and may be obtained from any dealers in chemical apparatus.

Bending Glass.—Inasmuch as this is one of the commonest operations in the laboratory, it is assumed that the reader knows how to perform it. However, it should be noted that in order to obtain the best results a broad (fish-tail burner) flame should generally be used, and the tube rotated on its axis during the heating, and allowed to bend mostly by its own weight. If large tubing is to be bent, one end must be stoppered and great care used. Whenever the tube shows signs of collapsing or becoming deformed, it must be gently blown out into shape, heating the desired spot locally if necessary. A blast-lamp is likely to be more useful here than the fish-tail burner.

Drawing Out a Tube.—Most students learn this the first day of their laboratory work in chemistry, but few take pains to do it well. The tube should be heated in the flame of a Bunsen burner, or blast lamp (preferably the latter) until it is very soft. During this time it must be continuously rotated about its axis, and so held that the edges of the heated zone are sharply defined; i.e., it should not be allowed to move back and forth along its own axis. When so hot that it cannot longer be held in shape, the tube is removed from the flame, and the ends slowly and regularly drawn apart, continuing the rotation of the tube about its axis. By regulating the rate of drawing and the length of tube heated, the desired length and diameter of capillary may be obtained. The tube should always be rotated and kept in a straight line until the glass has set, so that the capillary may have the same axis as the main tube. This capillary or "tail" is often a very necessary handle in glass-blowing, and if it is not straight and true, will continually make trouble.

In drawing out very large tubing, say from one to two inches in diameter, it is often necessary to draw the tube in the flame, proceeding very slowly and at a lower temperature than would be used with small tubing. This is partly on account of the difficulty of heating large tubing uniformly to a high temperature, and partly in order to prevent making the conical part of the tube too thin for subsequent operations.

Constricting a Tube.—Where a constriction is to be made in a tube, the above method must be modified, as the strength of the tube must be maintained, and the constricted portion is usually short. Small tubes are often constricted without materially changing their outside diameter, by a process of thickening the walls. The tube is heated before the blast lamp, rotating it about its axis as later described, and as it softens is gradually pushed together so as to thicken the walls at the heated point, as in a, Fig. 1. When this operation has proceeded far enough, the tube is removed from the flame, and the ends cautiously and gently drawn apart, continuing the rotation of the tube about its axis and taking care not to draw too rapidly at first. The resulting tube should have a uniform exterior diameter, as shown in b, Fig. 1.

Fig. 1.—Constricting a tube.

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