considerable value. This jet consists of a block of metal in which are drilled seven holes, one being central and the other six arranged in a close circle around the central hole. To each of these holes is a communication way leading to the gas supply, and an air jet is arranged centrally in each. Each hole has also an extension tube fitted into it, the whole effect being that of seven blowpipes. In order to provide a final adjustment for the flame, a perforated plate having seven holes which correspond in size and position to the outer tubes is arranged to slide on parallel guides in front of these outer tubes.

Fig. 2

The next piece of apparatus for consideration is the bellows, of which there are three or more types on the market, although all consist of two essential parts, the blower or bellows proper and the wind chamber or reservoir. Two patterns are shown in Fig. 2; a, is the form which is commonly used by jewellers and metal workers to supply the air blast necessary for heating small furnaces. Such a bellows may be obtained at almost any jewellers' supply dealer in Clerkenwell, but it not infrequently happens that the spring in the wind chamber is too strong for glass-blowing, and hence the air supply tends to vary in pressure. This can be improved by fitting a weaker spring, but an easier way and one that usually gives fairly satisfactory results, is to place an ordinary screw-clip on the rubber tube leading from the bellows to the blowpipe, and to tighten this until an even blast is obtained.

Another form of bellows, made by Messrs. Fletcher and Co., and common in most laboratories, is shown by b; the wind chamber consists of a disc of india-rubber clamped under a circular frame or tied on to a circular rim. This form is shown by Fig. 2, b.

The third form, and one which my own experience has caused me to prefer to any other, is cylindrical, and stands inside the pedestal of the blowpipe-table. A blowpipe-table of this description is made by Enfer of Paris.

There is no need, however, to purchase an expensive table for laboratory use. All the work described in this book can quite well be done with a simple foot bellows and a quick-change blowpipe. Nearly all of it can be done with a single jet blowpipe, such as that described first, or even with the still simpler apparatus mentioned on page 84, but I do not advise the beginner to practise with quite so simple a form at first, and for that reason have postponed a description of it until the last chapter.

Glass-blowers' tools and appliances are many and various, quite a number of them are better rejected than used, but there are a few essentials. These are,—file, glass-knife, small turn-pin, large turn-pin, carbon cones, carbon plate, rubber tube of small diameter, various sizes of corks, and an asbestos heat reflector. For ordinary work, an annealing oven is not necessary, but one is described on page 60 in connection with the special cases where annealing is desirable.

Fig. 3 illustrates the tools and appliances. a is an end view of the desirable form of file, and shows the best method of grinding the edges in order to obtain a highly satisfactory tool. b is a glass knife, shown both in perspective and end view, it is made of glass-hard steel and should be sharpened on a rough stone, such as a scythe-stone, in order to give a slightly irregular edge. c is a small turn-pin which may be made by flattening and filing the end of a six-inch nail. d is the large turn-pin and consists of a polished iron spike, about five inches long and a quarter of an inch diameter at its largest part. This should be mounted in a wooden handle. e and f are carbon cones. A thin rubber tube is also useful; it may be attached to the work and serve as a blowing tube, thus obviating the necessity of moving the work to the mouth when internal air pressure is to be applied. In order to avoid undue repetition, the uses of these tools and appliances will be described as they occur.

Fig. 3

Glass, as usually supplied by chemical apparatus dealers is of the composition known as "soda-glass." They also supply "hard" or "combustion" glass, but this is only used for special purposes, as it is too infusible for convenient working in the ordinary blowpipe flame.

Soda-glass consists primarily of silicate of sodium with smaller quantities of silicate of aluminum and potassium. Its exact composition varies. It is not blackened, as lead glass is, by exposure to the reducing gases which are present in the blue cone of a blowpipe flame, and hence is easier for a beginner to work without producing discolouration.

Further notes on glasses will be found on page 55, but for ordinary purposes soda-glass will probably be used.

CHAPTER II

Perhaps the most common need of the glass-blower whose work is connected with that of the laboratory is for a sealed tube; and the sealing of a tube is an excellent preliminary exercise in glass-blowing.

We will assume that the student has adjusted the blowpipe to give a flame similar to that shown in d, Fig. 1, and that he has learned to maintain a steady blast of air with the bellows; further, we will assume that the tube he wishes to seal is of moderate size, say not more than half an inch in diameter and with walls of from one-tenth to one-fifth of an inch thick.

Fig. 4

A convenient length of tube for the first trial is about one foot; this should be cut off from the longer piece, in which it is usually supplied, as follows:—lay the tube on a flat surface and make a deep cut with the edge of a file. Do not "saw" the file to and fro over the glass. If the file edge has been ground as shown in a, Fig. 3, such a procedure will be quite unnecessary and only involve undue wear; one movement with sufficient pressure to make the file "bite" will give a deep cut. Now rotate the tube through about one-eighth of a turn and make another cut in continuation of the first. Take the tube in the hands, as shown in a, Fig. 4, and apply pressure with the thumbs, at the same time straining at the ends. The tube should break easily. If it does not, do not strain too hard, as it may shatter and cause serious injuries to the hands, but repeat the operation with the file and so deepen the original