chief difficulty in learning to use the blowpipe properly is experienced in acquiring the habit of keeping up the blast of air for some time uninterrupted by the breathing. A little patient trial will, however, soon remove this difficulty, if it is borne in mind that the cheeks must be kept constantly inflated with air, and that the air must be forced through the blowpipe by the pressure of the cheeks alone and not by the action of the lungs, breathing being carried on meanwhile through the nose, and the mouth being occasionally replenished with air from the throat just before breathing out the air from the lungs through the nose. It is frequently necessary to have both hands free whilst using the blowpipe; this may be secured by resting the jet a on the top of the burner, and supporting the other end b, by the lips alone, as shown in fig. 15; or the blowpipe may be rested on a support of convenient height, such as the ring of a retort-stand properly adjusted. The bright flame obtained by closing the air-holes of the burner is much better suited for use with the blowpipe than the ordinary non-luminous flame. A burner with an elliptical crifice which gives a flat flame is commonly substituted for the ordinary round flame of the Bunsen-burner; it is easily obtained by slipping a brass tube down the tube of the burner. A further account of the uses of the blowpipe is given in par. 30. 5. Glass tube or rod is cut by laying it upon a flat surface, FIG. 16. and making a deep scratch with the edge of a three-cornered file at the point to be cut. The glass is then held with both hands, one on either side of the scratch and close to it, and a gentle pressure is exerted upon the glass as if trying to break it across (fig. 16). If the file mark has been made sufficiently deep, the glass will readily break at the scratch; the FIG. 17. sharp edges of a rod or tube should always be at once rounded by holding them in the Bunsen or blowpipe-flame until they are partly melted, or by rubbing them with the face of a file. 6. Glass tube is bent by holding the part to be bent in the upper edge of a common fish-tail gas-flame, so as to heat at least two inches of the glass (fig. 17). The Bunsen-flame must never be employed for bending glass-tubing; it produces a most unsatisfactory bend. The tube is supported by holding it with both hands, one on either side of the flame, and whilst being heated it is constantly turned slowly round on its axis so as to heat all sides equally. As soon as the glass is felt to be soft and pliable, it is taken out of the flame and quickly bent to the required angle. The heated part must not be allowed to touch anything until it is cold; the soot is then removed from it by a cloth or piece of paper. A bend, if properly made, should be a curve, and should not alter the bore of the tube (fig. 17 a); if a sharp angle is made, the bore will be narrowed, and the bend, besides being unsightly, is very liable to break under a small strain (fig. 17h). Glass rod may be bent in the Bunsen or blowpipe flame. FIG. 17 a. FIG. 17 b. 7. Glass tube is drawn out by holding it with both hands as for bending; and whilst turning it constantly round on its long axis, the part to be drawn out is strongly heated in the Bunsen, or better, in the blowpipe flame (see fig. 15); when the glass is well softened, the two ends are slowly pulled asunder in opposite directions. 8. Corks are bored usually by means of brass cork-borers, which are used as punches. A borer is selected of the same size as the glass tube which is to be inserted into the cork, مانسية FIG. 18. rather less than greater; the cork is then pressed against a wooden surface (best against the upright edge of a thick bench or table), and the perforation made in it by pushing the borer through whilst constantly turning it upon its axis (fig. 18). Caution and practice will enable the student to make a clean straight hole without damaging the surrounding parts of the cork. In boring a single hole through a cork, the easiest way to make it straight is to bore from the centre of one end halfway towards the other, then reverse the cork and bore a hole to meet this from the centre of the opposite end. A perforation may also be made by pushing a sharply pointed round file carefully through the cork.. The round file is required for smoothing the interior of holes made by the cork-borer, or for slightly enlarging them when they are too small; whilst doing this great care must be taken to leave the hole round in shape, and not to enlarge it so much that the glass tubing when inserted fits loosely. In perforating india-rubber stoppers, the borer used must be sharp, and must be kept well wetted with water, or better with solution of caustic soda or potash. The edge of the cork-borer is sharpened when necessary by rubbing the outer part of the edge obliquely with the face of a fine-toothed three-cornered or flat file. 9. Sulphuretted-hydrogen tube.-A tube of the requisite shape will have been used for the experiments in Section I.; it is seen as a bent-delivery tube in figs. 4 and 5 (pp. 12, 14). If not already made, bend a piece of glass tubing (6) ten or twelve inches in length at right angles into the shape shown in fig. 17 u, the shorter limb being about two inches long. This tube is to be kept for passing sulphuretted-hydrogen gas through liquids. 10. Small Ignition-tubes.-A piece of hard glass tubing, perfectly dry inside and out, not much less than the eighth of an inch in internal diameter, is drawn out at its middle point by heating it strongly in the blowpipe flame, as shown in fig. 15, p. 29. The narrowed portion of the tube (fig. 19) is then cut across (5) at its middle point, and by heating FIG. 19. FIG. 19 a. the conical part a, the narrow tube may be drawn off, and a small closed tube is obtained (fig. 19 a). If the closed end is strongly heated in the blowpipe flame and blown into whilst hot, it may be expanded into a small bulb. (See fig. 36, p. 115). 11. Glass Stirring Rods.-Three or four glass rods are made by cutting (5) a length of solid glass rod into pieces, some end (see fig. 20); the sharp edges are thus partially melted. The end of the rod must not be allowed to touch anything until it is cool. *To distinguish hard glass from soft (lead) glass, hold the tube towards the light with one end near the eye with hard glass the ring of glass will appear green or colourless, with soft glass black or dark. C 11 a. Sometimes a very thin glass rod is required; this may be made from a stouter rod by softening it in the blowpipe flame (fig. 15, p. 29), and then quickly drawing it out until the required fineness is produced. 12. Mounted Platinum Wires.-The platinum wire used here must not persistently colour the flame green. Draw out a FIG. 21. piece of glass tube six or seven inches in length at its middle point (fig. 15, p. 29) into the shape shown in fig. 19. Cut it across (5) at the middle of the narrow portion. Each piece of glass thus obtained serves for the handle to a wire. Break off the narrow part of the tube until it extends only about a quarter of an inch from the shoulder (a, fig. 19), and into the narrow opening insert the end of a piece of platinum wire about two inches in length and bent round at its other end into a circle about the eighth of an inch across by rolling it round a piece of thick wire; then hold the end of the wire surrounded by glass in the Bunsen or blowpipe flame until the glass melts and thickens around it, fixing it firmly when cold (fig. 21). At least two wires should be mounted in this FIG. 22. a way. They are then rendered less liable to be lost and much more easy to handle. The wire may also be mounted by softening the end of a short piece of glass-rod in the blowpipe flame, and sticking the wire into it. 13. Wash- Bottle.-A thin flat-bottomed flask is chosen (best of the triangular shape shown in fig. 22) and of 16 or 18 ounces capacity; the neck must not be less than an inch in diameter. Procure a sound cork which is slightly too large to enter the neck, soften the cork by placing it upon the floor and rolling it backwards and forwards under the foot with gentle pressure; when thus softened the cork must |