The use of the wheel-cutter calls for no special mention as it will cut at any angle, although the pressure required is somewhat greater than that needed by most diamonds. CHAPTER III Internal Seals, Air-Traps, Spray Arresters, Filter-Pumps--Sprays, Condensers; Plain, Double Surface, and Spherical--Soxhlet Tubes and Fat Extraction Apparatus--Vacuum Tubes, Electrode Work, Enclosed Thermometers, Alarm Thermometers, Recording Thermometers, "Spinning" Glass. _Internal Seals._--It is convenient to class those cases in which a glass tube passes through the wall of another tube or bulb under the heading of "Internal Seals." These are met with in barometers, spray arresters, and filter pumps, in condensers and some forms of vacuum tube. The two principal methods of making such seals will be considered first and their special application afterwards. _An Air Trap on a Barometer Tube._--This involves the use of the first method, and is perhaps the simplest example that can be given. Fig. 9, _a_, _a1_ and _a2_, show the stages by which this form of internal seal is made. For the first trials, it is well to work with fairly thick-walled tubing, which should be cut into two pieces, each being about eight inches long. [Illustration: Fig. 9] First seal the end of one tube as described on page 13, heat the sealed end and expand to a thick walled bulb. Fuse the end of the other tube, attach a piece of glass rod to serve as a handle, and draw out; cut off the drawn-out portion: leaving an end like _a_. Now heat a small spot at the end of the bulb, blow, burst out, and remove the thin fragments of glass. Heat a zone on the other tube at the point where the drawn-out portion commences and expand as shown by _a1_. The next stage is to join the tubes. Heat the ragged edges of the burst-out portion until they are thoroughly rounded. At the same time heat the drawn-out tube to just below softening point. Then, while the rounded edges of the burst-out portion are still soft, insert the other tube; rotate the join in the blowpipe flame until it is quite soft, and expand by blowing. If necessary, re-heat and expand again. The finished seal, which should be slightly annealed by smoking in a sooty flame, is shown by _a2_. _A Spray Arrester._--This is made by the second method, in which the piece of tube which projects inside the bulb is fused in position first and the outer tube is then joined on. The various stages of making are illustrated by _b_, _b1_ and _b2_, Fig. 9.