On Laboratory Arts Part 4

-- 27. Occasionally, when it is seen that in order to produce a joint closed all round, one side of the tube would be too much thickened, it is better to patch the open side. For this purpose take a gla.s.s rod about one-sixteenth inch in diameter, and turn the flame to give its greatest effect, still keeping rather an excess of air or oxygen. See that the side of the joint already made is kept fairly hot--it need not be soft; interrupt any other work often enough to ensure this.

Then, directing the flame chiefly on the thin rod, begin to melt and pull the gla.s.s over the edges of the gap. When the gap is closed get the lump very hot, so that all the gla.s.s is well melted together, and then, if necessary, pull the excess of gla.s.s off, as before described.

It must be remembered that this and the method of the previous section are emergency methods, and never give such nice joints as a manipulation which avoids them, i.e. when the ends of the tubes are perfectly straight and true to begin with. Also note that, as the tubes cannot be kept in rotation while being patched, it is as well to work at as low a temperature as possible, consistently with the other conditions, or the gla.s.s will tend to run down and form a drop, leaving a correspondingly thin place behind.

Fig. 20.

-- 28. A very common fault in cutting a tube of about an inch in diameter is to leave it with a projecting point, as shown. This can be slowly chipped off by the pliers, using the jaws to crush and grind away the edge of the projection; it is fatal to attempt to break off large pieces of gla.s.s all at once.

-- 29. It will be convenient here to mention some methods of cutting large tubes. With tubes up to an inch and a half in diameter, and even over this--provided that the gla.s.s is not very thick--we may proceed as follows: Make a good scratch about half an inch long, and pretty deep, i.e. pa.s.s the knife backwards and forwards two or three times. Press a point of melted gla.s.s exactly on one end of the scratch; the gla.s.s point even when pressed out of shape should not be as large as a b.u.t.ton one-twelfth of an inch in diameter. If this fails at first, repeat the operation two or three times.

Fig. 21.

If a crack does not form, touch the hot place with the cold end of the nail. If no success is obtained, try the other end of the scratch.

If failure still pursues the operator, let him make another cut on the opposite side of the tube and try again. In general, the tube will yield the first or second time the hot drop of gla.s.s is applied.

Never apply the drop at the centre of the scratch, or a ragged crack, which may run in any direction, will result. Very often, with a large tube, the crack formed by a successful operation will only extend a short distance. In this case it is desirable to entice the crack round the tube, and not trust to its running straight when the tube is pulled apart.

On the whole, the best method in this case is to employ a flame pencil, which should be kept ready for use. This merely consists of a bit of gla.s.s tube of about the same dimensions as an ordinary lead pencil, drawn down to a very fine jet at one end. The jet must not be very long or thin, or the gla.s.s will soon fuse up. A few trials will enable the operator to get the proper proportions, which are such that the tube has the general appearance of a pencil normally sharpened (say with a cone of 60'). This tube is best made of hard gla.s.s.

Connect it to a gas supply by light flexible tubing, and turn down the gas till the flame from the end of the jet is not more than one-tenth of an inch long. Then apply the jet, beginning from the end of the crack, and gradually draw it (the crack) round the tube. The operation will be a.s.sisted if a rubber ring is slipped on the tube to begin with, so that the eye has some guide as to whether the flame is being drawn round properly or not. The ring must, of course, be far enough away to escape the effect of the flame. The crack will be found to follow the flame in the most docile manner, unless the tube is thick or badly annealed. Some operators recommend a pencil of glowing charcoal, but the flame is undoubtedly better.

-- 30. To cut very thick Tubes.

A large number of methods have been proposed, and nearly everybody has his favourite. The following has always succeeded with me. First mark on the tube, by means of a little dead black spirit paint, exactly where the cut is to be. Then sharpen the gla.s.s knife and scratch a quite deep cut all round: there is no difficulty in making the cut one-twentieth of an inch deep. It will be proper to lubricate the knife with kerosene after the first mark is made. [Footnote: The edge of the knife may be advantageously saved by using an old file moistened with kerosene for the purpose. I find kerosene is not worse, but, if anything, better than the solution of camphor in turpentine recommended by Mr. Shenstone.]

If the gla.s.s is about one-eighth of an inch thick, the scratch maybe conveniently about one-twentieth of an inch deep, but if the gla.s.s is anything like one-quarter of an inch thick, the scratch must be much deeper, in fact, the gla.s.s may be half cut through. To make a very deep scratch, a wheel armed with diamond dust, which will be described later on, may be used. However, it is not essential to use a diamond wheel, though it saves time.

When the cut is made to a sufficient depth proceed thus: Obtain two strips of bibulous paper or bits of tape and twist them round the tube on each side of the scratch, allowing not more than one-eighth of an inch between them. Then add a few drops of water to each, till it is thoroughly soaked, but not allowing water to run away. Dry out the scratch by a shred of blotting paper.

Turn down the oxygas flame to the smallest dimensions, and then boldly apply it with its hottest part playing right into the nick and at a single point. Probably in about two seconds, or less, the tube will break. If it does not, rotate the tube, but still so that the flame plays in the nick. After making the tube very hot all round--if it has not broken--apply the flame again steadily at one point for a few seconds and then apply a bit of cold iron. If the tube does not break at once during these processes, let it cool, and cut the groove deeper; then try again. [Footnote: This method is continually being reinvented and published in the various journals. It is of unknown antiquity.]

Fig. 22.

If the tube breaks after great heating and long efforts, it will probably leave incipient cracks running away from the break, or may even break irregularly. A good break is nearly always one that was easily made. If a number of rings have to be cut, or a number of cuts made on gla.s.s tubes of about the same size, it will be found economical in the end to mount a glazier's diamond for the purpose. A simple but suitable apparatus is figured (Fig. 23).

Fig. 23.

The only difficulty is to regulate the position of the diamond so that it cuts. In order to do this, carefully note its cutting angle by preliminary trials on sheet gla.s.s, and then adjust the diamond by clamps, or by wriggling it in a fork, as shown. Weight the board very slightly, so as to give the small necessary pressure, and produce the cut by rotating the tube by hand. When a cut is nearly completed take great care that the two ends join, or irregularity will result. This is not always easy to do unless the tube happens to be straight.

Having got a cut, start a crack by means of a fine light watchmaker's hammer, or even a bit of fused gla.s.s, and entice the crack round the cut by tapping with the hammer or by means of the flame pencil.

If the cut is a true "cut" the tube will break at once. As a supply of electrical current for lighting will, in the near future, be as much a matter of course for laboratory purposes as a gas supply, I add the following note. To heat a tube round a scratch, nothing--not even the oxygas blow-pipe--is so good as a bit of platinum or iron wire electrically heated. If the crack does not start by considerable heating of the gla.s.s, stop the current, unwind the wire, and touch the gla.s.s on the crack either with a bit of cold copper wire or a wet match stem. I prefer the copper wire, for in my experience the water will occasionally produce an explosion of cracks. On the other hand, the cold wire frequently fails to start a crack.

Judging from the appearance of thick tubes as supplied by the dealers, the factory method of cutting off appears to be to grind a nick almost through the tube, and right round; and for really thick gla.s.s this is the safest but slowest way; a thin emery wheel kept wet will do this perfectly. Suitable wheels may be purchased from the "Norton" Emery Wheel Co. of Bedford, Ma.s.s, U.S.A.--in England through Messrs.

Churchill and Co. of London, importers.

-- 31. To blow a Bulb at the End of a Tube.

I must admit at once that this is a difficult operation--at all events, if a large bulb is required. However, all there is to be said can be said in few words. In general, when a bulb is required at the end of a tube it will be necessary to thicken up the gla.s.s. A professional gla.s.s-worker will generally accomplish this by "jumping up" the tube, i.e. by heating it where the bulb is required, and compressing it little by little until a sufficient amount of gla.s.s is collected. The amateur will probably find that he gets a very irregular ma.s.s in this way, and will be tempted to begin by welding on a short bit of wide and thick tubing preparatory to blowing out the bulb.

However, supposing that enough gla.s.s is a.s.sembled by-either of these methods, and that it is quite uniform in thickness, let the thickened part be heated along a circle till it becomes moderately soft, and let it then be expanded about one-fifth, say by gently blowing. It is perhaps more important to keep turning the gla.s.s during bulb-blowing than in any other operation, and this both when the gla.s.s is in the flame and while the bulb is being blown. It is also very important to avoid draughts. In general, a bulb is best blown with the tube in a nearly horizontal position, but sloping slightly upwards from the mouth. If it be noticed that a bulb tends to blow out more at one side than another, let the side of greatest protuberance be turned down, so that it is at the lowest point, reduce the pressure for an instant, and then blow again. It will be observed that the bulb will now expand at the top.

The reason of this is chiefly that the under side cools most rapidly (according to Faraday, Chemical Manipulation, -- 1194), and consequently can expand no further; but also it is not unlikely that the gla.s.s tends to flow somewhat from the upper side, which remains hot, and consequently the bulb, when the next puff reaches it, will tend to yield at this point. By heating several zones the tube will become gradually expanded.

Fig. 24.

Fig. 25.

Fig. 26.

When the length of the thickened part of the tube only slightly exceeds its diameter (Fig. 25), let the whole be brought to a temperature which, with flint gla.s.s, should be just short of that of perfect fluidity; and then, holding the tube horizontally and constantly turning it, let the bulb be blown out to its full size, noting the appearances and correcting too great protuberance on any side by the means above mentioned. If the bulb appears pear-shaped turn the tube so that the melted ma.s.s is directed upwards; if the bulb have the contrary fault, correct in the corresponding manner.

The bulb when finished may be lightly tapped on the table, when, if there is any weak place owing to inequality of thickness, the bulb will break, and the operation may be started afresh. "A good bulb is round, set truly on the tube, and free from lumps of thick gla.s.s or places of excessive thinness." When the amateur has succeeded in blowing a bulb two inches in diameter on the end of a strong bit of thermometer tube--say for an air thermometer--he may well seek the congratulations of his friends.

In case the bulb is not satisfactory on a first attempt, it may be melted down again, if the following precautions are taken. Directly creases begin to appear in the bulb let it be withdrawn from the flame, and gently blown till the creases come out. By alternate heating and blowing the gla.s.s can be got back to its original form, or nearly so, but unless the operator shows great skill and judgment, the probability is that the gla.s.s will be uneven. By heating and keeping the thicker parts in the higher position, and blowing a little now and again, the gla.s.s may be got even, and a new attempt may be made. It must not be supposed that this process can be carried on indefinitely, for the gla.s.s tends to lose its viscous properties after a time, or, at all events, it "perishes" in some way, especially if it has been allowed to get very thin; consequently too frequent attempts on the same gla.s.s are unprofitable. Two or three trials are as many as it generally pays to make. As a rule the largest possible flame may be used with advantage in this operation.

-- 32. To blow a bulb in the middle of a tube, the procedure is much like that already treated, but the manipulation is, if anything, more difficult, for the further end of the tube must be carried and turned as well as the end which is held to the lips.

-- 33. To make a side Weld.

This is by no means difficult, but is easier with lead gla.s.s than with soda gla.s.s. The tube to which it is desired to make a side connection having been selected, it is closed at one end by rubber tube stops, or in any other suitable manner. The zone of the proposed connection is noted, and the tube is brought to near softness round that circle (if the tube is made actually soft, inconvenience will arise from the bending, which is sure to occur). Two courses are then open to the operator, one suitable to a thick tube, the other to a tube of moderate thickness.

Taking the former first. Provide a piece of gla.s.s rod and warm its end. Direct a small flame against the spot on the thick tube where the proposed joint is to be. When the gla.s.s becomes almost incandescent at this spot, put the end of the rod against it and draw out a thread of gla.s.s till sufficient "metal" has been removed. Then fuse off the thread close to the tube.

Fig. 27.

The subsequent procedure is the same as for thin tubes. In this case heat the spot by the smallest flame available, and get the spot very hot. Blow it out gently into a bubble, perhaps extending to a height equal to its diameter. Then heat the top of the bubble till it is incandescent and blow violently. This will produce an opening fringed by gla.s.s so thin as to exhibit interference colours. Remove the filmy part, and heat the frayed edges till they cohere and form an incipient tube. If the flame has been of a correct size, the tube will now be of the same diameter as the tube to be welded on, and will project perhaps one-sixteenth of an inch from the surface of the main tube (Fig. 28).

Fig. 28.

Fig. 29.

When this stage is reached, again heat the tube all round till it nearly softens, and by means of the other hand heat the end of the other tube which it is proposed to weld. Just before the main tube actually softens, turn it so as to heat the edges of the aperture, and at the same time get the end of the side tube very hot. Take both out of the flame for an instant, and press the parts together, instantly slightly withdrawing the side tube. If the operation is well performed, it will be found that the point of maximum thickness of gla.s.s is now clear of the main tube. The joint is then to be heated all round and blown out--a rather awkward operation, and one requiring some practice, but it can be done.

Fig. 30.

If great strength is wanted, heat the main tube all round the joint bit by bit, and blow each section slightly outwards. If the operator is confident in his skill, he should then heat the whole joint to the softening point, blow it out slightly, and then adjust by pulling and pus.h.i.+ng. Cool first in the gas flame, and then plunge the joint into the asbestos and cover it up--or if too large, throw the asbestos cloth round it.

In the case of soda gla.s.s this final "general heat" is almost essential, but it is not so with flint gla.s.s, and as the general heat is the most difficult part of the job, it will be found easier to use lead gla.s.s and omit the general heating. With soda gla.s.s a very small irregularity will cause the joint to break when cold, but flint gla.s.s is much more long-suffering. It is easy to perform the above operation on small tubes. For large ones it will be found best to employ flint gla.s.s and use the clip stands as in the case of direct welds, treated above, but, of course, with suitable modifications.

Never let the main tube cool after the hole is made until the work is done.

-- 34. Inserted Joints.

In many instances the performance of apparatus is much improved by joints of this kind, even when their use is not absolutely essential.

There are two ways in which inserted joints may be made. The first method is the easier, and works well with flint gla.s.s; but when one comes to apply it to soda gla.s.s there is a danger of the gla.s.s becoming too thick near the joint, and this often leads to a cracking of the joint as the gla.s.s cools.

Fig. 31.

Suppose it is desired to insert the tube B into the tube A (Fig. 31).

Begin by reducing the size of the end of tube A till B will just slip in quite easily. With B about one-quarter inch in diameter, a clearance of about one-twentieth of an inch, or less, in all (i.e.