The Methods of Glass Blowing and of Working Silica in the Oxy-Gas Flame Part 4

Ozone generators of the form shown on next page (Fig. 19), afford an interesting example of the insertion of smaller tubes into larger.

On account of the small s.p.a.ce that may be left between the inner and outer tubes of an ozone generator, and of the length of the inner tube, its construction needs great care. I find the following mode of procedure gives good results. Select the pieces of tube for this instrument as free from curvature as possible. For the inner tube, a tube 12 mm., or rather more, in external diameter, and of rather thin gla.s.s, is drawn out, as for closing, until only a very narrow tube remains at _C_, the end of _C_ is closed the area round _C_ is carefully blown into shape, so that by melting off _C_ the tube _A_ will be left with a well-rounded end. A small bulb of gla.s.s is next blown on _A_ at _B_. This bulb must be of slightly greater diameter than the contracted end _E_ of the larger tube (II.), so that _B_ will just fail to pa.s.s through _E_. The length from _B_ to _C_ must not be made greater than from _E_ to _G_ on the outside tube. The end at _C_ is then to be cut off so as to leave a pin-hole in the end of _A_.

[Ill.u.s.tration: FIG. 19.]

The outer tube (II.), whose diameter may be 5 or 6 mm. greater than that of _A_, is prepared by sealing a side tube on it at _F_, after previously contracting the end _E_. For this purpose the end _E_ should be closed and rounded, and then re-heated and blown out till the bulb bursts. To ensure that the diameter of the opening is less than that of the tube, care must be taken not to re-heat too large an area of the end before blowing it out. It is very important that the cross section at _E_ shall be in a plane at right angles to the axis of the tube.

Wrap a strip of writing paper, one inch in breadth, closely round the end of _A_ at _C_ till the tube and paper will only just pa.s.s easily into the mouth _D_ of the outer tube, push the inner tube _A_, with the paper upon it, into _D_, and when the paper is entirely within _D_, withdraw _A_, and cautiously push the paper a little further into the outer tube. Insert _A_ into _DE_ through _E_, so that the bulb _B_ is embraced by _E_. Close _D_ with a cork. Ascertain that the paper does not fit sufficiently tightly between the two tubes to prevent the free pa.s.sage of air, by blowing into the mouth _K_ of _A_. Air should escape freely from _E_ when this is done. Gradually bring the line of contact of _B_ and _E_ and the surrounding parts of the tube before a pointed flame, after previously warming them by holding near a larger flame, and rotate them before the flame so that the gla.s.s may soften and adhere.

Then heat the joint spot by spot as usual. In blowing this joint, take care that the gla.s.s on each side of the actual joint is slightly expanded. It should present the form shown by the dotted lines in III.

(these are purposely exaggerated, however). Finally, heat the whole joint between the lines _JI_ till it softens, and simultaneously blow and draw it into its final shape as seen at III.

The side tube _F_ should not be too near the end _E_. If, however, it is necessary to have them close together, the joint _F_ must be very carefully annealed when it is made; it must also be very cautiously warmed up before the construction of the joint at _H_ is begun, and must be kept warm by letting the flame play over it from time to time during the process of making the latter joint.

A good joint may be recognised by its freedom from lumps of gla.s.s, its regularity of curve, and by a sensibly circular line at _H_, where the two tubes are united.

When the joint after annealing has become quite cold, the pin-hole at _C_ on the inner tube may be closed, after removing the paper support, by warming the outer tube, and then directing a fine pointed flame through _D_ on to _C_. And the end _D_ of the outer tube may be closed in the ordinary manner, or a narrow tube may be sealed to it. As the end of gla.s.s at _D_ will be too short to be held by the fingers when hot, another piece of tube of similar diameter must be attached to it to serve as a handle (see p. 35, Fig. 9).

=Blowing a Bulb or Globe of Gla.s.s.=--For this purpose it is very important that the gla.s.s tube employed shall be of uniform substance.

The size and thickness of the tube to be employed depends partly on the dimensions of the bulb desired, and partly on the size of neck that is required for the bulb. It is easier to blow large bulbs on large-sized tubes than on those of smaller size. When it is necessary to make a large globe on a small tube, it can be done, however, if great care be taken to avoid overheating that part of the small tube which is nearest to the ma.s.s of viscous gla.s.s from which the bulb is to be formed. For the purpose of blowing a very large bulb on a small tube, it is best to unite a wide tube to that which is to serve as the neck, as it will save some time in collecting the necessary ma.s.s of gla.s.s from which to form the globe.

[Ill.u.s.tration: FIG. 20.]

_To blow a Bulb at the End of a Tube._--Select a good piece of tube, say 15 cm. in diameter, and about 30 cm. long; draw out one end to a light tail (_a_, Fig. 20) about 3 inches in length. Then heat up a _short_ length of the tube at _b_, with a small brush flame, by rotating the gla.s.s in the flame, and gently press it together when soft to thicken it; blow into it if necessary to preserve the regularity of its figure.

Repeat this process on the portion of tube nearest to that which has been first thickened, and so on, till as much gla.s.s has been heated and thickened as you judge will serve to make a bulb of the size desired.

You should have a ma.s.s of gla.s.s somewhat resembling that shown at _B_ (Fig. 20), but probably consisting of the results of more successive operations than are suggested in that diagram. Apply the flame as before to the narrower parts _cc_ of _B_, gently compress and blow until all the small bulbs first made are brought together into a ma.s.s still somewhat resembling the enlarged end of _B_, but more nearly cylindrical, with the gla.s.s as regularly distributed as possible, and of such length from _d_ to the contracted part that the whole of it may easily be heated simultaneously with the large brush flame of your blow-pipe. Take great care in the foregoing operations not to allow the sides of the ma.s.s of gla.s.s to fall in and run together, and, on the other hand, do not reduce the thickness of the gla.s.s needlessly by blowing it more than is necessary to give the gla.s.s as regular a form as possible. When you are satisfied with the ma.s.s of gla.s.s you have collected, melt off the tail _a_, and remove the pointed end of gla.s.s that remains, as directed on page 33. Turn on as large a brush flame as is necessary to envelop the whole ma.s.s of gla.s.s that you have collected, and heat it with constant rotation, so that it may gradually run together to the form seen at _C_ (Fig. 20), taking care that it does not get overheated near _d_, or the tube which is to form the neck will soften and give way.

The position in which the ma.s.s of heated gla.s.s is to be held will depend upon circ.u.mstances; if the ma.s.s of gla.s.s be not too great, it is best to keep it in a nearly horizontal position. If the ma.s.s of gla.s.s be very large, it may be necessary to incline the end _B_ downwards; but as that is apt to result in an excess of gla.s.s acc.u.mulating towards _d_, avoid doing so if possible by rotating the gla.s.s steadily and rapidly. If at any time the gla.s.s shows indications of collapsing, it must be removed from the flame and gently blown into shape, during which operation it may be rotated in the perpendicular position; indeed, to promote a regular distribution of the gla.s.s by allowing it plenty of time to collect, it is well from time to time to remove the heated ma.s.s of gla.s.s from the flame, and slightly expand it by blowing. Finally, when a regular ma.s.s of gla.s.s, such as is shown at _C_ (Fig. 20) has been obtained, remove it from the flame, and blow it to its final dimensions.

A succession of gentle puffs _quickly_ succeeding each other should be employed, in order that the progress of the bulb may be more easily watched and arrested at the right moment. During the process of blowing, the hot gla.s.s must be steadily rotated.

To collect the gla.s.s for blowing a bulb of lead gla.s.s, employ the flame described on pp. 17-22 for heating lead gla.s.s.

If the tube be held horizontally whilst the globe is blown, its form will most nearly approach that of a true globe. If it be held in the perpendicular position, with the ma.s.s of gla.s.s depending from it, the form of the bulb will usually be somewhat elongated. If it be held perpendicularly, with the ma.s.s of gla.s.s upwards, the resulting bulb will be flattened.

When a bulb is not of a sufficiently regular form, it may sometimes be re-made by re-collecting the gla.s.s, and re-blowing it. The greatest care is needed at the earlier stages of re-heating to prevent the gla.s.s from collapsing into a formless and unworkable ma.s.s. This is to be prevented in all such cases by gently blowing it into shape from time to time whilst gathering the gla.s.s.

[Ill.u.s.tration: FIG. 21.]

_To blow a Bulb between two Points_ (Fig 21).--Select a piece of suitable tube, seal or cork one end, gather together a ma.s.s of gla.s.s at the desired part, as directed for blowing a bulb at the end of a tube; when a ma.s.s of gla.s.s has been collected of sufficient thickness, blow it into shape from the open end of the tube by a rapid succession of short blasts of air, till the expanding gla.s.s attains the desired dimensions.

The tube must be held horizontally, and must be rotated steadily during the process. By slightly pressing together the gla.s.s while blowing, the bulb will be flattened; by slightly drawing apart the two ends of the tube, it will be elongated.

A pear-shaped bulb may be obtained by gently re-heating an elongated bulb, say from _a_ to _a_, and drawing it out. It is easiest to perform this operation on a bulb which is rather thick in the gla.s.s.

If the tubes _bb_ are to be small, and a globe of considerable size is wanted, contract a tube as shown in Fig. 22, taking care that the narrow portions of the tube are about the same axis as the wider portions, for if this be not the case, the mouths of the bulb will not be symmetrically placed; seal at _C_, cut off the wider tube at _B_, and make the bulb, as previously described, from the gla.s.s between _AA_.

If, as probably will be the case, the contracted portions of the tube be not very regular, they may be cut off, one at a time, near the bulb, and replaced by pieces of tube of the size desired.

[Ill.u.s.tration: FIG. 22.]

When a bulb has to be blown upon a very fine tube, for example upon thermometer tubing, the mouth should not be employed, for the moisture introduced by the breath is extremely difficult to remove afterwards. A small india-rubber bottle or reservoir, such as those which are used in spray-producers, Galton's whistles, etc., securely attached to the open end of the tube, should be used. With the help of these bottles bulbs can be blown at the closed ends of fine tubes with ease, though some care is necessary to produce them of good shape, as it is difficult to rotate the hot gla.s.s properly when working in this way.

=Making and Grinding Stoppers.=--Apparatus which is to contain chemicals that are likely to be affected by the free admission of air, needs to have stoppers fitted to it. Making a good stopper is a much less tedious process than is commonly supposed.

Suppose that the tube I. of Fig. 23 is to be stoppered at _A_, it must be slightly enlarged by softening the end and opening it with a pointed cone of charcoal; or a conical mouth for the stopper may be made by slightly contracting the tube near one end, as at _B_, cutting off the cylindrical end of the tube at the dotted line _C_, and then very slightly expanding the end at _C_ with a charcoal cone after its edges have been softened by heat. In either case the conical mouth should be as long and regular as possible.

[Ill.u.s.tration: FIG. 23.]

For the stopper take a piece of rather thick tube, of such size that it will pa.s.s easily, but not too easily, into _A_ or _B_. Expand this tube at _D_, as shown in II., by softening the gla.s.s and gently compressing it. The configuration of the enlarged tube as shown at _D_ may be obtained by heating and compressing two or more zones of the tube that are adjacent, one zone being less expanded than the other, so as to give the sides of the imperfect stopper as nearly as possible the form shown at _D_, which, however, is much less regular than may easily be obtained. Seal off the head of the tube at _H_, and heat the gla.s.s till it runs together into a nearly solid ma.s.s; compress this with a pair of iron tongs to the flattened head _E_. In making _D_, aim at giving it a form which will as nearly as possible correspond to that of the tube into which it is to be ground, and make it slightly too large, so that only the lower part at _D_ can be introduced into the mouth of _A_ or _B_. Before it is ground, the stopper must be heated nearly to its softening-point and annealed.

Moisten _D_ with a solution of camphor in recently distilled turpentine, and dust the wet surface with finely-ground emery, then gently grind it into its place till it fits properly. In this operation the tail _G_, which should fit loosely into the tube _A_, will be of a.s.sistance by preventing _D_ from unduly pressing in any direction on _A_ in consequence of irregular movements. The stopper should be completely rotated in grinding it. It must not be worked backwards and forwards, or a well-fitting stopper will not be produced. Renew the emery and camphorated turpentine frequently during the earlier part of the grinding; when the stopper almost fits, avoid using fresh emery, but continue to remove the stopper frequently at all stages of the operation. That added at the earlier stages will be reduced to a state of very fine division, and will therefore leave the stopper and mouth of _A_ with smoother surfaces than fresh emery.[10]

[10] Mr. Gimmingham recommends giving stoppers a final polish with rotten-stone (_Proceedings of the Royal Society_, p. 396, 1876).

NOTE.--The addition of camphor to the turpentine used for grinding gla.s.s is very important. Notwithstanding its brittle nature, gla.s.s will work under a file moistened with this solution almost as well as the metals.

Small quant.i.ties should be made at a time, and the solution should be kept in a well-closed vessel, for after long exposure to the air it is not equally valuable.

If the stopper is to fit a tube contracted like _B_, it must be constructed from a piece of tube that will pa.s.s through the contraction at _B_. The tail _GF_ will not do such good service as it does in the case of a tube which has been opened out to receive its stopper, but it will help to guide the stopper, and should be retained.

When the stopper has been ground into its place, melt off the tail at _F_. The flame must be applied very cautiously, as gla.s.s which has been ground is particularly apt to crack on heating. To avoid all risk of this, the tail may simply be cut off, and its edges filed smooth with a file moistened freely with camphorated turpentine.

The stoppers of bottles are not made exactly in the manner described above, though, on occasion, a new stopper may be made for a bottle by following those directions. Ill-fitting stoppers, which are very common, can be very easily re-ground with emery and camphorated turpentine.

CHAPTER IV.

_MAKING THISTLE FUNNELS, U-TUBES, ETC.--COMBINING THE PARTS OF COMPLICATED APPARATUS--MERCURY, AND OTHER AIR-TIGHT JOINTS--VACUUM TAPS--SAFETY TAPS--AIR-TRAPS._

In Chapter III. the simpler operations used in making the separate parts of which apparatus is composed have been described. In this Chapter finished apparatus will be described, and the combination of the separate parts into the more or less complicated arrangements used in experiments will be so far explained as to enable the student to set up such apparatus as he is likely to require. I have thought it would be useful that I should add a short account of various contrivances that have come much into use of late years for experimenting under reduced pressure, such as safety taps, air-traps, vacuum joints, etc.

[Ill.u.s.tration: FIG. 24.]

=Electrodes.=--On page 38 (Fig. 13) is shown a simple form of electrode sealed into a gla.s.s tube, which for many purposes answers very well. But frequently, in order that there may be less risk of leakage between the gla.s.s and the metal, the latter is covered for a considerable part of its length with solid gla.s.s, which at one extremity is united to the apparatus. In Fig. 24 _W_ is the metal core of the electrode, and _G_ the gla.s.s covering around it. The wire is fused into the gla.s.s, and the gla.s.s is then united to the apparatus; a little white enamel should be applied at one end and combined with the gla.s.s by fusion.

=U-Tubes.=--A U-tube is but a particular case of a bent gla.s.s tube. It is scarcely possible when bending very large tubes in the manner described on p. 29 to produce regular curves of sufficient strength.

To make a U-tube, or to bend a large tube, close one end of the tube selected with a cork, soften and compress the gla.s.s in the flame at the part where it is to be bent till a sufficient ma.s.s of gla.s.s for the bend is collected, then remove the ma.s.s of gla.s.s from the flame, let it cool a little, and simultaneously draw out the thickened gla.s.s, bend it to the proper form, and blow the bend into shape from the open end of the tube. Small irregularities may be partly corrected afterwards.

To make a good U-tube of large size, and of uniform diameter from end to end, requires much practice, but to make a tolerably presentable piece of apparatus in which the two limbs are bent round till they are parallel, without any considerable constriction at the bend, can be accomplished without much difficulty.[11]

[11] Large tubes may also be bent by rotating a sufficient length of the tube in a large flame till it softens, and bending in the same manner as in the case of smaller tubes, and after filling them with sand, closing one end completely, and the other so that the sand cannot escape, though heated air can do so.

=Spiral Tubes.=--These may be made by twisting a tube gradually softened by heat round a metal cylinder. Spiral tubes made of small thin tubes possess considerable elasticity, and have been used by Mr. Crookes for making air-tight connections between separate pieces of apparatus when a rigid connection would have been unnecessary and inconvenient. By the use of such spiral tubes it is possible to combine comparatively free movement with all the advantages attached to hermetically-sealed joints.