GROUND JOINTS

It is sometimes required to join two pieces of tubing end to end, by means of a ground joint. Whenever possible, a regular sealed joint should be used instead of this ground joint, as it is quicker to make, and more certain to be tight. Where a ground joint is necessary, however, it is best made in the conical form shown in c, Fig. 17. If the wall of the tube to be used is not very thick, it is thickened by collecting glass as for a bulb on the ends of two tubes (Exercise No. 6), and drawing to form cones of suitable shape (a and b, Fig. 17) and of such relative sizes that a will slip about half way into b. In order to make a straight and give it the proper angle, it may be rolled when hot, upon a hot plate of carbon. Blowing during this rolling is often helpful to remove depressions. After b has been drawn to nearly the proper size and shape, it may be smoothed by the use of a small carbon rod, held inside it at a slight angle, or better by the use of a truncated hexagonal pyramid of carbon, whose edges have the proper slant to make the inside of the cone right. The proper taper for both these cones is the same as that used in stopcocks of similar size. The hexagonal carbon can easily be made by carefully filing down an electric light carbon, and finally impregnating it with paraffin or beeswax, and is extremely useful wherever a conical surface has to be formed from the inside of a tube.

The tail is allowed to remain on piece a, as a sort of guide in grinding, and should therefore be in the axis of the tube and have rather thick walls. Grind with emery or carborundum, as described under a previous head. (Regrinding plug for stopcock.) If many such joints are to be made, it will pay to have a little sleeve of brass made with the proper taper, and rough down the plug a in it to about the proper size, while b is roughed down by means of a brass or iron plug having the same taper. This prevents excessive grinding of one-half of the joint in order to remove a defect in the other half, and is the method commercially used in making stopcocks.

SEALING IN PLATINUM WIRE

Very often it is necessary to seal platinum wire into the wall of a tube. Professional glass-blowers usually use a special sort of glass ("Einschmelzglas") which is usually a lead glass, and is made of such composition that it has the same or practically the same coefficient of expansion as platinum. A little globule of this glass is sealed into the tube in such a way that it joins the platinum to the glass of the tube. To do this, the small globule of special glass is fused on the platinum wire at the proper point and the tube into which the wire is to be sealed is heated and a small tail drawn out at the point where the wire is to be inserted. The lump of the special glass should be from ³⁄32 to ¹⁄8 inch in diameter, and the tail drawn on the tube should have a slightly less diameter at the point (about ¹⁄8 inch or less from the tube) where it is cut off. There are now two ways of sealing in the wire. (1) The wire with the globule of glass is placed inside the tube and the latter revolved until the end of the wire sticks out of the cut tail (a, Fig. 18). The latter is now gently heated, and the two glass surfaces fused together, taking care to use only the end of the hissing flame, if the special glass contains lead. (See Chapter I, page 1.) The whole circumference of the tube is then heated and annealed carefully. (2) The end of the wire which is to be outside the tube is attached to the end of a thin scrap of glass, by heating the glass and thrusting the wire into it a very little way. Using this piece of glass as a handle, the wire is inserted in the cut tail (b, Fig. 18) and the globule brought near to the end of the tail. (If the main tube is cold, it must of course first be warmed.) With the end of the hissing flame, as in the first method, the globule of glass is melted and the end of the tail softened. The wire is now pushed into place, the handle removed by heating the end and withdrawing it, and the tail reheated a little if necessary to make it shrink back into line with the walls of the tube. The whole circumference of the tube is heated at that point and annealed as usual.

The use of this special glass is not absolutely necessary if the platinum wire is small (¹⁄4 millimeter or less in diameter), and in fact it is often better in such cases not to use it, unless the apparatus is to be subjected to a very high vacuum. On small tubes, especially, it is undesirable to use the special glass, as a lump of it will usually cause the tube to crack on cooling. When such glass is not at hand or is not to be used, the procedure is altered somewhat. The tail which is drawn out is very fine, having only a sufficient diameter so that when it is cut off the wire can be inserted in it. Such a fine tail is readily made by heating a small spot on the tube, touching it with a warm platinum wire, removing from the flame and drawing out the tail with the wire. After cutting off the tail the wire is inserted in it, being held on a scrap of glass as in the previous case, and the wire and tail heated until the latter shrinks back into line with the walls of the tube. If too great shrinkage occurs, the place may be blown out gently after reheating. Thus the wire is sealed through the wall of the tube without changing the thickness of the latter, and consequently without developing undue stresses at that point. Such a joint must of course be carefully reheated and annealed. With fine platinum wire there is very little risk of the tube cracking if care is taken to avoid formation of any lump and to reheat the whole circumference of the tube at that point.

Any glass adhering to the end of the platinum wire, where the scrap of glass was sealed on for a handle, may be removed when the glass has cooled by crushing it carefully with a pair of pliers.

SEALING VACUUM TUBES

Tubes which have been evacuated usually are sealed off while they are still connected to the vacuum pump. The connection should be through a small, rather thick-walled tube. When this is to be sealed, it is slowly heated toward the softening point. As the glass just begins to soften, the air-pressure will force it in, and care must be taken that the softening is uniform over the whole circumference of the tube. As the shrinking goes on, the tube is gently drawn out to make a thick-walled cone at that place, and the end is drawn off as soon as the tube is sealed. The principal point to be guarded is the thickness of the walls of the cone, and uniform heating. A thin place or a hot place will give way under the air-pressure and be sucked into the tube.

CLOSED TUBES FOR HEATING UNDER PRESSURE

(Carius method for determination of the halogens and sulphur.) In this case the tubing used must have thick walls (usually about ³⁄32 inch) to withstand the pressure. Its external diameter is usually about ³⁄4 inch. One length will usually make two tubes of standard length for the cannon furnace. Especial care must be taken in heating and cooling it on account of the thick walls. A length is gradually warmed in the center, finally heated at that point until soft, drawn out, cut apart and annealed. Taking one of the pieces, the cone is carefully heated and shrunk, as in Exercise 4, until its walls are as thick as those of the main tube. A flame with a little tinge of yellow should be used for this operation to prevent devitrification (page 2), as the thick glass shrinks slowly. The tail is now drawn off and the whole end heated and gently blown several times to make a rounded end, like a test-tube, with walls as thick as those of the main tube. This must be carefully annealed. It is more important that the walls be thick than that the end be nicely rounded: it may indeed be left somewhat conical in shape.

At a point about two inches from the open end of the tube, it is slowly warmed and finally heated to the softening point. Grasping the open end with a pair of crucible tongs, it is cautiously pulled out, a little at a time, usually during rotation in the flame, to make a constriction of moderate wall-thickness, but of sufficient internal diameter to admit the tube containing the substance. After annealing this, cooling and cleaning the tube, the acid and salt are introduced (the former by means of a long-stemmed funnel) and the tube is inclined and rotated about its axis so that the acid wets its surface about half way up from the bottom. The substance is now weighed out in a piece of thin-walled glass tubing, closed at one end, and about two inches long. Inclining the large tube at a suitable angle, the small one is introduced, closed end first, and allowed to slide down the walls of the large tube until it reaches the place where the acid has wet the tube. Here it will stop, and if the tube is kept inclined during the rest of the operation it will roll around inside the tube at this point and thus not get down where any acid is likely to get into it and produce any pressure by decomposing it before the open end of the tube is sealed. Now the tube is held in an inclined position, taking care that the acid does not reach up to the substance, the constricted portion cautiously warmed and shrunk. It is finally shrunk and drawn out into a somewhat elongated cone, with walls as thick as the rest of the tube, and when this is accomplished the end of the cone is sealed and the waste piece drawn off. Anneal with great care, and cool in such a position that the acid cannot reach the hot glass. The shrinking of this cone takes a good deal of patience, and is one of the most important parts of the process. If the walls are left too thin, the tube may burst when heated, and the whole labor is lost. If care is taken, the same tube can be used for a number of determinations, until it becomes quite short.

INDEX

Annealing glass, 4, 24,


Bellows, 4,

Bending glass, 8,

Blowing glass, 13, 19, 20, 21, 24, 29, 31,
  with a rubber tube, 22,

Blowpipe, 4,

Bulb at end of tube, 28,
  in middle of tube, 32,
  very large, 32,

Bulbs, string of, 33,


Capillary tube, drawing on larger tube, 9, 54,
  tubing, working, 43,

Carius method, tubes for, 55,

Closed circuits of tubing, 48,
  tubes, for heating under pressure, 55,

Collecting glass for bulb, 29, 31, 32,

Constricting a tube, 10,

Crystallization of glass, see Devitrification.

Cutting glass, 7, 25,


Devitrification, 1, 2,

Drawing out a tube, 9, 18, 19, 27,


Flanging a tube, 11, 14,
  tool, 11,


Gas-washing tube, 35,

Glass, annealing, 4, 24,

Glass, bending, 8,
  blowing, 13, 19, 20, 21, 24, 29, 31,
  collecting for bulb, 29, 31, 32,
  cutting, 7,
  defects, 2,
  grinding, 47,
  hard, 1,
  knife, 7,
  lead, 1,
  qualities desired, 1,
  rod and tube, joining, 45,
  rod, working, 44,
  shrinking, 18, 19, 22, 26,
  soft, 1,
  working temperature, 1, 13, 19, 27,

Grinding stopcock or joint, 47,

Ground joints, 51,


Handle on stopcock, mending, 45,

Hard glass, 1,

Holding tube, 13, 14,


Insertion of tube through another, see Sealing a tube through another tube.


Joints, ground, 51,

Joining rod and tube, 45,
  tubing end to end: first method, 16,
  second method, 20,

Joining tubes of different diameters, 25,
  a new tube to a stopcock, 48,


Kjeldahl trap, 41,


Lead glass, 1,

Lump of glass, removed, 18, 19, 20, 21, 24, 26, 30, 38,


Platinum wires, sealed into glass, 1, 52,

Position for glass-working, 5,

Pressure, tubes for heating under, 55,


Quality of glass, 1,


Rod, glass, working, 44,

Rotation of the tube, 13, 19,

Rounded end of tube, 35, 38,

Rubber tube used for blowing, 22,


Sealing a tube through another tube, 35, 39,

Sealing vacuum tubes, 55,

Shrinking glass, 18, 19, 22, 26, 31,

Side tube, blowing, 22, 25,

Soda glass, 1,

Soft glass, 1,

Spirals, making, 50,

Stopcocks, mending, 45,

Suction pump, 39, 42,

Sulphur dioxide tube, 28,


"Tail" of glass, drawing out, 9, 54,
  removed, 30, 35,

Tubes, closed, for heating under pressure, 55,

"Tee" tube, 22,
  on capillary tubing, 43,
  small side tube on a large tube, 24,


Vacuum tubes, sealing, 55,


Working temperature of glass, 1, 13, 19, 27,