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]
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.
To make a flexible spiral tube, mount a copper cylinder on a screw, so that the cylinder will travel in the direction of its axis when it is rotated. Fix a fine glass tube to the cylinder, and direct a flame towards the cylinder so as to heat and soften the glass, which will then bend to the form of the cylinder. Gradually rotate the cylinder before the source of heat, so that fresh portions of tube are successively brought into position, softened, and bent. Useful spirals may also be made by hand without a cylinder. As each length of tube is bent, a fresh length may be united to it until the spiral is completed. The fine tubes employed are prepared by heating and drawing out larger tubes.
Fig. 25.
Thistle Funnels ([Fig. 25]).—Seal a moderately thick piece of small glass tube at A, then heat a wide zone of it a little below A by rotating it horizontally in the blow-pipe flame till the glass softens, and expand the glass to a bulb, as shown at B of 1; during the operation of blowing this bulb, the end A must be directed to the ground.
Soften the end A and a small portion of B as before, and, holding the tube horizontally from the mouth, blow out the end C as at 2. Heat the end of C gradually, till the glass softens and collapses to the dotted line dd, and at once blow a steady stream of air into the open end of the tube, rotating it steadily, till it is about to burst; finally clean off the thin glass from round the edges of the funnel, which should have the form shown at 3, and round them. An inspection of a purchased thistle funnel will generally show that the head B has been formed from a larger tube sealed to E at f.
Closing Tubes containing Chemicals for experiments at high temperatures.—Tubes of the hard glass used for organic analyses answer best for this purpose; the operation of drawing out the end of such a tube is practically identical with what has been described under the head of [choking], [p. 35]. A well-sealed tube presents the appearance of that shown by [Fig. 26].
