If now g is cut off at x a few inches below the junction of e with the arm / (the part h being no longer used), and a glass cock be inserted at xt then by means of a short rubber tube this cock can be connected with the open end of the barometer tube to be filled, which latter will take the general position of the whole tube g.
The rubber tube must be covered with melted sealing wax. The impure mercury in a should first be washed in acids and dried before introduction. At the beginning of operations, a is full of impure mercury, but the rest of the apparatus contains only air. The Sprengel pump is set in motion and gradually exhausts the air from b, c, d, e and the barometer tube, until no air bubbles can be seen in the running mercury of the Sprengel pump, and until the sharp click is heard when the drops of mercury fall. The tube / is then sealed or a stop cock in it is turned, cutting off the Sprengel pump; the Bunsen burner under c is lighted, and the mercury will distil over into the barometer tube, which will thus be filled without allowing the mercury to come into direct contact with the air.
The barometer tube should be constantly watched in order to detect any air bubbles that may be carried over; when seen they must be cooked out by heating the tube slightly by means of a Bunsen burner. When the barometer tube has become filled with the mercury, the cock at x can be closed, the sealing wax is broken, and the tube is replaced by another. (F. Waldo).
(5) Cleaning barometer tubes. To clean the tube of a film, etc, get a piece of iron wire and fix on the end of it a piece of wash-leather. It must be very fine wash-leather, cut into narrow strips; wrap the iron wire from end to end, leaving a thicker piece at the end to tightly fit the tube.
Clean with warm water, soda, and soap-powder, afterwards with cold water, using the covered wire all the time, of course replacing the wet with dry leather to finish. If the wire is not covered, the tube will most assuredly break, if not at the time, certainly within 48 hours after using the wire. Clean the mercury with nitric acid and water, say, for 4-51b. of mercury, 4 tea-spoonfuls of acid, and 20 teaspoonfuls of water; put the whole into a soup-plate, and put it in the oven or before the fire, and heat up to about 140°-150° F., stirring it at intervals until the acid forms a sort of a powder or refuse on the top of the mercury. When cold, run the mercury through a fine paper cone a few times, and then it is fit for
Filling barometer tubes.
(6) Use double cotton-covered copper wire, such as is used for electrical purposes. With such, I hare never broken a tube. The idea that suggested such to me was a most ingenious form of diamond for catting gauge-tubes. It was a short metal rod covered with cotton with the diamond arranged at one end. An adjustable stop at the end next the hand gave the length required. By inserting, it in the tube and gently turning it with the diamond part against the inside of the tube, the inner skin was cut through and the tube easily parted. It was found in practice au advantage to subsequently anneal the cut end. This gave me the idea of using a metal rod with safety to clean a tube with; and, as already stated, I hare never had a smash. But I need hardly say 1 never attempted to force in too tight a wad of washleather. On the contrary, it fitted very loosely.
(7) Siphon Urometer. A few words must fust be said regarding the selection of the glass tube, as on its fitness for the purpose the instrument's future excellence will very much depend. Ordinary white, easily fusible glass tube should not be used, as the mercury is apt to attract its oxide of lead, and not only become impure, but by adhesion to the inside of the bore hinder the free oscillation of the barometric column. The proper kind of tubing is that which shows a greenish tinge in the glass when looked at endways. For either of the instruments shown in Figs. 363 or 364, it should not be less than 3/8 in. outside diameter and 1/4-in. bore; and if slightly larger may still be used with advantage.
For the siphon barometer. Fig. 363, a piece of tube about 38 in. long is required. This is to be well cleaned by running through It plenty of warm soft water, while at the same time a little swab made from a piece of soft, fine tinea, tied in the middle of a cord, is S pulled through the bore from end to end. After the water has drained out, alcohol, in which precipi'ated chalk is suspended, should be applied to the inside by means of the swab. A clean swab, moistened with alcohol, will remove the particles of chalk, when, the cord being withdrawn, distilled water is to be poured through, after which the tube must stand in an upright position till it has drained perfectly dry, a little cap of paper, meantime, being placed on its upper end to exclude dust. The inner surface of the tube must finally be polished with a small piece of soft washleather fixed on the end of a clean, smooth brass wire.
Siphon and cistern barometers.
The tube thus cleaned and dried is now to be closed at one end by drawing it apart in a gas flame about 2 in. from the extremity. The narrow pointed end, which forms when the tube is drawn asunder, should be pressed and rotated in the flame till a substantial and well-rounded closing has been obtained. About 32 in. from the sealed extremity a y-shaped bend is to be made. Care must be taken to make the curve a gradual one, as failure in this respect would not only'mar the appearance of the instrument, but might also tend to narrow the bore and make the bend a weak point. The arc of the curve is to be 1 1/4 in. The longer limb of the siphon is thus 32 in. long, and the shorter one about 3 in. The short limb is not to be bent down quite parallel with the longer one, but should make a slight angle with it, to render the subsequent introduction of the mercury more easy. The tube a is to have adapted to it a supporting stand b, which may be a piece of dressed walnut, 34 in. long, 3 1/2 in. wide, and about 3/8 in. thick, rounded off at the top, and furnished with a brass screw and ring for hanging up. A shallow groove, curved to correspond with the bent tube, is made on the wood.