The tubes intended for barometers ought to be sealed hermetically on both ends, immediately after they are made at the glass-house, and to be kept in this state until they are fitted up. Without this precaution they are apt to be sullied with dust, moisture, and other impurities, which it is afterwards almost impossible to remove on account of the smallness of their diameters. When they are opened, which may be done with a file, care should be taken not to breathe into them, nor to wash them with spirit of wine, or other fluid, experience having proved that in tubes so treated, the mercury always stands a little below its proper level; this is owing to the adhesion of a little of the spirit of wine to the sides of the tube. When cleaning is necessary, it must be done with a fine linen rag that has been previously well dried.
The tubes ought to be as perfectly cylindrical aa possible, though, in some cases, this is not absolutely necessary. They should be about 33 inches in length, and the diameter of their bore should be at least 2 or 2 1/2 lines, otherwise the friction, and the capillary action will be apt to affect the free motion of the mercury. The glass should not be very thick, as it is apt in that case to break; when the mercury is boiled in the tube half a line is sufficient.
The mercury ought to be perfectly pure and free from all foreign metals. The best is that which has been recently revived from cinnabar; the common mercury of the shops being often adulterated intentionally with tin, lead, and bismuth, stands at various heights in the tube, according to the nature and quantity of the foreign substances with which it is amalgamated.
To Obtain the Mercury Pure:
For this purpose take a pound of cinnabar and reduce it to powder, mix it well with 5 or 6 oz. of iron or steel filings; and, having put the mixture into an iron retort, expose the whole to the heat of a reverberatory furnace; the mercury will soon pass over in a state of great purity, and may be obtained by adapting to the retort an earthen receiver, which has been previously half filled with water. Commercial mercury may be purified by distilling it over a portion of cinnabar. These are put into an iron bottle with an iron tube attached; to the end of the iron tube is one made of leather or India-rubber which dips beneath the surface of water constantly renewed.
Process of Filling the Tube:
Before being introduced into the tube, the mercury ought to be well heated, or even boiled in a glazed earthen pipkin, in order to drive off any moisture which may adhere to it, but this will be unnecessary if the mercury has been recently reduced.
The mercury ought likewise to be boiled in the tube to expel any air or moisture which may still remain attached to it, or to the inside of the tube. This is done in the following manner: Pour as much mercury into the tube as will make it stand to the length of 3 or 4 inches, and introduce a long wire of iron to stir it during the boiling. Expose the mercury in the tube gradually to the heat of a chafing-dish of burning charcoal, or a well regulated gas flame; and when it begins to boil, stir it gently with the iron wire to facilitate the disengagement of the bubbles of the air. When the first portion of the mercury has been sufficiently boiled, and all the air extricated, remove the tube from the chafing-dish and allow the whole to cool, taking care not to bring it into contact with any cold substance. Introduce an equal quantity of mercury, and treat it in the same manner, withdrawing the wire a little so that it may not reach below the upper part of the mercury already freed from air. The chafing-dish must also be placed immediately under the mercury which has been last poured in. Repeat the same process with each successive portion of mercury till the tube is filled, always applying the heat very cautiously; and be equally careful in allowing it to cool before a fresh portion of mercury is poured in.
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