For certain philosophical instruments it is necessary to employ flat tubes. These are formed of flint-glass, are very small, and have a canal or bore, which, instead of being round, as in common tubes, has the form of a long and very flat oval. This disposition has the advantage of rendering more perceptible the column of liquid that may be introduced, and which in a round canal would scarcely be visible. In choosing this sort of tubes, carefully avoid those of which the canal is twisted, and not found to be in the same plane, in the whole length of the tube.
The tubes should be sorted, according to their sizes and qualities, and should be deposited in large drawers or on long shelves, in such a manner as to be equally supported through their whole extent. They should also be sheltered from dust and from moisture. If you cannot conveniently warehouse them in this manner, you should tie them up in parcels, and support them in a perpendicular position. It is a very bad plan to place them in an inclined direction, or to support them by their extremities on wooden brackets, as it is the fashion to do in chemical laboratories; because, as the tubes are then supported only at certain points, they bend, in course of time, under the influence of their own weight, and contract a curvature which is extremely prejudicial in certain instruments, and which it is almost impossible to correct.
PREPARATION OF TUBES BEFORE HEATING THEM.
Before presenting a tube to the flame, you should clean it well both within and without, in order to remove all dust and humidity. If you neglect to take this precaution, you run the risk of cracking or staining the glass. When the diameter of the tube is too small to permit of your passing a plug of cloth or paper to clean its interior, you can accomplish the object by the introduction of water, which must, many times alternately, be sucked in and blown out, until the tube is deemed clean. One end of it must then be closed at the lamp, and it must be gradually exposed to a charcoal fire, where, by raising successively all parts of the tube to a sufficiently high temperature, you endeavour to volatilize and expel all the water it contains. In all cases you considerably facilitate the disengagement of moisture by renewing the air in the tube by means of a bottle of Indian-rubber fastened to the end of a long narrow tube, which you keep in the interior of the tube to be dried during the time that it is being heated. You can here advantageously substitute alcohol for water, as being much more volatile, and as dissolving greasy matters; but these methods of cleansing should only be employed for valuable objects, because it is extremely difficult fully to expel moisture from a tube wherein you have introduced water, and because alcohol is too expensive to be employed where there is no particular necessity.
When the tubes no longer contain dust, or moisture, you measure them, and mark the divisions according to the sort of work which you propose to execute.
METHOD OF PRESENTING TUBES TO THE FIRE, AND OF WORKING THEM THEREIN.
The two arms are supported on the front edge of the table, and the tube is held with the hands either above or below, according as it may be necessary to employ more or less force, more or less lightness. You ought, in general, to hold the tube horizontally, and in such a manner that its direction may be perpendicular to that of the flame. Yet, when you wish to heat at once a large portion of the tube, or to soften it so that it shall sink together in a particular manner, as in the operation of sealing, you will find it convenient to incline the tube, the direction of which, however, must always be such as to turn the heated part continually towards you.
We are about to give a general rule, upon the observance of which we cannot too strongly insist, as the success of almost every operation entirely depends upon it. The rule is, never to present a tube to the flame without CONTINUALLY TURNING it; and turning it, too, with such a degree of rapidity that every part of its circumference may be heated and softened to the same degree. As melted glass necessarily tends to descend, there is no method of preventing a heated tube from becoming deformed but that of continually turning it, so as to bring the softened part very frequently uppermost. When you heat a tube near the middle, the movement of the two hands must be uniform and simultaneous, or the tube will be twisted and spoiled.
When the tubes have thick sides, they must not be plunged into the flame until they have previously been strongly heated. You expose them at first to the current of hot air, at some inches from the extremity of the jet; you keep them there some time, taking care to turn them continually, and then you gradually bring them towards, and finally into, the flame. The thicker the sides of the tubes are, the greater precaution must be taken to elevate the temperature gradually: this is the only means of avoiding the fractures which occur when the glass is too rapidly heated. Though it is necessary to take so much care with large and thick tubes, there are, on the contrary, some tubes so small and so thin that the most sudden application of the fire is insufficient to break them. Practice soon teaches the rule which is to be followed with regard to tubes that come between these extremes.
Common glass ought to be fused at the maximum point of heat; but glass that contains oxides capable of being reduced at that temperature (such as flint-glass) require to be worked in that part of the flame which possesses the highest oxidating power. If you operate without taking this precaution, you run the risk of decomposing the glass. Thus, for example, in the case of flint-glass, you may reduce the oxide of lead, which is one of its constituents, to the state of metallic lead. The consequence of such a reduction is the production of a black and opaque stain upon the work, which can only be removed by exposing the glass, during a very long time, to the extremity of the jet.