The pipette is supported by two horizontal arms H K ([fig. 87.]) moveable about a common axis A A, and capable of being drawn out or shortened by the aid of two longitudinal slits. They are fixed steadily by two screw nuts e e′, and their distance may be varied by means of round bits of wood or cork interposed, or even by opposite screw nuts o o′. The upper arm H is pierced with a hole, in which is fixed, by the pressure of a wooden screw v, the socket of the pipette. The corresponding hole of the lower arm is larger; and the beak of the pipette is supported in it by a cork stopper L. The apparatus is fixed by its tail-piece P, by means of a screw to the corner of a wall, or any other prop.
The manner of filling the pipette is very simple. We begin by applying the fore-finger of the left hand to the lower aperture c; we then open the two stop-cocks R and R′. Whenever the liquor approaches the neck of the pipette, we must temper its influx, and when it has arrived at some millimetres above the mark a b, we close the two stop-cocks, and remove our forefinger. We have now nothing more to do than to regulate the pipette; for which purpose the liquid must touch the line a b, and must simply adhere externally to the beak of the pipette.
This last circumstance is easily adjusted. After taking away the finger which closed the aperture c of the pipette, we apply to this orifice a moist sponge m, [fig. 88.], wrapped up in a linen rag, to absorb the superfluous liquor as it drops out. This sponge is called the handkerchief (mouchoir), by M. Gay Lussac. The pipette is said to be wiped when there is no liquor adhering to its point exteriorly.
For the convenience of operating, the handkerchief is fixed by friction in a tube of tin plate, terminated by a cup, open at bottom to let the droppings flow off into the cistern C, to which the tube is soldered. It may be easily removed for the purpose of washing it; and, if necessary, a little wedge of wood, o, can raise it towards the pipette.
To complete the adjustment of the pipette, the liquid must be made merely to descend to the mark a, b. With this view, and whilst the handkerchief is applied to the beak of the pipette, the air must be allowed to enter very slowly by unscrewing the plug V, [fig. 85.]; and at the moment of the contact the handkerchief must be removed, and the bottle F, destined to receive the solution, must be placed below the orifice of the pipette, [fig. 88.] As the motion must be made rapidly, and without hesitation, the bottle is placed in a cylinder of tin-plate, of a diameter somewhat greater, and forming one body with the cistern and the handkerchief. The whole of this apparatus has for a basis a plate of tinned iron, moveable between two wooden rulers R R, one of which bears a groove, under which the edge of the plate slips. Its traverses are fixed by two abutments b b, placed so that when it is stopped by one of them, the beak of the pipette corresponds to the centre of the neck of the bottle, or is a tangent to the handkerchief. This arrangement, very convenient for wiping the pipette and emptying it, gives the apparatus sufficient solidity, and allows of its being taken away, and replaced without deranging any thing. It is obvious that it is of advantage, when once the entry of the air into the pipette has been regulated by the screw V, to leave it constantly open, because the motion from the handkerchief to the bottle is performed with sufficient rapidity to prevent a drop of the solution from collecting and falling down.
Temperature of the Solution.—After having described the manner of measuring by volume the normal solution of the sea salt, we shall indicate the most convenient means of taking the temperature. The thermometer is placed in a tube of glass T, [fig. 89.], which the solution traverses to arrive at the pipette. It is suspended in it by a piece of cork, grooved on the four sides to afford passage to the liquid. The scale is engraved upon the tube itself, and is repeated at the opposite side, to fix the eye by the coincidence of this double division at the level of the thermometric column. The tube is joined below to another narrower one, through which it is attached by means of a cork stopper B, in the socket of the stop-cock of the pipette. At its upper part it is cemented into a brass socket, screw-tapped in the inside, which is connected in its turn by a cock, with the extremity, also tapped, of the tube above T, belonging to the reservoir of the normal solution. The corks employed here as connecting links between the parts of the apparatus, give them a certain flexibility, and allow of their being dismounted and remounted in a very short time; but it is indispensable to make them be traversed by a hollow tube of glass or metal, which will hinder them from being crushed by the pressure they are exposed to. If the precaution be taken to grease them with a little suet and to fill their pores, they will suffer no leakage.