After having immersed the beak c of the pipette in the solution, we apply suction by the mouth, to the upper orifice, and thereby raise the liquid to d above the circular line a b. We next apply neatly the forefinger of one hand to this orifice, remove the pipette from the liquid, and seize it as represented in [fig. 84.] The mark a b being placed at the level of the eye, we make the surface of the solution become exactly a tangent to the plane a b. At the instant it becomes a tangent, we leave the beak c of the pipette open, by taking away the finger that had been applied to it, and without changing any thing else in the position of the hands, we empty it into the bottle which should receive the solution, taking care to remove it whenever the efflux has run out.
If after filling the pipette by suction, any one should find a difficulty in applying the forefinger fast enough to the upper orifice, without letting the liquid run down below the mark a b, he should remove the pipette from the solution with its top still closed with his tongue, then apply the middle finger of one of his hands to the lower orifice; after which he may withdraw his tongue, and apply the forefinger of the other hand to the orifice previously wiped. This mode of obtaining a measure of normal solution of sea salt is very simple, and requires no complex apparatus; but we shall indicate another manipulation still easier, and also more exact.
In this new process the pipette is filled from the top like a bottle, instead of being filled by suction, and it is moreover fixed. [Fig. 85.] represents the apparatus. D and D′ are two sockets separated by a stop cock R. The upper one, tapped interiorly, receives, by means of a cork stopper L, the tube T, which admits the solution of sea salt. The lower socket is cemented on to the pipette; it bears a small air-cock R′, and a screw plug V, which regulates a minute opening intended to let the air enter very slowly into the pipette. Below the stop-cock R′, a silver tube N, of narrow diameter, soldered to the socket, leads the solution into the pipette, by allowing the air, which it displaces, to escape by the stop-cock R′. The screw plug, with the milled head V′, replaces the ordinary screw by which the key of the stop-cock may be made to press, with more or less force, upon its conical seat.
[Fig. 86.] represents, in a side view, the apparatus just described. We here remark an air-cock R, and an opening m. At the extremity Q of the same figure, the conical pipe T enters, with friction. It is by this pipe that the air is sucked into the pipette, when it is to be filled from its beak.
