74

ASSAY.

of which 100 grammes will precipitate exactly one gramme of silver. But instead otpure salt, which is to be procured with difficulty, and which besides may be alteredreadily by absorbing the humidity of the air, a concentrated solution of the sea salt ofcommerce is to be preferred, of which a large quantity may be prepared at a time, to bekept in reserve for use, as it is wanted. Instruction de Gay Lnssac. •

Preparation of the Normal Solution of Sea Salt, when measured, by Volume .—-Themeasure by weight has the advantage of being independent of temperature, of havingthe same degree of precision as the balance, and of standing in need of no correction.The measure by volume has not all these advantages; but, by giving it sufficientprecision, it is more rapid, and is quite sufficient for the numerous daily assays of themint. This normal solution is so made, that a volume equal to that of 100 grammesof water, or 100 centimetres cube, at a determinate temperature, may precipitate exactlyone gramme of silver. . The solution may be kept at a constant temperature, and inthis case the assay stands in want of no correction; or if its temperature be variable,the assay must be corrected.according to its influence. These two circumstances makeno change in the principle of the process, but they are sufficiently important to occasionsome modifications in the apparatus. Experience has decided the preference in favorof applying a correction to a variable temperature.

We readily obtain a volume of 100 cubic centimetres by means of apipetle, fig. 83,

so gauged that when filled with water up to theI mark a, b, and well dried at its point, it will runout, at a continuous efflux, 100 grammes of waterat the temperature of 15 C. (59 Fah.). We say. purposely at one efflux, because after the cessation“ of the jet, the pipette may still furnish two or& three drops of liquid, which must not be countedor reckoned upon. The weight of the volume ofthe normal solution, taken in this manner with' suitable precautions, will be uniform from oneextreme to another, upon two centimetres and ahalf, at most, or to a quarter of a thousandth, andthe difference from the mean will be obviouslytwice less, or one half. Let us indicate the most' simple manner of taking a measure of the normalsolution of sea salt.

After having immersed the beak c of the pipettein the solution, we apply suction by the mouth,to the upper orifice, and thereby raise the liquid tod, above the circular line a b. We next applyneatly the forefinger of one hand to this orifice,remove the pipette from the liquid, and seize itas represented in fig. 84. The mark a b beingplaced at the level of the eye, we make the sur*face of the solution become exactly a tangent tothe plane a b. At the instant it becomes a tangent, we leave the beak c of the pipetteopen, by taking away the linger that had been applied to it, and without changinganything el%e in the position of the hands, we empty it into the bottle which shouldreceive 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 theforefinger fast enough to the upper orifice, without letting the liquid run down belowthe mark a b, he should remove the pipette from the solution with its top still closedwith 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 tothe orifice previously wiped. This mode of obtaining a measure of normal solution ofsea salt is very simple, and requires no complex apparatus : but we shall indicate anothermanipulation still easier, and also more exact.

In this new process the pipette is filled from the top like a bottle, instead of beingfilled by suction, and it is moreover fixed. Fig. 85 represents the apparatus, d andrf are two sockets separated by a stop-cock n. The upper one, tapped interiorly,receives, by means of a cork stopper r., the tube T, which admits the solution of seasalt. The lower socket is cemented on to the pipette; it bears a small air-cock n', anda screw plug v, which regulates a minute opening intended to let the air enter veryslowly into the pipette. Below the stop-cock it', a silver tube N, of narrow diameter,soldered to the socket, leads the solution into the pipette, by allowing the air, which itdisplaces, to escape by the stop-cock n'. The screw plug, with the milled head v',replaces the ordinary screw by xvhich the key of the stop-cock may be made to presswith more or less force, upon its conical seat.