SILVER ASSAYING 



825 



1842 



1844 1843 



readily by absorbing the humidity of the air, a concentrated solution of the salt of 

 commerce is to be preferred, of which a large quantity may be prepared at a time to 

 be kept in reserve for use, as it is wanted. 



Preparation of the Normal Solution of Salt when measured by Volume. The 

 measure by weight has the advantage of being independent of temperature, of having 

 the same degree of precision as the balance, and of not standing in need of correction. 

 The measure by volume has not all these advantages ; but, by giving it sufficient 

 precision, it is more rapid. This normal solution is so made, that a volume equal to 

 that of 100 grammes of water, or 100 cubic centimeters, at a determinate temperature, 

 may precipitate exactly one gramme of silver. The solution may be kept at a con- 

 stant temperature, and in this case the assay stands in no want of correction ; or if its 

 temperature be variable, the assay must be corrected according to its influence. These 

 two circumstances make no change in the principle of the process, but they are 

 sufficiently important to occasion some modifications in the apparatus. 



We readily obtain a volume of 100 cubic centimeters by means of a, pipette, fig. 

 1841, so gauged, that when filled "with water up to the mark a b, and well dried at 

 its point, it will run out, at a continuous efflux, 100 grammes of water at the tem- 

 perature of 15 C. (59 F.) We say purposely at one efflux, because after the cessa- 

 tion of the jet, the pipette may still furnish two or three drops of liquid, which must 

 not be counted or reckoned upon. The weight of the volume of the normal solution, 

 taken in this manner with suitable precautions, will be uniform from one extreme to 

 another, upon two centimeters and a half, at most, or to a quarter of a thousandth, 

 and the difference from the mean will be obviously twice less, or one half. Let us 

 indicate the most simple manner of taking a measure of the normal solution of 

 salt. 



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. 1842. 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 

 anything 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, anyone 

 should find a difficulty in applying the fore- 

 finger 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 with- 

 draw his tongue, and apply the forefinger of 

 the other hand to the orifice previously wiped. 

 This method 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 much 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. 1843 represents the apparatus. D 

 and D' are two sockets, separated by a stopcock B. 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 B', 

 and a screw-plug v, which regulates a minute opening intended to let the air enter 

 very slowly into the pipette. Below the stopcock B', 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 stopcock, B'. The screw-plug, with the 

 milled head, v, replaces the ordinary screw by which the key of the stopcock may 

 be made to press, with more or less force, upon its conical seat. 



Fig. 1844 represents a side view of the apparatus just described. We here remark 

 an air-cock B, and an opening m. At the extremity Q, of tho same figure, the conical 



