SILVER.—Humid Assay. Volumetric Methods.

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the saturated solution to each gallon of commonwater will give approximately the.requisite strength,and further adjustment must be made after an assayon pure silver has shown the condition of the liquid.

(b) The decimal salt solution may be easily pre-pared by introducing 100 c.c. of the normal solution,by means of the pipette subsequently described,into a litre flask, and diluting the whole to 1 litre.

(c) For the decimal solution of silver, 1 gram, ofchemically pure silver, prepared by one of themethods already described, is dissolved in a smallquantity of pure nitric acid, and the liquid diluted

Fig. 13.

to 1 litre.

The necessary apparatus is extremely simple,consisting principally of a pipette capable of accu-rately measuring 100 c.c. of liquid; tube pipettes,graduated in c.c.s, and having a fine opening at thelower end; and bottles, holding about 10 ozs., withcarefully ground stoppers. The 100 c.c. pipette,originally recommended by Gay-Lussac , is rarely,if ever, employed at the present day. Its use in-volved the careful adjusting of the level of theliquid to a graduation on a fine tube, and was thusliable to introduce considerableerror in the results obtained.An excellent and extremelysimple form of pipette has beenproposed by Stas, which rendersall adjustment by the eye un-necessary ; and when carefullymade it gives very accurateresults. It is shown, accom-panied by the necessary acces-sories, in Fig. 13, from whichit will be seen that both endsare drawn out to a fine opening,which is about 009 inch indiameter at the lower extremity,and 0-04 inch at the upperextremity. It is found that if care be taken ingrinding, the drop of liquid retained at the lowerend is of a constant magnitude, and thus the amountdelivered by the pipette is invariable.

The solution is introduced at the lower extremityby means of an india-rubber tube connected with alarge vessel, which is placed at an elevation abovethe level of the pipette. A perforated glass dish isaffixed to the upper extremity, to collect any liquidwhich may escape at the opening; and when thepipette is full, the flexible tube is closed by a tap ora spring clip, the finger is placed on the upperopening, and the tube removed. AVhen the lowerportion of the pipette has been wiped, by drawingthe fingers over it, the bottle is placed below, andthe upper end opened by sliding the finger off—amotion which insures the same amount being always

left within the tube.

The pipette employed for the decimal solution isa simple glass tube, about 10 inches in length and0-35 inch in diameter, graduated in c.c.s, andterminating in a fine opening at its lower end. Anarrangement for more delicately measuring smallquantities of this solution has been devised by

Mulder. It consists of a pear-shaped vessel, filledwith the liquid, and fixed in a vertical position. Itis provided with two glass taps for regulating theflow, and 20 drops are exactly equal to 1 c.c.

The agitator shown inFig. 14 is a convenientappliance for shaking tenbottles at a time, andmuch saving of labourwill attend the use ofit where a considerablenumber of assays is fre-quently made. It con-sists of a metal frame, m,divided into compart-ments, b, 6, b, to receivethe bottles, and a lid,provided with springslides, prevents the bot-tles from being displacedand the stoppers frombeing shaken out. Theframe hangs from a shortcarriage spring, s, firmlyfixed to the wall, by aleather strap or by india-rubber door springs, andit is connected with the floor by a long india-rubberspring, p.

A dead black shelf should be fixed in front of awindow (which may be glazed with yellow glass) toreceive the bottles during the addition of the deci-mal solution; and the shelf is provided with a blackback of such a height that the light just passesthrough the upper part of the liquid contained in thebottles.

Additional Details with reference to the above Method.—If a small quantity of mercury be present in themetal under examination, it will, unless its preju-dicial effect be neutralized, render the result of theassay inaccurate. Its presence may be at oncedetected by — (1) The permanence of the whitecolour of the chloride of silver, even when exposedto a strong light; and (2) The difficulty experi-enced in rendering the supernatant liquid clear byshaking. As already explained, the pure chloriderapidly becomes blue on exposure to chemicallyactive light, and this is the reason for the employ-ment of yellow glass or blinds to the windows bysome assayers, as mentioned above. Should mer-cury be suspected, and the discoloration still takeplace, the question may be set at rest by taking 1gram, or more of the metal, and only adding suffi-cient salt solution to precipitate, say, a quarter ofthe metal. The mercury will be first thrown down,and its effect will thus be concentrated on a smallquantity of chloride of silver.

A convenient method of neutralizing the effect ofmercury, even if only found to be present in thecourse of an assay, consists in adding acetate of sodaor caustic ammonia and acetic acid, when, accordingto Debray, the nitrates of silver and mercury aretransformed into acetates, an alkaline nitrate being

Fig. 14.