SILVER. —Blowpipe Assay.

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also formed. This authority, however, prefers toremove the mercury by previous fusion, in conse-quence of the difficulty in “clearing” which isoccasioned by the use of acetates.

Mulder has shown that when the most exactchemical proportions of silver and salt react on eachother, the addition of a solution of either will pro-duce a precipitate—a fact which he considers due tothe presence of nitrate of soda, but this explanationis doubted by Stas. Mulder points out, therefore,that an assay may be completed in- either of thethree following ways:—(1) By adding decimal saltsolution until it just ceases to produce a cloudi-ness ; (2) By adding a slight excess of salt, andthen decimal silver until no more precipitate occurs;or (3) By finding the “ neutral ” point—that is,the point at which either the decimal salt or silversolution will produce the same amount of precipi-tate. He considers the last of these the mostaccurate, but the first is preferable from its rapidity;and if check assays be made from time to time, verygreat accuracy can be attained by its means..

It may be well to mention that as a rise of tem-perature occasions an expansion of the solution, itwill render the “ normal” liquid weaker, and there-fore will tend to make the results appear too high ,-and a fall of temperature will have the reverseeffect. It is very essential that the solutions be keptin well-closed vessels (of glass or earthenware), toavoid evaporation; and the exact strength of thenormal solution should be ascertained each morningby one or two assays on fine silver. If it is foundnecessary to add salt solution, in order to reach thepoint at which no cloudiness is produced, it indicatesthat the solution is weak; the results therefore appeartoo high, and the excess must be deducted. Whenit is necessary to add silver solution, the reverse isthe case ; and the solution being too strong, theresults appear low. Therefore it is necessary to addto all the results the amount represented by theadditional silver solution introduced.

Pisam's Method. —This, which is specially applicablewhen only small quantities are present, cannot beused in presence of bodies which exert a decom-posing action on iodide of starch—such as salts ofmercury, and many of those of tin, antimony,arsenic, iron, manganese, and gold, or of substanceswhich colour the solution. The process dependsupon the fact that if solutions of iodide of potassiumand starch be gradually added to a liquid containingnitrate of silver, no permanent blue colour is pro-duced until all this salt is decomposed. Pisani’smethod, as modified by Vogel, is very convenientfor photographers. The following solutions arerequired:—10 grams, of commercially pure iodideof potassium are dissolved in distilled water, anddiluted to a volume of 1023-4 c.c.; 1 c.c. of thissolution will suffice to precipitate 0-01 gram, ofsilver. One gram, of pure sulphate of monoxide ofiron is added to 1000 grams, of nitric acid, of specificgravity 1-200, in order to obtain free nitrous acid;a small quantity of the iron salt must be addedfrom time to time. The starch solution is made by

dissolving 1 part of starch in 100 of warm water.This is allowed to settle ; and after the clear liquidhas been decanted, 20 parts of pure nitrate of potashare added. It will remain fit for use for about sixweeks.

The following is the method of procedure :—1c.c. of the nitric acid is added to 1 c.c. of the solutionunder examination, then 10 or 12 drops of starchsolution and a few drops of the iodide, and thewhole shaken. The addition of the latter is con-tinued until a permanent pale blue coloration isjust produced, and the number of c.c.s addedindicates, without calculation, the number of centi-grammes present per centimetre of the solution—that is, grammes per litre.

Voliiard’s method is applicable even when aconsiderable quantity of copper is present. If asolution of sulphocyanate of potassium or ammoniumbe added to an acid solution of a silver salt con-taining a small quantity of ferric sulphate, thewhole of the silver is precipitated; and the leastexcess of the precipitant causes a blood-red colora-tion, owing to its combination with the iron salt.Hence if a standard solution of sulphocyanate beused, it will at once indicate the amount of silverpresent. As the sulphocyanate of ammonium isexceedingly hygroscopic, a standard solution mustbe prepared as follows:—10 c.c. of a solution ofnitrate of silver, containing 10-8 grams, of puresilver per 1000 c.c., are introduced into a beaker,together with 5 c.c. of a solution of ferric sul-phate (50 grams, per litre of water), and a solutioncontaining about 8 grams, of the ammonium saltadded gradually until a permanent pale red tint isobtained. A hundred times the amount so addedis diluted to 1 litre, and each c.c. of the resultingsolution is equivalent to 10 milligrammes of silver.

Preliminary Volumetric Assay. —As already men-tioned, it is essential that the composition be approxi-mately known, in order to successfully practise thewet method of Gay-Lussac . This may convenientlybe determined by dissolving 1 gram, of the alloy innitric acid, and very carefully precipitating with thenormal salt solution. Thus suppose 70 c.e.s arerequired for this purpose, we have, 100 :1-0 :: 70 : x,or x = 0-7; and, to ascertain the amount of alloy tobe taken for the assay proper, 0-7 :1:: 1: x', whencex' = 1-428 gram.

Blowpipe Assay. —This account of the methods ofassaying silver would not be complete without somedescription of the method of determining the amountof the precious metal in either ores or alloys bymeans of the blowpipe, which was first proposedin 1827 by IIarkort, and considerably improvedby Plattner. A very complete account of themethod will be found in the Chemical News for 1867,by David Forbes, who practised it with great suc-cess. It is exactly analogous to the furnace assaybefore described, the fusion with lead, scorification,and subsequent cupellation being performed in allcases before the blowpipe. In the cupellation, how-ever, the process is somewhat modified.

If the silver be already alloyed with lead, and be