J2C 



UNDULATORY FORCES. ELECTKO-METALLURGY. [GOLD SOICTIONJ, 



colour. It should bo kept covered as much M possible, 

 lly allowing it to stand undisturbed a few minutes 

 towanla the latter part of the .;>!. it;. .11, and occasion- 

 ally tapping the ados of the ladlo or crucible, tlio iron 

 of the fcrro-cyanido will settle at the bottom as a tino 

 Mack jHiwdor; the colourless cyanide of potassium may 

 then U' ]K>urod off into :i roM iron pan, or upon a thick 

 and cold inni plate; it should bo broken up whilst still 

 warm, and preserved in a well-stopped jar. 



Tlio black sediment, which contains much cyanide of 

 potassium, should be scraped out of the vessel while 

 still hot, and preserved, as water will at any time dis- 

 solve the cyanide. If the process bo well condn 

 the product will bo of a clear white colour, or, nt 

 most, but very slightly gray. A larger proportion of 

 cyanide of potassium "is obtained by this process than 

 when forro-cyanido alone is employed, because, in the 

 former case, one-third of the cyanogen (that which was 

 combined with the iron) combines with the potassium of 

 the carbonate of potash, whilst in the latter case it is 

 lost ; the cyanide produced by the fusion of the ferro- 

 cyauido of potassium alone is of a grayish-black colour, 

 and is termed "black cyanide." 



'.40. Cyanide of Potttssium. Commercial cyanide of 

 potassium varies very much in price and quality. By 

 dissolving several specimens of commercial black cyanide 

 in water, and filtering the solution, we found the pro- 

 portion of black impurity in them varied from one- 

 fourth to one-sixth of their weight ; and by experiments 

 with the commercial white cyanide, we found that one 

 part (200 grains) of it dissolved in about one-aud-a- 

 quartcr parts (230 grains) of distilled water at 60 Fall. , 

 and tliat it dissolved much more freely iu water con- 

 taining hydrocyanic acid. 



147. Impurities in Cyanide of Potassium. According 

 to tlio researches of Messrs. Glassford and Napier,* 

 commercial white cyanide generally contains about thirty - 

 five per cent, of impurities, and often as much as fifty 

 per cent., in the form of carbonate and .sulphate of 

 potash, chloride of potassium, cyauato of potash, ferro- 

 cyanide of potassium, and silica ; and if the mixture of 

 salts from which it is made be not dry, ammoniacal 

 compounds will be also formed. The sulphate of potash 

 and chloride of potassium occur in the commercial car- 

 bonate of potash ; the silica is present when we operate 

 with an earthen crucible : and even when the process is 

 well conducted and pure materials used, the product 

 contains twenty per cent, of cyauato of potash, pro- 

 duced partly by the contact of the air with the melted 

 mixture. 



148. Toting Cyanide of Potassium. According to the 

 same experimentalists, the quantity of pure cyanide in 

 any given sample of cyanide of potassium may be cor- 

 rectly ascertained thus : Make two solutions, one of the 

 given cyanide and one of nitrate of silver, each contain- 

 ing known weights of the salts say one ounce of the 

 cyanide dissolved in six ounces of distilled water in a 

 graduated glass vessel, and 175 grains of the crystallised 

 nitrate dissolved in about two or three ounces of distilled 

 water; add the cyanide solution carefully and slowly to 

 the nitrate of silver liquid, until the precipitate first 

 formed is all redissolved. The quantity of the cyanide 

 solution required to effect this (with the above quantity 

 of nitrate of silver) will have contained 130 grains of 

 pure cyanide ; and, from the quantity used, we may easily 

 calculate the amount of pure cyanide ii. the whole <>uin <. 

 It is said, that "when nitrate of silver is added to a 

 solution of cyanide of potassium, so long as the ] 

 pitato formed is all redissolved, we obtain the whole of 

 the cyanide of potassium in combination with the silver : 

 none of the other salts in solution take any part in i In- 

 action, even though they bo present in a large propor- 

 tion. This enables us to text the exact quantity of 

 cyanide of potassium in any sample." 



143. Chemical Character! of Cyanide of Silver. In the 

 presence of cyanide of potassium, as we are informed by 

 Ifatn. Glassford and Napier, cyanogea has a greater 

 affinity for silver than anything else has decomposing 



arine, ISM. 



every salt of silver except the sulphide, and forming 

 cyanide of silver. In dissolving the oxide, . 

 chloride, or forro-cyanido of silver, in a solution 

 of potassium, they are all deo imposed, and cyanide nt 

 silver always funned. Cyanide of silver should bo dried 

 below 200 Fall. : hydrochloric acid decomposes it with a 

 solution of hydrocyanic acid gas ; cold nitric acid lias no 

 action upon it ; a boiling mixture of sulphuric aeid and 

 water decomposes it, with escape of hydrocyanic acid 

 gas, and format ii HI of sulphate of silver; it is soluble in 

 the alkaline chlorides, but its lst solvent is an aqueous 

 solution of cyanide of potassium, of each of whi< I 

 equivalent is required. The resulting solution, when 

 evaporated, yields crystals of double cyanide of silvi-r 

 and potassium, which are soluble in Ngot parts of i-uld 

 and in one' part of boiling-water. The solution of thin 

 double salt, which is nearly the same as the ordinary 

 plating solution, may be boiled for any length of time 

 without being decomposed, and it is very little affected 

 by light; it is decomposed by all acids, and th< ;, 

 cipitnte the silver as cyanide of silver : the hydro-acids 

 hydrochloric acid, for example decompose the cyanide 

 of silver also; sulphuretted hydrogen precipitates the 

 silver as sulphide of silver. 



160. GM Solution. Various salts of gold have been 

 used for electro-deposition ; among which arc the hypo- 

 sulphite, sulphite, iodide, bromide, terchloride, cyanide, 

 and sulpho-cyanido. Finely-divided gold, which is Rome- 

 times used for dissolving, may bo obtained by adding a 

 solution of protosulphato of iron to a solution of 

 chloride of gold, as long as a greenish-brown precipitate 

 occurs ; this is gold in a state of very minute division. 

 Oxide of gold is obtained by adding to a solution of ter- 

 chloride of gold a cold solution of caustic potash, until it 

 A to produce a precipitate; or by digesting tcr- 

 cldoride of gold with magnesia, washing the fresh pre- 

 cipitate, first with dilute nitric acid, and then with 

 water only. Iodide of gold is formed, either by digest- 

 ing oxido of gold in hydriodic acid, or by adding a 

 solution of .iodide of potassium to a solution of ter- 

 chloride of gold as long as a precipitate is produced 

 washing the precipitate with water ; it is of a yellow 

 colour, insoluble in cold water, but freely soluble in a 

 solution of iodide of potassium. Bromide of gold may 

 be formed either by digesting finely-divided gold, or 

 oxide of gold, in liquid bromine contained in a stop- 

 pered bottle. It is a salt of a rich red colour, and is 

 soluble in water. 



151. Chloride of Gold. Terchloride of gold, commonly 

 known as chloride of gold, is the most usual salt of the 

 metal, and its preparation requires separate explanation. 

 It is formed as follows: Take a mixture of two or 

 three measures of hydrochloric acid, and one measure 

 of nitric acid ; make the mixture hot, and add pure 

 grain gold to it as long as it will dissolve : it evolves 

 gas whilst dissolving. When it is all dissolved, slowly 

 evaporate the liquid until it crystallises to a dark ruby- 

 red mass ; or it may be of a yellow colour, according to 

 the proportions of the ingredients. This is tcrchlorido 

 of gold, and contains one equivalent (l!i~ parts) of jrold, 

 auu three equivalents (loti-.'i parts) of chlorine ; if it be 

 too much evaporated, chlorine gas will bo evolved, the 

 gold will bo set free and be mixed with the salt, and 

 will precipitate on dissolving the salt iu water. To 

 produce the yellow chloride, mix together, in a glass or 

 stoneware vessel, one part, by weight, of nitric acid, 

 throe parts of hydrochloric acid, one part of tine gold, 

 and one part of water : cover the vessel with a glass 

 dish, make the liquid quite hot, and maintain the heat 

 until the rod vapours cease; if some of the gold remain 

 uudissolved, add more of the liquid mixture, and treat 

 aa before: when the vapours cease, remove tlio glass 

 cover, and replace it by folds of blotting-paper, ir.nl 

 rate until it crystallises, on cooling, into yellow 

 ilil. .iido of gold. The red chloride is formed iu the 

 same manner, except that the liquid mixture should be 

 composed of one part of nitric, and two parts of hydro- 

 chloric acid, more being added than is necessary to dis- 

 solve all the gold. One ounce of gold will dissolve in 



