COPPER, SILVER, AND GOLD 421, 



silver is here formed, which he called allotropic silver. It may be 

 obtained by taking 200 c.c. of a 10 per cent, solution of AgNO 3 and 

 quickly adding a mixture -(neutralised with NaHO) of 200 c.c. of a 

 80 per cent, solution of FeSO 4 and 200 c.c. of a 40 per cent, solution 

 of sodium citrate. A lilac precipitate is obtained, which is collected 

 on a filter (the precipitate becomes blue) and washed with a solution of 

 NH 4 NO 3 . It then becomes soluble in pure water, forming a red 

 perfectly transparent solution from which the dissolved silver is preci- 

 pitated on the addition of many soluble foreign bodies. Some of the 

 latter for instance, NH 4 N0 3 , alkaline sulphates, nitrates, and citrates 

 give a precipitate which redissolves in pure water, whilst others for 

 instance, MgS0 4 , FeS0 4 , K 2 Cr 2 O 7 , AgNO 3 , Ba(N0 3 ) 2 and many others- 

 convert the precipitated silver into a new variety, which, although no 

 longer soluble in water, regains its solubility in a solution of borax 

 and is soluble in ammonia. Both the soluble and insoluble silver are 

 rapidly converted into the ordinary grey-metallic variety by sulphuric 

 acid, although nothing is given off in the reaction ; the same changd 

 takes place on ignition, but in this case CO 2 is disengaged ; the latter 

 is formed from the organic substances which remain (to the amount of 

 3 per cent.) in the modified silver (they are not removed by soaking in 

 alcohol or water). If the precipitated silver be slightly washed and 

 laid in a smooth thin layer on paper or glass, it is seen that the soluble 

 variety is red when moist and a fine blue colour when dry, whilst the in- 

 soluble variety has a blue reflex. Besides these, under special conditions 18 



18 When solutions of AgNO' 3 , FeSO 4 , sodium citrate, and NaHO are mixed together 

 in the manner described above, they throw down a precipitate of a beautiful lilac 

 colour; when transferred to a filter paper the precipitate soon changes colour, and 

 becomes dark blue. To obtain the, substance as pure as possible it is washed with a 

 5-10 p.c. solution of ammonium nitrate; the liquid is decanted, and 150 c.c. of water 

 poured Over the precipitate. It then dissolves entirely in the water. A small quantity of 

 a saturated solution of ammonium nitrate is added to the solution, and the silver 

 in solution again separates out as a precipitate. These alternate solutions and 

 precipitations are repeated seven or eight times, after which 'the precipitate is trans- 

 ferred to a filter and washed with 95 jp.c. alcohol until the filtrate gives no residue oil 

 evaporation. An analysis of the substance so obtained showed that it contained from 

 97'18 p.c. to 97'31 p.c. of metallic silver. It remained to discover what the remaining 

 2-3 p.c. were composed of. Are they merely impurities, or is the substance sonje com- 

 pound of silver with oxygen or hydrogen, or does it contain citric acid in combination 

 which might account for its solubility ? The first supposition is set aside by the fact' 

 that no gases are disengaged by the precipitate of silver, either under the action of gases 

 or when heated. The second supposition is shown to be impossible by the fact that 

 there is no definite relation between the silver and citric acid. A determination of the 

 amount of silver in solution showed that the amount of citric acid varies greatly for one 

 and the same amount of silver, and there is no simple ratio between them. Among 

 other methods of preparing soluble silver given by Carey Lea, we may mention the 

 method published by him in 1891. AgNO 3 is added to a solution of dextrine in caustic 

 soda or potash ; at first a precipitate of brown oxide of silver is thrown down, but the 



