422 PEINCIPLES OF CHEMISTKY 



a golden yellow variety may be obtained, which gives a brilliant golden, 

 yellow coating on glass ; but it is easily converted into the ordinary 

 grey- metallic state by friction or trituration. There is no doubt 18 bis 

 that there is the same relation between ordinary silver which is per- 

 fectly insoluble in water and the varieties of silver obtained by Carey 

 Lea 18 tri as there is between quartz and soluble silica or between 



brown colour then changes into a reddish chocolate, owing to the reduction of the silver 

 by the dextrine, and the solution turns a deep red. A few drops of this solution turn 

 water bright red, and give a perfectly transparent liquid. The dextrine solution is pre- 

 pared by dissolving 40 grams of caustic soda and the same amount of ordinary brown 

 dextrine in two litres of water. To this solution is gradually added 28 grams of AgNO s 

 dissolved in a small quantity of water. 



The insoluble allotropic silver is obtained, as was mentioned above, from a solution 

 of silver prepared in the manner described, by the addition of sulphate of copper, 

 iron, barium, magnesium, &c. In one experiment Lea succeeded hi obtaining the 

 insoluble allotropic Ag in a crystalline form. The red solution, described above, after 

 standing several weeks, deposits crystals spontaneously in the form of short black 

 needles and thin prisms, the liquid becoming colourless. This insoluble variety, when 

 rubbed upon paper, has the appearance of bright shining green flakes, which polarise 

 light. 



The gold variety is obtained in a different manner to the two other varieties. A 

 solution is prepared containing 200 c.c. of a 10 p.c. solution of nitrate of silver, 200 c.c. 

 of a 20 p.c. solution of Rochelle salt, and 800 c.c. of water. Just as in the previous" case 

 the reaction consisted in the reduction of the citrate of silver, so in this case it consists 

 in the reduction of the tartrate, which here first forms a red, and then a black precipitate 

 of allotropic Ag, which, when transferred to the, filter, appears of a beautiful bronze 

 colour. After washing and drying, this precipitate acquires the lustre and colour 

 peculiar to polished gold, and this is especially remarked where the precipitate comes 

 into contact with glass or china. An analysis of the golden variety gave a percentage 

 composition of 98'750 to 98'749 Ag. Both the insoluble varieties (the blue and gold) 

 .have a different specific gravity from ordinary silver. Whilst that of fused silver is 10'50, 

 and of finely-divided silver 10'62, the specific gravity of the blue insoluble variety is 9'58, 

 and of the gold variety 8'51r The gold variety passes into ordinary Ag with great ease. 

 This transition may even be remarked on the filter in those places which have acciden- 

 tally not been moistened with wate^r. A simple shock, and therefore friction of one 

 particle upon another, is enough to convert the gold variety into normal white silver. 

 Carey Lea sent samples of the gold variety for a long distance by rail packed in three 

 .tubes, in which the silver occupied about the quarter of their volume ; in one tube only 

 he filled up this space with cotton-wool. It was afterwards found that the shaking of 

 the particles of Ag had completely converted it into ordinary white silver, and that only 

 the tube containing the cotton-wool had preserved the golden variety intact. 



The soluble variety of Ag also passes into the ordinary state with great ease, the 

 heat of conversion being, .as Prange showed in 1890, about + 60 calories. 



18 bio The opinion of the nature of soluble silver given below was first enunciated in 

 the Journal of the Russian Chemical Society, February 1, 1890, Vol. XXII., Note 78. 

 This view is, at the present tune, generally accepted, and this silver is frequently known 

 AS the ' colloid ' variety. \ may add that Carey Lea observed the solution of ordinary 

 molecular silver in ammonia without the access of air. 



18 trt It is, however, noteworthy that ordinary metallic lead has long been considered 

 soluble in water, that boron has been repeatedly obtained in a 'brown solution, and that 

 observations upon the development of, certain bacteria have shown that the latter die in 

 water which has been for some time in contact with metals. This seems to indicate the 

 passage of small quantities of metals into water (however, the formation of peroxide of 

 hydrogen may be supposed to have eorae influence in these oases) 



