66 



NATURE 



[May 23, 1872 



ON THE CRYSTALLISATION OF SILVER, 

 COLD, AND OTHER METALS* 



THERI*: are few chemical experiments so well known 

 as the growth of the "lead tree" a specimen of 

 which is on the table, together with a " silver tree" that is 

 said to have been made by the late Professor Faraday. 

 These carry our minds back to the time of the alchemists, 

 who called the (irst " arbor Saturni," and the second " arbor 

 Dian;e ;" and they may be looked upon as the types of a 

 large number of phenomena, in which the salt of one 

 metal in solution is decomposed by some other metal. 

 My assistant, Mr. Tribe, and myself have been lately ex- 



form that can be detected by a powerful microscope. A 

 stronger solution gives white crystals from the commence- 

 ment, which frequently assume the appearance of fern- 

 leaves ; while the growth from a still stronger liquid re-- 

 minds us r.ither of a furze bush. If the nitrate of silver- 

 amount to 15 per cent., or thereabouts, there occurs a 

 steady advance of brilliantly white ir.oss ; and if the 

 solution be saturated, or nearly so, say 40 per cent., this 

 moss is very sturdy, often ending in solid crystalline 

 knobs, or strrctching out into the liquid as an arborescent 

 fringe. ■ e ^ 



In all these cases, however, when the solution in tront 

 of the growing crystals has been soniewhat exhausted, 

 certain prominent or well-circumstanced er^s'ials seem to 



Xv^^-V 



amining these replacements, the metallic crystals which 

 are thus produced, and the forces that act through the 

 liquid. 



Our more special attention has been given to the mutual 

 action of copper and nitrate of silver. If these two sub- 

 stances be broviL;ht into contact by the intervention of 

 water, th:ie grow upon the red metal what may be well 



calUd ''trees," and though the analogy between crystals 

 and plants is a very superficial one, yet the resemblances 

 of external form are striking enough, and a nomenclature 

 drawn from the garden seems the most expressive. 



It is very beautiful to watch the growth of these silver 

 crystals round a piece of copper under the microscope ; 

 a blue glass underneath adds to the effect, and they are 



best seen when they reflect a strong light thrown upon 

 them. They may also be thrown upon a screen as opaque 

 objects, but the beauty and lustre of their surface is in 

 this way lost. , , „ , , • 



The crystals of silver thus produced differ both in 

 colour and form, according to the strength of the solution. 

 If it be very weak, say one per cent , the copper is fringed 

 with black bushes of the metal, which, in growing, change 

 their colour to white without any alteration of crystalline 



• I ectute delivered .-it the Roy.il Institution of Great Britain, Febru.iry 

 16, 187-', by Juhii H.ill C.ladstonc, F.R.S. 



monopolise the power, and to push forward through the 

 remaining portions of the liquid. This gives rise to 

 beautiful branches which assume a variety of gTaceful 

 forms, which it is hopeless to attempt to pourtray by 

 diagrams, but of which the subjoined Sgures give some of 

 the more characteristic outlines greatl>' magnified. The 

 weak solutions produce featliery crystals i«>mewhat as net 



Fig. I, consisting of a straight central stem from which 

 grow on either side crystalline rays that terminate in a 

 sharp point, and frequently become themselves the centre 

 stems of a similar crystalline structure. In the outlying 

 growth of a moderately strong solution the apparent 

 regularity of the crystalline form is lost ; the tnain stem 

 is built up of a confused mass of hexagonal plates, while 



the side branches are an agglomeration of minute pointed 

 crystals turning in every direction, and producing such 

 jagged outlines as are drawn in Fig. 2. In stronger solu- 

 tions still the branches lose every appearance of straight- 

 ness, and they are built up of hexagonal plates so studded 

 with' crystalline specks that the whole has the rounded 

 appearance depicted in Fig. 3. The arborescent crystals 

 that succeed the fringes from a saturated solution, are 

 smaller in their fohage than the last, and end m httle 

 spherical or botryoidal knobs. 



