132 ON THE FOEMATION OF MOSS GOLD A5fD SILVEE. 



4. Crystallizatio7i hy Electrolysis. "When solutions of the saltfc- 

 of the heavier metals are submitted to the action of electric cur- 

 rents, they undergo decomposition, and the metal which is 

 deposited at the negative pole is usually more or less crystallized. 

 A current of low intensity, cceteris paribus, seems to favour the 

 formation of well-developed crystals. The reduction of a metallic 

 solution by a more electro-positive element may probably be 

 classed under this head, as stannic chloride by zinc, or silver 

 nitrate by lead, and so on. 



5. Spontaneous Crystallization, as it is usually termed, e.g., 

 the gradual passage of amorphous plastic sulphur into the crys- 

 talline state, also the similar change undergone by barley sugar. 

 Many well known chemical precipitates apparently undergo 

 spontaneously a similar change. Again, the gradual conversion 

 of tough fibrous wrought iron into hard brittle iron with short 

 grain, by repeated concussion and vibration, seems to be a variety 

 of crystallization ; certainly a great molecular change has taken 

 place — but this matter requires further investigation. Then we 

 have the passage of blocks of tin, which had been exposed to 

 intenspe cold, from the malleable and non-crystalline to a fibro- 

 crystalHne and brittle state, — in fact, so brittle does the tin 

 become that it more or less completely falls to powder. 



The devitrification of glass may also be here mentioned. 



6. Crystallization hy tlierono -reduction. I think that we may safely 

 regard the forms exhibited by the artificially produced moss 

 metals as varieties of crystalline forms, and with as much reason 

 as the mineralogist assigns a place for the similar natural forms 

 amongst crystals ; the arborescent and other group forms assumed 

 by native metals can be traced from normal and primary forms, 

 such as of the octohedron and rhombic dodekahedron through 

 various degrees of elongation and attenuation until we arrive at 

 the filiform and capillary threads, a number of which aggregated 

 together give in the velvet or plush-like mass of moss copper or 

 other metal. Moreover, some portions of the gold reduced from 

 the mispickle showed branching and arborescent groups which 

 had all the appearance of elongated dodekahedra placed end to 

 end in no way differing from natural specimens except in 

 minuteness and perhaps greater brilliancy of lustre. 



But these crystals have been produced by a process differing 

 considerably from the methods enumerated in the first five 

 divisions ; hence the necessity for forming the sixth and last group. 



The artificially prepared moss metals are produced by a pro- 

 cess of reduction, aided neither by vaporization, solution, fusion, 

 or electrolysis, neither are they produced " spontaneously," but 

 they are prepared by the aid of a heated re-agent. Hence I 

 have for convenience ventured to form a special class, i.e., 

 crystallization hy tliermo-reduction. 



