438 THE POPULAR SCIENCE MONTHLY. 



also, there is a luxuriant growth of large, flat leaflets, or symmetrical 

 structures resembling fern-leaves, but with the fronds arranged at right 



angles, or combinations of these with 

 octohedra, as shown in Fig. 8, These 

 fern-leaves often begin of a dull-gray 

 color, but, as they advance, suddenly 

 change to a brilliant white. The 

 particular form of these crystalline 

 growths depends, therefore, primarily 

 on the specific character of the metal, 

 ^J^^^^^^^^^^' but this is greatly modified by the 



strength of the solution. 

 The forms assumed by native metals resemble those produced by 

 the process of substitution. In some cases, indeed, it seems almost 

 certain that the deposition of these minerals was effected in the same 

 way — as, for instance, the silver on the native copper of the Lake 

 Superior district. 



Gold is frequently found in cubes more or less rolled, but the leaf- 

 gold from Transylvania bears a striking likeness to the crystals that 

 form in our laboratory experiments. 



Silver is often found native as twisted hairs or wires of metal — a 

 form that never occurs in the decomposition of its nitrate by copper, 

 but which can be artificially produced in another way. There has been 

 noticed a singular tendency in old silver ornaments and coins to be- 

 come crystalline and friable. I have an ancient fibula from the island 

 of Cyprus, supposed to be at least fifteen hundred years old, which, 

 through the greater portion of its substance, presents a fracture some- 

 thing like that of cast iron, and its specific gravity has been reduced in 

 round numbers from ten to nine. 



It contains a little copper. This property of certain metals, or their 

 alloys, to change in condition and in volume, is worthy the attention of 

 those whose duty it is to make our standards. Experiments should be 

 instituted for the purpose of learning what metals or combinations of 

 metals are least subject to this secular change. 



These metallic crystals are Nature's first attempt at building. The 

 material is the simplest possible — in fact, what chemists look upon as 

 elementary. But how is the building carried on ? What are the tools 

 employed ? Where are the bearers of burdens that bring and prepare 

 the pieces and lay them together according to the plan of the Great 

 Architect ? We must imagine what is taking place in the transparent 

 solution. The silver, of course, existed at first in combination with 

 the nitric element, and for every particle of silver deposited on the 

 growing tree an equivalent particle of copper is dissolved from the 

 surface of the plate. The nitric element never ceases to be in combi- 

 nation with a metal, but is transferred from the one metal to the other. 

 On the " polarization theory " the positive and negative elements of 



