314< Mr. E. Solly, Jun., on Voltaic Frecipitation, 
reached the opposite electrode they complete the metallic 
circuit and prevent further decomposition. In the same way 
I expected that when copper was deposited against a badly- 
conducting surface, it would increase much more rapidly in 
the direction towards the zinc rod, and that it would have 
but very little tendency to increase sideways ; but I found, on 
the contrary, that the deposited copper had a remarkable pro- 
perty of extending by its edges far more rapidly than it in- 
creased in thickness ; seeming to creep along or cling to the 
surface of the plaster or other non-conducting substance, 
against which it was being precipitated : and even when the 
plaster surface was placed at an angle of 45° to the zinc rod, 
and the deposition commenced in the centre, where a piece of 
gold-leaf had been applied, the copper extended equally all 
round and quite as fast on the side, where it receded from 
the zinc as on that part where by increasing it approached 
it. Following out this circumstance, I soon found that by 
very slightly improving the conducting power of the surface 
of the plaster or other non-conducting substance, I was 
enabled to precipitate copper without any gold-leaf or other 
metallic surface for commencement. The degree of conduct- 
ing power requisite for this purpose was very slight, all that 
was necessary being to wash the surface over once or twice 
with a solution of nitrate of silver or muriate of gold, drying 
and well blacking each successive coat by exposure to light, 
the surface having previously been well rubbed over with a 
small quantity of plumbago. When thus prepared and placed 
in the solution of sulphate of copper, it was sufficient to touch 
any part of it with the wire attached to the zinc to cause the 
precipitation; a small ring of copper soon formed on the 
blackened surface round the wire, which increasing in size, in 
time covered the whole surface which had been prepared. 
When the deposit of copper had reached the edge of the pre- 
pared surface it still continued to increase, but more slowly, 
extending around the edges, even on to the back of the plaster, 
and accommodating itself to all the inequalities of its surface 
almost as perfectly as if it were metallic. In this manner I 
have caused it to be precipitated along the surface of card, 
paper, and on a variety of the most delicate and easily de- 
structible organic substances. Indeed I have frequently seen 
it, when arrested by an air-bubble, gradually surround and 
envelope it, and thus form a perfect cast; the process going on 
with sufficient rapidity, and yet without disturbing so frail a 
form. 
Some of these experiments are interesting when viewed in 
relation to certain phaenomena of fossilization, not merely in 
