18 PAPERS, ETC. 
nature uses in her vast and varied laboratory. Moreover, 
there are other conditions besides electrical action, neces- 
sary to be observed, such as a more or less even temperature, 
absence of light, and in many cases a constant motion of the 
fluid holding the crystallizable matter in solution, either 
by dropping from the roof of a cavern, or by water 
constantly flowing, or by the continual alternate elevation 
and depression of the surface of the subterranean waters, 
which surface is for ever varying—low in summer, or more 
or less overflowing in winter—but constantly in motion. 
It is this eternal motion that greatly facilitates the growth 
of erystals. This would seem a strange doctrine in a 
chemical laboratory, where perfect rest is more or less 
essential to the formation of well-defined saline crystalli- 
zations ; but such is by no means the case with metallic 
and earthy matters. I have kept up a constant electrical 
action for three successive months, upon fluids in a state of 
unceasing ebullition, in a sand heat furnace, day and night 
without a moment’s rest, the evaporated fluid being duly 
replenished and watched in the most careful manner ; yet 
the erystals formed were as perfectly solid and regular 
as similar ones taken from a mine, and were much 
accelerated in their growth both by the heat employed and 
by the motion communicated by such heat. There is 
another condition essential to the production of nearly, if 
not quite all, regularly-formed metallic, and most earthy, 
erystallizations. It is the interposition of a porous medium 
between the two opposite electrical poles engaged in the 
work of forming minerals. In art this is brought to pass 
by the intervention of tabular surfaces, or cups of porous 
earth, or other porous material, which is used to separate 
the fluids or substances acted on, so as to bring them 
together slowly and regularly into a solid form. It is 
