HOLWELL CAVERN. 29 
producing the cerystallized red oxide of copper, either in 
cubes or octohedrons, at pleasure ; but I can give no 
reason why the one always made its appearance in one 
case and the other formation in the other. The day will 
probably arrive when more will be known of those matters. 
I now come to another point, most intimately connected 
with geology ; I mean the formation of fissures in the 
earth, on which I have made many experiments. If you 
take a porous pot—(a common garden pot will do, but it 
is better to cork up the aperture at the bottom of it)—and 
fill it with clay kneaded with water to the consistence of 
moist putty, (pipe-clay is the best for the experiment), 
and stand it upright in a large basin of water, in such a 
manner as that the surface of the clay in the pot shall be 
elevated four or five inches above the surface of the water 
inthe basin; then connect the water in the basin by a 
platinum wire with the positive pole of a constant, but 
weak, voltaic battery; and lastly, force a platinum wire 
into the middle of the moistened pipe-clay in the pot, 
vertically, and at about half the depth of the clay, con- 
necting the other end of the wire with the negative pole of 
the battery; the following effeets will take place : If the 
battery is ‚feeble, after a few days action two fissures, 
forming with each other two obtuse and two acute angles, 
will appear in contact with the wire, and will divide the 
clay vertically from top to bottom. The size of these 
fissures is comparatively great; and they will quickly be 
filled with water, drawn upwards from the moistened clay, 
which will form a small pool on the top of the clay, where 
it will shortly increase till it drops over the side of the 
pot, through a notch which should be cut in it, into the 
basin below. The clay would thus be soon emptied of its 
water, were it not replenished by that in the basin, which 
