22 Scientific Proceedings, Royal Dublin Society. 
given) and to make it highly probable that the increasing tem- 
perature is more than able to hold its own against the increasing 
pressure, as faras diminishing the conductivity of the less inward 
parts is concerned. The above “auxiliary considerations” shew 
that is not necessary for our purpose that the increased tempera- 
ture should be a match for the increased pressure, as regards the 
condensation produced thereby. This it probably is not. And 
yet we must not concede too much even on this point. It is a 
very reasonable belief that the unquestionable high density of 
the more inward parts of our earth is partly due to the greater 
specific gravity of the materials there—that is to say, for all 
that appears to the contrary, the expansive power of the heat 
may, even there, be a match for the condensing power of the 
pressure, which, however, we do not wish it to be. However, 
as regards the present question we may be indifferent as to the 
thermal conductivity of the more inward parts if, as seems so 
very probable, we have a sufficient thickness of material of low- 
ered conductivity between it and the surface. Its comparatively 
small magnitude, also, makes it of little importance. 
It would be very interesting to know more than we do on the 
thermal expansibilities and the compressibilities of rocks. It 
would enable us to form some sort of opinion as to how the con- 
ductivity of the geological crust is affected by the conflicting 
heat and pressure to which it is subject. It is, however, wart 
while to mention that iron, copper, and slate, under the conditions 
of heat and pressure which must obtain for a iew miles, at least, 
beneath the surface, would be in a more expanded state than at 
the surface; that is to say, their thermal conductivities would 
decrease with the increase of depth. As slate has been produced 
under enormous pressure, the effect of which it still retains in its 
high density, its behaviour as regards expansion by heat and 
condensation by (further, artificial) pressure is very significant 
and important, as regards the present matter. It should be 
acknowledged that marble would probably become more con- 
densed when subjected to the conditions that exist for some 
distance beneath the surface. 
We now pass to another consideration which is essentially 
unconnected with what we have just been considering, but which 
tends in the same direction. Sir William Thomson, in forming 
