MR. HOPKINS’S RESEARCHES IN PHYSICAL GEOLOGY. 
55 
it be independent of the pressure to which the fused matter is subjected. For if the 
terrestrial temperature be due to that source, it must undoubtedly be sufficient at the 
depth of one-fortieth or one-fiftieth of the earth’s radius, to fuse all the rocks com- 
posing- the superficial solid portion of the globe placed under the atmospheric press- 
ure. Consequently matter must exist at such depths in a state of fusion, and the 
crust of the earth must be extremely thin, unless its solidification has been promoted 
by pressure. I have shown, however, that the crust of the globe cannot be very thin, 
and therefore the truth of our proposition is manifest. 
There is also another mode, independent of our results respecting precession, by 
which we arrive at the same conclusion. Making the assumption just stated respect- 
ing the origin of the actual terrestrial heat, there is no doubt of its being immensely 
greater at the earth’s centre than that which would be necessary to reduce the matter 
composing the earth’s surface, under the atmospheric pressure, to a state of fusion. 
It would probably reduce a large portion of it to a state of vapour. Now this actual 
central temperature must necessarily be at least something less than that which ex- 
isted at the time of the earth’s incipient solidification, whether the solidification com- 
menced at the centre or surface (Mem. I.). If it began at the centre it must have 
been owing to the predominance of pressure in promoting solidification over high 
temperature in opposing it, and the truth of our proposition is therefore involved in 
this hypothesis. Again, suppose the solidification to have commenced at the surface. 
In this case it has been shown (Mem. I.) that the whole mass would arrive at that 
state in which the fluidity would just become imperfect, at nearly the same time. 
The superficial temperature would then be just that of 'perfect, fusion under the at- 
mospheric pressure for the matter constituting the earth’s surface, which, as just stated, 
must be small compared with the actual central temperature, and, a fortiori, small 
compared with the central temperature at the epoch referred to. Consequently, at 
that epoch, the central and superficial parts of the earth, under widely different 
temperatures, would have the same degree of fluidity, viz. that at which it just be- 
came imperfect, or that at which the component particles would cease to move among 
themselves in the process of cooling. If then denote the temperature of perfect 
fusion for a point of the earth’s mass at any depth beneath its surface (or the tempe- 
rature at which the mass would there acquire perfect fluidity),^ must be some func- 
tion of the pressure at the proposed point. Also let r 2 denote the temperature of 
incipient fusion at the same point (or that at which the matter then under the same 
pressure would just lose its property of solidity), then the question is, whether r 2 be 
a function of r x or not. Now that there should be some necessary relation between 
r l and r 2 would scarcely seem to admit the possibility of a doubt. But in such case 
r 2 , being a function of r l5 must depend on the pressure. Hence it follows, as before, 
that the temperature of fusion of the earth’s mass must depend on the pressure to 
which it is subjected, assuming always that the fusibility of the matter composing 
the central portion of the globe is not extremely different from that which constitutes 
its surface. 
