30 THE RIGHT HON. LORD KELVIN^ G.C.V.O., OX 



the liquor is less, by something like 15 per cent., than the 

 specific gravity of the solid crystals, it must te)id to find its 

 Avay upwards, and will actually do so, hoAvever slowly, until 

 stopped by the already solidified impermeable crust, or until 

 itself becomes solid on account of loss of heat by conduction 

 outwards. If the upper crust were everywhere continuous 

 and perfectly rigid the mother liquor must, inevitably, if 

 sufiicient time be given, find its way to the highest places of 

 the lower boundary of the crust, and there form gigantic 

 pockets of liquid lava tending to break the crust above it 

 and burst up through it. 



§ 37. But in reality the upper crust cannot have been 

 infinitely strong ; and, judging filone from Avhat we know of 

 properties of matter, we should expect gigantic cracks to 

 occur from time to time in the upper crust tending to shrink 

 as it cools and prevented from lateral shrinkage by the non- 

 shrinking imcooled solid below it. AVIkju any such crack 

 extends downwards as far as a pocket of mother liquor 

 underlaying the wholly solidified crust, we should have an 

 outburst of trap rock or of volcanic lava just such as have 

 been discovered by geologists in great abundance in many 

 parts of the world. Wc might even have comparatively 

 small portions of Jiigli plateaus of the primitive solid earth 

 raised still higher by outbm'stsof the mother liquor squeezed 

 out from below them in virtue of the pressure of large sur- 

 rounding portions of the superincumbent crust. In any such 

 action, due to purely gravitational energy, the centre of 

 gravity of all the material concerned must sink, although 

 portions of the matter may be raised to greater heights ; but 

 we must leave these large questions of geological dynamics, 

 having been oidy brought to think of them at all just now 

 by our consideration of the earth, antecedent to life upon it. 



§ 38. The temperature to which the earth's surface cooled 

 witliin a few years after the solidification reached it, must 

 have been, as it is now, such that the temperature at Avhich 

 heat radiated into space during the night exceeds that 

 received from the sun daring the day, by the small difference 

 due to heat conducted outwards from within.* One year 



* Sup])ose for example the cooling and thickening of the npper crust 

 has proceeded so far, that at the surface and therefore ajjproximately for 

 a few decimetres below the surface, the rate of augmentation of tem- 

 perature downwards is one degree per centimetre. Taking as a rough 

 average '005 c.g.s. as the thermal conductivity of the surface rock, we 

 should hav^e for the heat conducted outwards -005 of a gj-amme water 



