22 LECTURES TO SCIENCE TEACHERS. 



heat from the hotter to the colder part, and consequently, 

 we may rest assured that there is continual flux of heat 

 from the interior to the exterior of the earth itself by 

 radiation into space that the earth in fact is getting 

 gradually cold. Geologists and natural philosophers have 

 at times come into collision on this matter, and various 

 opinions have been held about it. Sir Charles Lyell sup- 

 posed that by the aid of chemical and electrical action in 

 the interior of the earth a continual supply ^f heat could 

 always be maintained, and that this could go on for ever, 

 so that there would always be a gradual flux of heat from 

 the interior to the exterior, which was always to be com- 

 pensated by this chemical and electrical action going on 

 in the interior. This, I need hardly say, is completely 

 opposed to all our knowledge of the theory of heat and to 

 Sir William Thomson's grand theory of the dissipation of 

 energy. It might be possible to account for the difference 

 of temperature as we descend on a chemical hypothesis, 

 provided it was only a local fact, but it is a fact which is, 

 to the best of my belief, perfectly universal all over the 

 world, that the deeper you descend the greater is the in- 

 crease in temperature, and consequently we must assume 

 that there is this regular flux of heat from the earth, and 

 that the earth is gradually cooling. Following out the 

 analysis of Fourier, Sir William Thomson was led, about 

 twelve years ago, to calculate the condition of the earth, as 

 to temperature, backward, for a vast number of years. If 

 we know what is the conductivity of the rocks, that is, K, 

 and if we know the difference in temperature at an interval, 

 say D, of 100 feet, then we can find out the flux of heat 

 through a given area of the earth's surface in a given time, 

 simply by this formula, and consequently we can measure 

 the quantity of heat which is being lost by the earth every 

 year, and can sum this up by a mathematical process and 

 find the quantity of heat lost in past ages, and so we can 

 find the temperature which the earth has had at different 

 epochs in its history. Following out this idea, Sir William 

 Thomson was led to conclude that, if we assume 7,000 F. 

 to be the temperature of molten rocks, then 98 million years 

 ago the earth was in the condition of a molten mass of rock 

 with a crust just beginning to form upon the outside. This 

 is a very important conclusion, geologically as well as 



