president's address— section a. 21 



Sources of heat — the radio-active elements — are now known,, 

 which Lord Kelvin did not, of course, take into account. The 

 earth can no longer be regarded as a body possessing only its 

 sensible heat, to supply the stream continually flowing from the 

 interior to the surface — heat which it presumably radiates into 

 space. 



The above treatment of the problem might be modilied by 

 taking into account this additional supply of heat. But the dis- 

 coveries of radio-activity, which require the modification, at the 

 same time afford, it appears to the writer, an alternative treatment 

 of the history of the earth which is more convincing. This treat- 

 ment consists in accepting the antiquity of the earth as found by 

 Professor Strutt.usmg a radio-active method, and in then examining 

 the heat of the earth in the light of that result. Every calculation 

 of the age of the earth assumes to some degree a uniformity of 

 present phenomena throughout the whole life of the earth, either 

 this uniformity is assumed in the rate of deposition of sediments,^ or 

 for the addition of sodium to the sea,^ or for the conduction of heat 

 by rocks, or finally the rate of accumulation of helium in minerals or 

 rocks. 3 Now of all these processes the last is the only one which is 

 not altered by temperature, pressure, or other physical conditions . 



Professor Strutt has concluded from a very refined determina- 

 tion of (1) the present rate of production of helium and (2) the 

 total accumulated helium in thorianite that it has taken 280 million 

 years for the helium to accumulate. As the earth is presumably 

 older than the mineral, this is a minimum age for the earth. 



The Heat of the Earth. — The question at once arises, how is it 

 that the temperature gradient of the crust is not less than it actually 

 is, for according to Lord Kelvin after only 40 million years the 

 gradient would have fallen to the present value. During the 

 remaining 240 million years it would have gone on decreasing, 

 and at any time would be inversely proportional to the square 

 root of the time. It becomes evident then that there is actually 

 a need for the heat supplied by the radio-activity of the crust 

 if all these deductions are to be reconciled. 



The Heat Stream from the Interior. — The heat stream from the 

 interior is that flowing through the Earth's surface layers. This is 



If=Wr^KdO/flr 



= 5-1 XlQi^x -004x1/3200 

 = 6-4x10" ca//sec. 



where r is the earth's radius, do/dr the temperature gradient of the 

 crust at the earth's surface and *: its thermal conductivity. We 

 must attempt to substitute such numerical values for the tempera- 

 ture gradient, and the conductivity as will give a correct result for 



1. Geikie, Brit. Ass. Rep., 1899. 



2. Joly, Brit. Ass. Rep. 



3. Strutt, Proc. Roy.Soc, 84. 379, 1910. 



