22 SCIENCE PROGRESS 



such a depth that at its base pressure and temperature would 

 attain certain critical values. Below that zone radioactive 

 processes would be inhibited by the excessive physical con- 

 ditions. Only in the outer shell would radioactive matter be 

 allowed to decay, and consequently only the rocks within that 

 shell could be appealed to as an active source of radiothermal 

 energy. 



It is clearly of the greatest importance to geologists to 

 decide between these alternative but not mutually exclusive 

 views. Unfortunately it is impossible to come to a securely 

 founded conclusion, but such evidence as is now available may 

 profitably be reviewed. Let us take first the possibility of 

 radioactive inhibition. The suggestion is based on the well- 

 known reaction law of Le Chatelier, which states that the 

 internal reactions within a material system are such as will 

 tend to diminish the effect of any external influences by which 

 its equilibrium may be disturbed. Thus, under a rising tem- 

 perature, elements change their state or form new compounds 

 in such a way as to absorb energy, and so oppose the tendency 

 of the temperature to increase further. Similarly, under high 

 pressure, the atoms of a compound rearrange themselves so 

 that the molecular volume is reduced, and the ultimate stresses 

 by which the system would have been constrained are therefore 

 also reduced. 



Dr. F. C. S. Schiller 1 suggests that uranium does not dis- 

 integrate in the earth's deep interior, or does so more slowly 

 than near the surface, and he thinks that radioactivity may 

 be an acquired habit of the substances that exhibit it. Dr. Leigh 

 Fermor 2 points out that the change from uranium to radium, 

 resulting as it does in an emission of energy, and, presumably, 

 an increase in atomic volume, is the kind of action which would 

 be inhibited by high pressure and temperature. Mr. H. S. 

 Shelton 3 is not content with inhibition only; he postulates 

 complete reversal. He thinks that " radioactive substances, 

 particularly uranium compounds, are synthesised from other 

 elements as a result of the conditions of great temperature and 

 pressure found in the earth's interior." This idea was originally 

 due to Dr. Barrell, 4 and has also been held by no less an 



1 Nature, June 26, 1913, p. 424. 2 Nature, July io, 1913, p. 476. 



3 Science Progress, No. 31, p. 456, 1914. 



4 Rutherford, Radioactive Transformations, p. 194, 1904. 



