NOTE ON EPEIROGENY 189 



ficial transfer of material combined with a deep-seated solid flow in the 

 nature of an undertow would establish a system analogous to incipient 

 convective circulation in a mass of hyperviscous liquid. Such an un- 

 equally heated globe reduces to a species of heat engine. The outer layer 

 down to about the level of isostatic compensation takes in heat energy of 

 very high temperature, discharging it at little above zero, while a part 

 of the energy thus rendered available is converted into the mechanical 

 work implied in uplift (partly balanced by erosion) rupture and plica- 

 tion of the continents. The prominence of the continents above sea- 

 bottom indicates that the mean density of the subcontinental columns of 

 rock down to the level of compensation is lower than the density of the 

 suboceanic columns. This difference might be due to a moderate excess 

 in the proportion of voids beneath the continents (about 3 per cent), or 

 to an excess in mean temperature (some hundreds of degrees), or to any 

 appropriate combination of the two causes. The general features of the 

 dynamical system resulting in isostasy thus become intelligible if the one 

 simple postulate of non-uniform conductivity he accepted.-" 



Eecent Advances in EADiOLoay 



Only a few years since one of the most remarkable features of radiology 

 was the extreme simplicity of the known relations between the elements 

 of the uranium-radium group. It was known that after a moderate time 

 an equilibrium must be established between the various members of this 

 group, since each member could decay only as fast as it was generated, 

 and the law of decay was considered as absolutely established. This law 

 for a single element is expressed by 



where I„ is the initial intensity of radiation I, the intensity at time 

 t and X is the radioactive constant. Many experiments had been made 

 to ascertain whether the value of A was modified, for instance, by high 

 temperature or high pressure; but no evidence of variability was dis- 

 covered then and, for that matter, none has been detected up to the 

 present time. So confident have radiologists been of the constancy of A 

 that they have not hesitated to extrapolate the law of decay, verified for 

 a year or two, over millions or thousands of millions of years. 



When the law of decay had been established and the a particles had 

 been identified with helium, it became practicable In compute the Jimonrii 



"^ Itplvin discussed the restoration of mechanical energy from an unequally heated 

 space. Phil. Mag., vol. 5, 1853, p. 102. 



