RELATIONS TO OTHER SCIENCES 85 



stances a rise in temperature of ten degrees or a one per cent increase 

 in temperature may be expected to produce a one per cent increase 

 in the kinetic pressure at the original volume; but as the external 

 pressure is constant and the cohesion is insignificant, we may expect 

 a one per cent increase in the volume in which the molecular motions 

 take place or an increase in the mean distance between molecules of 

 one third of one per cent. Such an expansion will be accompanied 

 by slightly lessened cohesive force, less rigidity, and less viscosity, 

 probably; but nothing like a sudden change of state is suggested. 

 The fact that at pressures greater than the critical pressures there 

 can be observed no sharp transition from the liquid to the gaseous 

 state with rise of temperature is quite in accord with the above con- 

 siderations, and it seems probable that in case of solids under great 

 pressure nothing like melting will be observed, but rather a gradual 

 loss of rigidity or transition to great viscosity, and that the viscosity 

 will decrease steadily with rise in temperature. 



But a new aspect is now given to the problem of the age of the 

 earth by the discovery of radioactivity and its attendant phe- 

 nomena. The earth, instead of being thought of as a cooling body, 

 is now conceived as having within itself a source of almost un- 

 limited energy. Locked up in each atom is believed to be a store of 

 energy so vast that the breaking down of comparatively few of them 

 in the radioactive process will supply the known outflow of heat 

 from the earth. 



Rutherford has shown that the observed dissemination of radio- 

 active substances in the earth's crust is probably sufficient to ac- 

 count for the outflow of energy from its surface. Thus the method 

 of estimating the age of the earth from the consideration of it as a 

 cooling body, a method which until lately seemed to physicists to be 

 based on essentially sound premises, and deserving of confidence 

 because of its greater simplicity as compared with the methods by 

 which geological and biological estimates are obtained, is now by the 

 very progress of physics itself abandoned as unreliable. 



So also has the study of radioactivity thrown new light on the 

 question of the maintenance of the sun's heat. It is now seen 

 that possible atomic transformations accompanied by the liber- 

 ation of the vast stores of energy locked up within the atoms of 

 matter may permit an enormous extension of the time during 

 which the sun may have been radiating with something like its 

 present intensity. 



In conclusion it may be remarked that a new world is opened 

 to the investigator by the discovery of radioactivity. The atoms 

 of matter are no longer thought of as necessarily fixed and un- 

 changeable. Besides the older problems of matter questions now 

 arise as to evidences of atomic disintegration and change from 



