JANUAEY 29, 1915] 



SCIENCE 



159 



evidence. The sub-continental temperature 

 is comparatively high; for the thermo- 

 metric gradient shows that at the level of 

 average sea bottom the rocks below the 

 continents have a temperature of about 

 100°, while at the bottom of the sea the 

 thermometer stands near zero. As for 

 crushing and jointing, recent experiments 

 in my laboratory show that the volume of 

 a brittle substance such as sulphur, con- 

 fined in a brass tube, may be increased to 

 the extent of more than six per cent, by 

 bending the tube. There can be no ques- 

 tion that rocks would behave in much the 

 same way under such confinement as that 

 to which deep-seated rocks are subject. 



If the average subcontinental mass down 

 to the compensation level had 3 per cent, 

 more voids than the sub-oceanic mass, this 

 would account for the present mean eleva- 

 tion of the land. The same result would 

 follow if the average temperature under the 

 continents were 40° higher than under the 

 ocean. Or again, the combination of 20° 

 excess of temperature and 1^ per cent. 

 excess of voids would account for the con- 

 tinents. 



If the areas occupied by the continents 

 were originally bounded by the same level 

 surface as the ocean bottoms, but possessed 

 a smaller conductivity, so that they cooled 

 more slowly, then it can be shown that the 

 earth would constitute an imperfect heat 

 engine and that abundant energy would 

 be available for crumpling and crushing 

 of the rocks or for the elevation of the 

 continents. 



Passing now to the recent developments 

 of radiology, that wonderful branch of 

 physics has very recently developed fresh 

 surprises. Rutherford has put forward a 

 nuclear theory of the atom, and van den 

 Broek has shown that the place of an ele- 

 ment in the periodic table is determined not 

 by its atomic weight, but by the number of 



positive electric charges carried by the nu- 

 cleus. This number of charges is known as 

 the atomic number. 



Now comes the astounding feature of the 

 subject. It has been definitely discovered 

 by Mr. Soddy, Sir Ernest Rutherford and 

 others that a single atomic number may be 

 borne by each of several substances which 

 may have different atomic weights and, in 

 the case of radioactive substances, differ- 

 ent stabilities, but which are inseparable by 

 ordinary chemical or physical properties. 

 They display the same chemical reactions, 

 the same electrochemical behavior, the 

 same spectrum, the same volatility. It 

 would appear, according to Rutherford, 

 that the charge on the nucleus is the funda- 

 mental constant which determines the phys- 

 ical and chemical properties of the atom. 

 Soddy calls the members of a group of ele- 

 ments bearing a single atomic number and 

 occupying therefore a single place in the 

 periodic table "isotopes." 



So far as lead is concerned, this revolu- 

 tionary doctrine has been authoritatively 

 confirmed by T. "W. Richards, who actually 

 finds the atomic weight of lead from urani- 

 nite deposits unmistakably lower than that 

 of ordinary lead. 



The discovery of isotopism sufficiently 

 explains the great discrepancies in the ages 

 of minerals as computed from the uranium- 

 helium ratio and the uranium-lead ratio. 

 These ratios also no longer seem adapted 

 to age determinations. It seems very pos- 

 sible, however, that the growing knowledge 

 of atomic structure may eventually lead to 

 trustworthy methods of age determination 

 from radioactive phenomena; but in the 

 meantime other methods must be resorted to. 



If the earth has cooled externally from 

 a high temperature, there must be a cer- 

 tain level at which the temperature of the 

 rock most closely approaches the melting 

 point at the prevailing pressure. This may 



