October 28, 1904.] 



SCIENCE. 



547 



great as that of hard brittle pitch near the 

 freezing point of water. From the point 

 of view of modern physical chemistry and 

 in consideration of Professor Arrhenius's 

 opinions, the matter requires further con- 

 sideration. In particular it is most im- 

 portant to know whether the earth is sub- 

 stantially a crystalline solid or an amor- 

 phous substance, for many modern phys- 

 ical chemists consider amorphous matter 

 as liquid. This opinion is far fx'om being 

 established, however, and recent experi- 

 ments by Mr. Spring show that mere de- 

 formation at ordinary temperatures, at- 

 tended by only a very small absorption of 

 energy, sufSces to convert crystalline metals 

 into substances exhibiting characteristics of 

 amorphous bodies. Since Nordenskiold's 

 great discovery of large masses of ter- 

 restrial iron, or, rather, nickel steel, in 

 Greenland, and the wide distribution since 

 proved for similar metal imbedded in 

 igneous rocks, a great amount of evidence 

 has accumulated that a large part of the 

 earth is composed of material indistin- 

 guishable from that of metallic meteorites. 

 Meteoric iron is of course a highly crystal- 

 line material. 



It is a very striking fact that the mean 

 rigidity of the earth is about that of steel, 

 for the only substance likely to occur in 

 extensive continuous masses and display- 

 ing such rigidity at ordinary temperatures 

 and pressures is steel itself. Nevertheless, 

 the conclusion can not yet be drawn from 

 the resistance to deformation displayed by 

 the earth, that it is chiefly composed of 

 steel. Elastic resistance is known to be a 

 function both of pressure and of tempera- 

 ture, and until this function has been de- 

 termined by theory and experiment, the 

 bearing of the evaluation of rigidity by 

 tidal action can not be ascertained. 



Having shown the earth to be a solid 

 globe. Lord Kelvin calculated its age from 

 one of Fourier 's theorems, assuming for 



purposes of computation an initial tem- 

 perature of 7000° F. (nearly 3900° C.) 

 and that the thermal difEusivity of the 

 earth is that of average rock. These as- 

 sumptions, with the observation that the 

 temperature near the surface of the earth 

 increases at the rate of 1° F. for every 50 

 feet of depth, lead to an age of 98,000,000 

 years; but on account of the uncertainty 

 as to conductivities and specific heats in 

 the interior, the conclusion drawn by Lord 

 Kelvin was only that the time elapsed 

 since the inception of cooling is between 20 

 and 400 million years. 



Clarence King subsequently took a fur- 

 ther important step on the basis of data de- 

 termined at his request by Professor Carl 

 Barus on the volume changes which take 

 place in diabase during congelation, and 

 on the effects of pressure in modifying the 

 melting and solidifying points. Assuming 

 that the earth can never have had a crust 

 floating on a liquid layer of inferior den- 

 sity, computation leads him to 24 million 

 years as the maximum period for the time 

 since superficial consolidation was effected, 

 provided that the superficial temperature 

 gradient and conductivity are correctly 

 determined. 



These researches, together with Helm- 

 holtz's investigation on the age of the solar 

 system, which is incomplete for lack of 

 knowledge of the distribution of density in 

 the sun, have had a restraining influence 

 on the estimates drawn from sedimenta- 

 tion by geologists. Many and perhaps most 

 geologists now regard something less than 

 100 million years as sufficient for the de- 

 velopment of geological phenomena. Yet 

 the subject can not be regarded as settled 

 until our knowledge of conductivities is 

 more complete. An iron nucleus, for exam- 

 ple, would imply greater conductivity of 

 the interior and a higher age for the earth 

 than that computed by King, though prob- 

 ably well within the range explicitly al- 



