676 T. C. CHAM BERLIN 



course of the surface changes it has undergone. Radioactive sub- 

 stances have freely entered into solution in the natural waters and 

 have thus been carried wherever the hydrosphere reaches, and in 

 turn they have been deposited therefrom. Their singular property 

 of passing spontaneously from certain states into gaseous forms 

 (emanations) and then back into the solid or liquid form, on defi- 

 nite time schedules, has caused them to be given forth freely into 

 the atmosphere, and, drifting in this, to be later precipitated in 

 the solid or liquid form, and this has naturally been dispersive 

 in an extreme degree. Radioactive matter is therefore found in 

 practically all the rocks of the surface of the earth, in practically 

 all the waters, and in practically all the atmosphere. 



But this highly diffusive distribution has not been uniform. 

 There have been special tendencies toward concentration running 

 hand in hand with the general tendencies to diffusion, and these 

 concentrative tendencies constitute a critical element in this dis- 

 cussion. 



So far as the accessible part of the earth is concerned, the 

 igneous rocks may be taken as the original source of the radio- 

 active substances. How the igneous rocks themselves came to 

 have their present content will be considered later. Whence the 

 radioactive substances came still more remotely is problematical. 

 There may be even now accessions of radioactive substances from 

 without the earth for aught that is known, and indeed this is prob- 

 able; but, except in the form of meteorites whose content appears, 

 from the few tests made, to be relatively meager, 1 such accessions 

 are not yet demonstrated. 



The cycle of distribution on the earth's surface is simple. From 

 the igneous rocks the radioactive substances are dissolved and 

 disseminated through the waters and carried wherever they go; 

 while from both the rocks and the waters the emanations are given 

 forth into the atmosphere. From the air and the waters in turn 

 the radioactive derivatives are reconcentrated into the earth, 

 except as their disintegration becomes complete and they pass 

 permanently, in the form of helium, into the atmosphere or are 

 lost from the atmosphere into the cosmic regions outside. 



1 Strutt, Proc. Roy. Soc, LXXVII A, 480. 



