60 ILLINOIS ACADEMY OF SCIENCE. 
tions. None of the parent radioactive metals is known to occur 
in the earth in a native state. In the form of compounds they 
have become widely distributed over the face of the globe in the 
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 
definite 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 
probable, but, except in the form of meteorites whose content 
appears from the few tests made to be relatively meager,^ 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. 
The special distribution of the radioactive substances among 
"Strutt, Proc. Roy. Soc. 77A, p. 480. 
