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ORIGIN OF URANIUM AND THORIUM 125 



^ence of helium and lead, considered in connection with the absence of 

 uranium, is extremely suggestive. 



Meteors bring to the earth matter most of which, at all events, belongs 

 to the solar system ; it is in part of cometary origin and in all probability 

 resembles the raw material of which the earth is built up. In the table 

 the elements identified gravimetrically in meteorites are indicated by 

 hyphens. Of these a few are not found in the sun or stars, namely, 

 phosphorus, sulphur, chlorine, arsenic, antimony, and lithium. Meteor- 

 ites must reach the sun, and the absence of lithium lines from the solar 

 spectrum is surprising. Many of the elements of high atomic weight are 

 also absent from the meteorites, or are present in quantities so small as to 

 escape chemical analysis — all of those, in fact, whose atomic weights 

 exceed those of antimony (120). 



The plausibility of the evolution hypothesis is increased rather than 

 diminished by the spectroscopic investigations of the last 35 years, and 

 the relations of the cosmic distribution of elements to the periodic law 

 add force to the suggestion. There is room in the table for just one very 

 light, inert, or nullivalent element with an atomic weight below that of 

 hydrogen. Possibly this protylic gas may be coronium. Assuming its 

 existence, the facts known seem to point to an orderly and gradual devel- 

 opment of members of the higher groups and the higher series. 



The results of the electronic theory of matter add greatly to the force 

 of the suggestion, as is apparent from the conclusion of Mr J. J. Thomson 

 quoted above. 



Eadioactive phenomena strengthen the evidence immensely. Since it 

 is now an established fact that some atoms can be resolved into at least 

 two others, it can not be denied that under appropriate conditions the 

 process might be reversed. This is recognized by Mr Rutherford, who 

 considers it reasonable to suppose that under some circumstances heavy 

 atoms were built up from the lighter and more elementary substances.^' 



These several lines of evidence shift the burden of proof from those 

 who maintained the probability of evolution to the shoulders of those who 

 deny it. Uranium and thorium have not been detected in the stars or the 

 sun, even with the most powerful modem apparatus and under special 

 scrutiny; yet in the sim numerous allied elements very rare on the earth 

 have been identified, including cerium, lanthanum, neodymium, erbium, 

 and europium. It seems highly improbable that if uranium and thorium 

 existed in any quantity in the sun their spectra should fail to appear. 

 When the reasonableness of the evolution hypothesis is considered, this 



»» Radioactive transformations, p. 194. 



