THE NEBULAR HYPOTHESIS 229 



If I interpret this quotation correctly, here is an in- 

 timation that if ever astrophysicists succeed in finding 

 even so much as an indubitable trace of radium existing 

 in the sun, the whole vexing problem of the source of the 

 solar heat will be labeled ' i solved ' ' and put away on the 

 shelf for keeps. But the radium hypothesis can never 

 solve this problem finally, no matter if the sun be proved 

 to contain any desired percentage of radium, or radio- 

 active materials. Even if it successfully disposed of the 

 quantitative question, it would still leave all the qualita- 

 tive difficulties untouched. But it does not by any means 

 measure up to the quantitative requirements. Professor 

 Joseph Barrell, in a recent Bulletin of the Geological 

 Society of America, says : 



But the sun is only one of the numberless host of stars, and 

 the source of its energy is a far-reaching cosmic problem. To 

 warm the earth through the vast length of geologic time, gravi- 

 tational condensation of the solar mass is found to be totally in- 

 adequate. The energy supplied by the atomic degeneration of 

 uranium and thorium would have ample endurance in time ; but, 

 even if the sun were composed entirely of these elements, their 

 decay could not supply the quantity of energy which is daily 

 expended. Geologic time brings to light, consequently, the evi- 

 dence of unknown sources of energy, cosmic forces which must 

 constitute a fundamental factor in any satisfactory hypothesis 

 of stellar evolution, a factor which has not as yet been taken 

 into full consideration in its bearings. Even if, as a lesser diffi- 

 culty, it should be sought to deny the validity of the radioactive 

 measurements of the earth's age, escape cannot be had from 

 this conclusion, for various lines of purely geological evidence 

 indicate an age many times greater than that which could be 

 granted if the solar energy were due simply to contraction of the 

 sun's mass. The depths of geological time leave us face to face 

 with the unknown. 



One of the most baffling problems of the geologists 

 is to explain the now well-established periodical recur- 

 rence of ice ages throughout geological history. As to 

 these I quote from the pen of Professor T. C. Chamber- 

 lin of the University of Chicago. He says (Origin of the 

 Earth, p. 4) : 



But this theory of a simple decline from a fiery origin to a 

 frigid end, from a thick blanket of warm air to a thin sheet of 



