52 ILLINOIS ACADEMY OF SCIENCE. 



of inertia have become that of an electrical effect rather than 

 that of an inexplicable datum of experience; I say nothing could 

 be more unfortunate than for us to hastily conclude that scientific 

 truth is something transient, or to think that the facts of the 

 case are, as Heraclitus might say, "in a flux." 



The granite blocks that were put in position by Lavoisier and 

 Dalton, by Faraday and Helmholtz, by Joule and Maxwell, are 

 just as essentially a part of the foundations of physical science 

 as ever. 



Other stones have been added to the building by Roentgen, 

 Goldstein, Becquerel, the Curies, Thomson, Rutherford; but 

 practically nothing has been torn away from the structure erected 

 out of "knowledge which has acquired impersonal validity." 



II. RADIUM FROM THE ASTRONOMICAL POINT OF 



VIEW. 

 By Edwin B. Frost, 

 University of Chicago. 



When I was requested by Dr. Coulter to take part in this 

 symposium my reply to him was that my remarks would be as 

 brief as the chapter on "Snakes in Ireland." 



There is no direct evidence of radium or the other radio-active 

 metals, uranium and thorium, in the spectrum of the sun or in 

 the spectra of other celestial bodies. But it is proper that I 

 should explain why there are no snakes in Ireland. I must also 

 refer to the indirect evidence of the presence of radium inferred 

 from the abundance of helium in certain celestial spectra. 



The critical test for the presence of elements in celestial bodies 

 is given by their spectra. If all or most of the characteristic lines 

 of an element are found in the spectrum of a celestial body, we 

 feel justified in asserting with great positiveness that that ele- 

 ment is present. The converse, however, is not true : the absence 

 of the lines of a given element from a celestial spectrum does not 

 prove that the element may not be present in the body, but that 

 its spectrum is for some reason suppressed. Hence the fact that 

 no lines of the three elements referred to have yet been proven 

 to exist in celestial bodies by no means justifies us in stating that 

 those elements are not present in celestial objects. 



Professor Crew has shown you the spectrum of radium and 

 of the radium emanation in the slides that he has thrown upon 

 the screen. Although there were some inaccuracies in the deter- 



