A HISTORY OF SCIENCE 



iant sparks or points can be seen. As the radium is 

 brought closer and closer these sparks increase in 

 number, until, as Sir William Crookes says, we seem 

 to be witnessing a bombardment of flying atoms hurled 

 from the radium against the surface of the blende. 

 A little instrument called a spinthariscope, devised by 

 Dr. Crookes and on sale at the instrument and optical- 

 goods shops, may be had for a trifling sum. It is fitted 

 with a lens focused upon a bit of Sidot's blende and 

 radium nitrate, and in a dark room shows these beau- 

 tiful scintillations " like a shower of stars." A still less 

 expensive but similar device is now made in the form 

 of a microscopic slide, to be used with the ordinary 

 lens. 



As we said a moment ago, radium appears to be an 

 elementary substance, as shown by its spark-spectrum 

 being different from that of any other known substance 

 the determinative test as fixed by the International 

 Chemical Congress. A particle of radium free from 

 impurities should, therefore, according to the conven- 

 tional conception of an element, remain unchanged and 

 unchangeable. If any such change did actually take 

 place it would mean that the conception of the Dal- 

 tonian atom as the ultimate particle of matter is 

 definitively challenged from a new direction. This is 

 precisely what has taken place. In July of 1903 Sir 

 William Ramsay and Mr. Soddy, in making some ex- 

 periments with radium, saw produced, apparently from 

 radium emanations, another quite different and dis- 

 tinct substance, the element helium. The report of 

 such a revolutionary phenomenon was naturally made 

 with scientific caution. Though the observation seem- 



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