256 The Outline of Science 



ether, 1 and, as we shall see, these ether disturbances are all of the 

 same kind ; they only differ as regards wave-lengths. The X-rays 

 which Rontgen discovered, then, are light, but a variety of light 

 previously unknown to us; they are ether waves of very short 

 length. X-rays have proved of great value in many directions, 

 as all the world knows, but that we need not discuss at this point. 

 Let us see what followed Rontgen's discovery. 



While the world wondered at these marvels, the men of 

 science were eagerly following up the new clue to the mystery of 

 matter which w r as exercising the mind of Crookes and other in- 

 vestigators. In 1896 Becquerel brought us to the threshold of 

 the great discovery. 



Certain substances are phosphorescent they become lumi- 

 nous after they have been exposed to sunlight for some time, and 

 Becquerel was trying to find if any of these substances give rise 

 to X-rays. One day he chose a salt of the metal uranium. He 

 was going to see if, after exposing it to sunlight, he could photo- 

 graph a cross with it through an opaque substance. He wrapped 

 it up and laid it aside, to wait for the sun, but he found the 

 uranium salt did not wait for the sun. Some strong radiation 

 from it went through the opaque covering and made an impres- 

 sion of the cross upon the plate underneath. Light or darkness 

 was immaterial. The mysterious rays streamed night and day 

 from the salt. This was something new. Here was a substance 

 which appeared to be producing X-rays; the rays emitted by 

 uranium would penetrate the same opaque substances as the 

 X-rays discovered by Rontgen. 



Discovery of Radium 



Now, at the same time as many other investigators, Professor 

 Curie and his Polish wife took up the search. They decided to 



1 We refer throughout to the "ether" because, although modern theories dispense 

 largely with this conception, the theories of physics are so inextricably interwoven with 

 it that it is necessary, in an elementary exposition, to assume its existence. The modern 

 view will be explained later in the article on Einstein's Theory. 



