Sir J. J. Thomson, Sir E. Rutherford 183 



During the last few weeks of the year 1896 some remark- 

 able experiments of W. C. Roentgen revealed to us a new 

 and quite unexpected class of phenomena. The electric 

 discharges in a highly-exhausted vessel were found to be 

 capable of generating a radiation now known to be due 

 to very short waves which could penetrate many bodies 

 opaque to ordinary light. This was the X-radiation which 

 has proved to be of such enormous value in surgery. Their 

 investigation indirectly led to our knowledge of a still more 

 remarkable class of phenomena. The French physicist, 

 Becquerel, while trying to find whether the sun emitted 

 X-rays, observed a most surprising effect, which could only 

 be accounted for by assuming the existence of a new form 

 of radiation, essentially different from that of the X-rays. 

 Separating the substance that was mainly responsible for it, 

 M. and Mme. Curie discovered the new metal radium. This 

 is the typical radio-active element, but two other known 

 chemical elements uranium and thorium proved to resemble 

 radium in its peculiar properties. A new science then opened 

 out. 



The effects of radio-activity show themselves by then* 

 power of ionizing air and affecting photographic plates, but 

 the first results were extremely puzzling, and experimenters 

 were being led away on a wrong track when Sir Ernest 

 Rutherford took up the work. He first discovered that 

 thorium and radium gave up gases the so-called emana- 

 tions which themselves were radio-active. It was the 

 disturbing effect of these gases which, diffusing through 

 the air of the laboratory, had affected the instruments, and 

 led Becquerel and Curie astray; it had to be separated 

 from that of the parent substance before the different 

 phenomena could be disentangled. By a series of remarkable 

 experiments, Rutherford soon cleared up the essential features 

 of radio-activity. In conjunction with Frederick Soddy he 

 then developed his theory, which now stands on a firm 

 basis. Radio-activity was shown to be the result of the 

 ejection of corpuscles from the parent body, which thereby 

 became transformed into another substance which was 

 generally itself subject to further decomposition through the 

 emission of other corpuscles. The decomposition proceeds 



