RADIO-ACTIVITY ' 165 



obtained could traverse certain substances opaque 

 to light — revealed the bones in his hand to the 

 man in the street. 



General attention was first directed to the 

 subject of radio-activity when in 1903 M. Curie 

 demonstrated that the stream of energy proceed- 

 ing constantly from the newly-discovered element 

 radium could be detected by a measurable rise of 

 temperature in a small quantity of the substance 

 protected from loss of heat. From then onwards, 

 an unbroken series of successful experimental 

 researches and brilliant theoretical generalisations 

 have together vastly extended our knowledge of 

 nature and revolutionised the outlook of physical 

 science. 



In this case also the essential phenomena have 

 been under investigation longer than is generally 

 known ; and their detection naturally arose from 

 a knowledge of the properties of Rontgen rays. 

 These rays produce fluorescent effects on suitable 

 screens ; and it was natural to examine phosphor- 

 escent and fluorescent substances, to determine if 

 they were the source of similar radiation. For 

 some time no definite results were obtained ; but, 

 in the year 1896, M. Henri Becquerel discovered 

 that compounds of the metal uranium, whether 

 phosphorescent or not, affected a photographic 

 plate through an opaque covering of black paper, 

 and rendered the air in their neighbourhood a 

 conductor of electricity. 



Such were the first observations on the 

 property of radio-activity ; but the rapid develop- 

 ment of the subject which has followed could only 

 have taken place with the aid of our previous 

 knowledge of the electrical properties of gases. 



