84 CHEMICAL PHYSICS. 



and have a decided physiological action on various parts of the 

 human body. 



Radio-activity. In 1896 the French scientist Becquerel dis- 

 covered that the metal uranium and its compounds exert sponta- 

 neously and continuously a certain influence upon their surroundings. 

 This influence was found to be due to rays (now called Becquerel 

 rays) of a peculiar kind. They act upon the photographic plate ; they 

 pass through black paper and metals ; they render the gases through 

 which they pass conductors of electricity ; they cannot be reflected or 

 refracted. Any substance exhibiting the power of sending out such 

 rays is said to be radio-active. Besides uranium and its compounds 

 those of thorium were found to possess the same properties. 



The subject of radio-activity was next studied by Madam Curie, 

 who showed that certain minerals (such as pitch-blende) contain- 

 ing uranium are more radio-active than uranium itself, and in the 

 course of her investigation demonstrated that in uranium ores are 

 several substances which show radio-activity to a remarkable extent. 



The work, carried yet farther by a number of scientists, has 

 brought out the following results : Three substances, named polonium, 

 actinium, and radium, have been obtained from pitch-blende and 

 show a radio-activity a million times greater than that of uranium 

 or thorium. 



Of these three substances at least one the radium has been posi- 

 tively proven to be an elementary substance of metallic nature form- 

 ing well-defined salts, such as radium chloride and bromide. Radium 

 salts show all the previously mentioned properties of Becquerel rays, 

 but in addition they exhibit some other features. 



Thus, radium salts are permanently luminous and render luminous 

 (or phosphorescent) for a shorter or longer period a great number of 

 other substances. The most sensitive are barium platinocyanide, zinc 

 sulphide, diamond, etc. Not only do radium salts impart luminosity 

 to other substances, but these salts communicate little by little their 

 radio-active properties to substances in their neighborhood, and these 

 in turn emit Becquerel rays for some time. 



Another startling discovery was made when it was shown that 

 radium salts develop heat continuously, and consequently are in 

 themselves warmer than their surroundings. A small flask containing 

 0.7 gramme of radium bromide shows a temperature of 3 C. higher 

 than the surrounding air. Calorimetric measurements have shown 

 that radium bromide evolves enough heat to convert per hour its own 



