156 ESSAYS BIOGRAPHICAL AND CHEMICAL 



decomposed by heat. Indeed, some compounds are so un- 

 stable that they decompose at the ordinary temperature, 

 not into their elements, it is true, but into other com- 

 pounds of carbon, hydrogen and oxygen. Such compounds 

 are stable only at a low temperature, and the higher the 

 temperature the more readily they decompose. Judging 

 by analogy, we should expect elements of high atomic 

 weight to show tendency to decomposition, granting, of 

 course, that any element at all is capable of decomposing. 

 Now among the three elements of highest atomic weight 

 known is radium, an element belonging to the barium 

 column, of which the atomic weight is 226. This remark- 

 able substance exists in a mineral named pitchblende, 

 an oxide of uranium ; its discovery by Madame Curie, of 

 Paris, is one of the most remarkable of recent events in 

 chemical history. 



The second element of high atomic weight is thorium 

 (232'5). It was noticed by Dr. Schmidt, and indepen- 

 dently by Professor Rutherford, of Montreal, that if air 

 was passed over a salt of thorium, or bubbled through its 

 solution, it carried with it an ' emanation ' which possessed 

 for a short time the power of discharging an electroscope. 

 Radium salts also give off such an emanation, or gas, 

 which, however, retains its properties for more days than 

 the thorium gas does for minutes. Uranium, the chief 

 constituent of pitchblende, too, has also the power of 

 discharging an electro^ope, but it gives off no emana- 

 tion. Its. atomic weight is 239'5 : it is the highest 

 known. 



The gases evolved from compounds of thorium and 

 radium can be condensed to solid or liquid by passing 

 them through a tube cooled with liquid air. But they 

 are present in such excessively minute quantity that they 

 have never been seen, even as a minute drop. They are 

 as inert as argon, and they are members of that group of 



