EMANATIONS. 



117 



may be condensed from the air with which it is mixed by 

 the action of extreme cold. The radium emanation con- 

 denses on the glass walls of the vessels into a brilliant patch 

 of phosphorescent light at 150 degrees below zero, Centi- 

 grade. If, therefore, it behaves in all these respects like an 

 ordinary gas the reader may wonder why it should not be 

 definitely called a gas, but the difficulty of it is that the 

 emanation exists in so minute a quantity that it has not 

 yet been brought within the range of the spectroscope or 

 the balance. The emanation has been detected, and in- 

 vestigated by the electroscope [Figs. 38 (a) and 38 (6)], which 



Fig. 38 (a). 



Curie's Electroscope for the 

 study of radio-activity. 



Fig. 38 (6). 



Diagrammatic representation of 

 Fig. 38 (a). 



An electroscope will detect one one-millionth of a millionth of a milligram. 



measures the radium rays by the power to discharge its 

 electrified gold leaves. The electroscope is about a mil- r? p 

 lion times more sensitive than the most sensitive spectro- 

 scope and yet the spectroscope is capable of detecting easily 

 the millionth part of a milligram of matter. So it is that 

 history repeats itself. The spectroscope, when placed new 



