8o SCIENCE PROGRESS 



standardised against the first. A mixture of hydrogen and 

 emanation was then introduced into the hollow space in the 

 bulb. The two thermometers were wrapped in cotton-wool 

 and placed in silvered vacuum vessels ; daily readings were 

 taken for three weeks. The difference was 0*52° on the first 

 day, rose to 073° on the second, then slowly grew less ; the 

 thermometers registered the same temperature at the end of 

 a fortnight. 



Pegram has shown that thorium also evolves a very small 

 quantity of heat. 



It has been shown by various experimenters that radium 

 is scattered throughout the earth in minute quantity, though 

 sufficiently large to account for the natural ionisation of the 

 air. Some interesting speculations have been put forward by 

 G. H. Darwin, Rutherford, Strutt, and others, connecting this 

 universality of radioactive matter, each minute particle of which 

 evolves a definite amount of heat, with the total heat of the 

 earth. It has been shown that if radioactive matter exists in the 

 same proportion throughout the earth as it does at the surface, 

 then the heat it evolves is far more than is required to maintain 

 the earth's present temperature. Consequently, various hypo- 

 theses have been suggested to show that radioactive matter 

 may be non-existent, or unchanging, in the earth's interior. 

 It also follows that the age of the earth may be much greater 

 than 100,000,000 years, the figure calculated by Lord Kelvin 

 from the observed temperature gradient at the earth's surface. 

 Provided similar radioactive changes are taking place in the 

 sun, it will emit heat at its present rate for a period many 

 times greater than that obtained by calculations based on 

 dynamical data. 



The duration of the life of radium has been estimated in two 

 ways. Rutherford has calculated it from an estimation of the 

 number of atoms of radium disintegrating in one second. He 

 has shown experimentally that 6'2 x io 10 a particles are expelled 

 per second. From considerations of the heat effect he concluded 

 that each atom in disintegrating emits one a particle. Hence 

 6*2 x io 10 atoms of radium break up per second. It has also 

 been shown that one cubic centimetre of hydrogen at o° C. 

 and 760 mm. pressure contains 3*6 x io 19 molecules. Hence the 

 number of atoms of radium considered as a monatomic gas under 

 the same conditions can be calculated ; if its atomic weight is 



