i86 Transactions of the Canadian Institute. [vol. ix 



bered that carbon occludes more air at a low temperature than at a high 

 and that the a raj/s excite secondary in the air layer on the surface of 

 the carbon as well as in the carbon itself.* When the temperature of 

 the carbon is high the air readily comes away from the surface of the car- 

 bon and hence the secondary radiation decreases. When the temper- 

 ature of the carbon is low the air does not come away but becomes denser 

 and denser at the surface of the carbon as it oozes out of the interior and 

 also as it comes out of the cooled brass walls of the apparatus. (It will 

 be shewn later that air is expelled from brass as it is cooled.) There- 

 fore instead of a decrease there is set up a gradual increase in the sec- 

 ondary radiation as the air layer becomes denser. 



The final value of the secondary radiation from the carbon and the 

 air layer at its surface was obtained in each case by taking the difference 

 between the constant rates of charging of the polonium without and with 

 the applied magnetic field. In the following table the values of the sec- 

 ondary radiation with the carbon at the several different temperatures 

 are set down. 



Table V. — Fresh Carbon in Air. 

 Secondary radiation from Carbon and Air Layer. 



Temperature of Carbon .. . iio°C 23°C -yS^C -I92°C 



Secondary Radiation -124.5 -120.6 -151 -228 



It will be seen from the table that when the carbon was at the temp- 

 erature of 23° C the secondary radiation excited by the a rays was 

 -120.6 and when the carbon was at 110° C the secondary radiation was 

 -124.5. These numbers are very nearly the same and accounting for 

 their difference by experimental error, they shew that the amount 

 of a ray excited secondary radiation from carbon was the same when 

 the carbon was at 23° C as when it was at 110° C. Since this secondary 

 radiation most probably comes from both the carbon and its air layer, 

 the coincident values of the secondary radiation from carbon at the' two 

 temperatures would indicate that the density of the gaseous layer at 

 the surface of the carbon v/as the same at the two temperatures. When 

 the carbon was at -78° C however, the secondary radiation from it 

 amounted to -151 i.e., an increase of about 24% over the value of the 

 secondary radiation when the carbon was at room temperature. Again 

 when the carbon was at -192° C the secondary radiation was -228 or an 

 increase of 87% above the value at room temperature. These large 



*V. E. Pound, Trans. Canadian Institute, 1912. 

 Phil. Mag. 1912. 



