408 Mr. V. E. Pound on the Secondary Rays 



at the temperature of 23° C. the secondary radiation excited 

 by the « 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 show 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 was the same at the two temperatures. When the 

 carbon was at — 78° C, however, the secondary radiation 

 from it amounted to — 151, t. e. an increase of about 24 per 

 cent, 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 in- 

 crease of 87 per cent, above the value at room temperature. 

 These large increases in the secondary radiation when the 

 carbon was at the lower temperatures is readily accounted 

 for by the increase in the secondary radiation from the 

 gaseous air layer at the surface of the carbon, which increases 

 in density as the temperature is lowered. 



In order to find out whether this same effect would occur 

 when the carbon was in other gases than air experiments 

 were also tried with oxygen and hydrogen. A fresh piece 

 of carbon was used for each gas. The air which was occluded 

 in the fresh piece of carbon on account of its being in an 

 atmosphere o£ air was got rid of by putting the carbon in 

 the apparatus, and exhausting and leaving it for a long time. 

 The apparatus was then filled with the gas which was to be 

 experimented upon, and left for some time in order that the 

 carbon might take up the new gas as much as possible. 

 After this the carbon was cooled or heated to the desired 

 temperature, the gas was pumped from the apparatus, and a 

 series of readings was taken, in the manner indicated above, 

 of the rate of charging of the polonium, both with and without 

 the magnetic field. The same characteristic results were 

 obtained with these gases at the different temperatures as 

 with the air. The readings taken with the carbon at tempe- 

 ratures 115° C, 23° C.,'-78° C., and -192° C, in an 

 atmosphere of oxygen are given in Tables VI., VII., VIII., 

 and IX. below, and the readings taken with the carbon at 

 temperatures 24° C. and —192° 0. in an atmosphere of hydro- 

 gen are given in Tables X. and XI. 



