54 
PROFESSOR E. RUTHERFORD AND MR. R. K. McCLUNG 
In the case of uranium it has been shown (loc. cit.) that apparently two types of 
radiation are emitted, one of which is readily absorbed in air. The ionization due to 
the more j)enetrating rays is in general a small part of the total, especially for thin 
layers of uranium ; so that in the present calculation we will only consider the 
energy given out by uranium in the production of the more absorbed type of 
radiation. 
The intensity of the radiation emitted from uranium falls to half its value after 
passing through 4‘3 millims. {loc. cit., p. 128). Therefore X = 1‘6. 
Since d = 4'1, therefore e~^'^ is small and may be neglected. 
Thus the energy given out into the air is — W. Now for a thick layer of uranium 
oxide (3‘6 grammes spread over a surface of 38 scp centims.) the current i — 'Oolo 
electrostatic unit. Thus the energy emitted per unit area of uranium surface 
per second 
•0515 
= X 1-90 X 10“i° = -0004 erg 
o'o X 10 X 38 ° 
= 10"^^ calorie per second, approximately. 
This amount of energy wmuld suffice to raise 1 cub. centim. of water 1° C., assuming 
no radiation of lieat, in about 3000 years. 
It is a difficult matter to determine the total energy given off in the radiation by a 
given weight of uranium on account of the ease with which the radiation is probably 
absorbed by the heavy metal uranium itself in its jDassage through it. Some experi¬ 
ments were made on the current due to a given surface of uranium oxide when 
different depths of the active material were spread over it. The following are some 
of the results :— 
Surface of Uranium Oxide = 38 sq. centims. 
Weight of uranium 
oxide in grammes. 
Current in E.S. units 
per second. 
•138 
•0201 
•365 
•0365 
•718 
■0471 
1-33 
•0515 
3-63 
•0560 
Tlie uranium oxide in the form of a fine powder was dusted on uniformly by means 
of a fine wire gauze. The results show that the current per gramme of uranium 
oxide is greater for small than for large thicknesses. Even with a very thin layer of 
uranium oxide in tlie form of powder it is probable that a large proportion of the 
