ATMOSPHERIC ELECTRICITY 
4S5 
air during the voyage of the Terra Nova, As niiglit be 
expected (dry frozen surfaces can hardly disengage any 
great quantity of radium emanation), the variations 
in radium content were not large, but of tlic same order 
of magnitude as observed over the sea. 
{b) 3. Natural ionisation in closed vessels. 
The paper before mentioned contains the results ob- 
tained on the voyage of the Terra Nova from England to 
New Zealand, of which the section on Natural Ionisation 
may be summarised as follows : 
(i) Variations in natural ionisation are due primarily 
to varying amounts of radioactive products in the air 
(disengaged, chiefly at least, from land surfaces). 
(ii) These radioactive products are too difTusely dis- 
tributed in the atmosphere to have any direct effect on 
the natural ionisation, and only become operative when 
deposited in the neighbourJiood of the experimental 
station by precipitation, or by the earth's electric field 
(potential gradient). 
(iii) Tiiere exists a minimum value to this natural 
ionisation (about 4 ions per c.c. per sec.) wJiich has 
not by any method been reduced in value. This minimum 
would seem to be independent of the si/e or material of 
retaining vessel, and may therefore be best ascribed to a 
spontaneous breakdown of the enclosed gas, very similar 
to the spontaneous breakdown of radioactive substances. 
From the above one can sec that in places where the 
radium content of the air is very small (as over the sea), 
the variations in natural ionisation will also be small. 
Further work on the minimum value m the Antarctic 
