ATMOSPHERIC ELECTRICITY 313 



can hardly disengage any great quantity of radium emanation), 

 the variations in radium content were not large, but of the same 

 order of magnitude as observed over the sea. 



(b) 3. Natural ionisation in closed vessels. 



The paper before mentioned contains the results obtained 

 on the voyage of the Terra Nova from England to New Zea- 

 land, of which the section on Natural Ionisation may be sum- 

 marised 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 diffusely distributed 

 in the atmosphere to have any direct effect on the natural ionisa- 

 tion, and only become operative when deposited in the neighbour- 

 hood of the experimental station by precipitation, or by the 

 earth's electric field (potential gradient). 



(iii) There exists a minimum value to this natural ionisation 

 (about 4 ions per c.c. per sec.) which has not by any method been 

 reduced in value. This minimum would seem to be independent 

 of the size or material of the 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 see 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 in the Antarctic (carried on in an ice cave at con- 

 stant temperature) gave a value very slightly lower than that 

 found over the sea, and showed, with a self-recording instrument, 

 no variations greater than the probable errors of observation. 



For further details of the above, and in respect to measure- 

 ments of the ionisation of the air, the reader is referred to the 

 paper before mentioned. 



Samples of sea water were also collected from various depths 

 for radium analysis, and will be worked out by Prof. Joly, but the 

 results are not yet available. 



(c) Pendulum observations. 



It will be known to the general reader that the weight of any 

 substance as measured by the pull of the earth upon it is not an 



