304 Mr. C. S. Wright on Variations in the 



4. Preliminary Observations on Conductivity. 

 (a) General Conditions of the Different Experiments. 



It has been noted by Prof. McLennan and other observers 

 that when a metallic receiver has been thoroughly scoured 

 with emery- or glass-paper, in Order to remove any active 

 coating which may have been deposited on its surface by 

 exposure to the atmosphere, and after being washed with 

 hydrochloric acid, ammonia, alcohol, and distilled water, 

 then filled with freshly filtered air and allowed to stand, the 

 ionization of the enclosed air gradually increases for some days, 

 and ultimately reaches a steady value. Owing to this effect 

 it was found necessary, in comparisons of the ionizing power 

 of radiations existing in any two localities or under any two con- 

 ditions, to make the observations with the ionization chamber 

 under precisely the same conditions in both circumstances. 



The different comparisons were made, therefore, either 

 with a receiver freshly cleaned and freshly filled with filtered 

 air immediately before taking the observations, or else with 

 a receiver containing air which had been allowed to remain 

 in it till the steady state had been reached. 



Further, as it was impossible always to obtain observations 

 at different times with the atmospheric conditions the same 

 as to pressure and temperature, it was assumed that the 

 ionization obtained in all localities would vary directly with 

 the density of the air in the receiver, and in making any 

 reductions which were necessary in order to reach values 

 which were comparable, this assumption, which is amply 

 warranted by the measurements of McLennan and Burton % 

 on the ionization of air at different pressures, has been 

 adopted. 



In this connexion it may be well to emphasize the extreme 

 importance of taking every precaution, in making observa- 

 tions such as are described in the present paper, to secure 

 absolute uniformity in the conditions of the measuring 

 receivers. With the different receivers used in the present 

 investigation, it was found that when the cylinders were 

 thoroughly scoured and washed in the manner described 

 above, the conductivity of freshly filtered air admitted into 

 the chamber was always the same at any particular observing 

 station, and thus by always working under these definite 

 conditions it was possible to obtain very definite results. 



Numerous investigators in this field of research have ex- 

 perienced considerable difficulty in arriving at concordant 



* McLennan and Burton, Phys. Rev. 3. 1903. 



