o8 The Philippine Journal of Science i9u 



thorium-decay products in the open air, has been of great utility 

 in the examination of the radioactivity of the atmosphere. 

 Nevertheless, its adequacy for the absolute determination of 

 the amount of emanation present is questionable, for the amount 

 of active deposit collected by a charged wire undoubtedly de- 

 pends upon other factors in addition to the amount of emanation 

 present. Rutherford " states : 



It has generally been supposed that only the positively charged atoms 

 of radium A are collected on the wire, and that they travel in an electric 

 field at the same rate as the positive ions. Account has to be taken 

 of the rate of re-combination of the charged atoms of radium A with 

 the nejjative ions of the air, for only those atoms reach the wire which 

 retain their charpre. The constant of re-combination is no doubt affected 

 hy atmospheric conditions, and the number of nuclei present. 



At high elevations the air, owing to its reduced density, should 

 cause a smaller resistance to the motion of the charged particles, 

 so they should be swept in more rapidly and from a greater 

 distance than at a lower level. Therefore, the amount of active 

 deposit collected should increase with altitude even if the ema- 

 nation remained constant. Saake • found that the active deposit 

 at Arosa, elevation 1,800 meters, was three times as great as 

 that of Wolfenbiittel. Other observers have obt-ained similar 

 results. That this is not due to an increase in the emanation 

 content is indicated by the determinations of Wright and Smith," 

 who made an investigation of the variation of the radium emana- 

 tion with altitude by the charcoal absorption method and found 

 that the emanation content at Manila, sea level, was approx- 

 imately four times that obtained on Mount Pauai, elevation 2,460 

 meters. 



Many observers have found that, for a given locality, the 

 amount of deposit collected depends upon the direction of the 

 wind, but there has been considerable disagreement as to the 

 effect of the variation of the wind velocity. Kinoshita, S. 

 Nishikawa, and S. Ono ' deduced the lines of flow of the charged 

 particles for different wind velocities from theoretical considera- 

 tions, and found that, above a critical velocity, the variation 

 should not affect the amount collected. 



The diurnal variation of the active deposit has been the subject 



* Radioactive sub.stances and their radiations. Cambridge University 

 Press (1913). 



' Phys. Zcitschr. (1903), 4, 426. 



• This Journal Sec. A (1914), 9, 51. 

 ' PhU. Mafj. (1911), 22, 821. 



