358 Mr. Briggs on Distribution of Active Deposits of 



initially positively charged in air and hydrogen, 78*9 in 

 carbon dioxide, the remainder being neutral, and in ether 

 vapour all the deposit atoms were neutral. The term 

 " initial " refers to the instant when the recoil atom has 

 reached the end of its recoil path, and the values quoted 

 represent the fraction of the recoil atoms which possess- 

 a positive charge at the end of this path before either 

 volume or columnar recombination has had a chance to 

 become operative. The chief advantage of the type of 

 vessel used by Wellish is that a delicate test of the presence 

 of negative deposit atoms Cifn be made if the central 

 electrode is positive. In his first paper Henderson * de- 

 scribed the results of experiments on thorium active deposit. 

 The electrodes were parallel plates with a guard ring, and 

 were 3 cm. apart. In the air the cathode activities at 

 120 and 12,000 volts were 98*6 and 99'8 per cent. As the 

 latter value differed from 100 by less than the experimental 

 error, Henderson concluded that all the deposit atoms from 

 thorium emanation are initially positively charged in air. 

 When ether vapour was added to the air the percentage 

 decreased, becoming zero for pure ether vapour. In two 

 succeeding papers, however, Henderson t described expe- 

 riments on radium emanation, and concluded from the 

 results he obtained when using a new form of: parallel plate 

 exposure vessel and strong uniform electric fields, that all 

 the deposit particles from radium emanation in air, carbon 

 dioxide, and sulphur dioxide are positively charged. In 

 column D of Table I. are given values calculated from 

 those of Henderson, who expressed his results as the ratio 

 of the cathode activity to the total activity. 



As the values were still increasing at the highest potential 

 used, he concluded that there is no limit to the percentage 

 cathode activity, and hence that initially all the deposit 

 atoms are positively charged. 



II. Experiments with Radium Emanation in Strong Fields. 



Fig. 1 shows an apparatus designed by the author for 

 use with strong fields. A and B are two circular brass 

 i disks cemented to an ebonite ring P. The inner surface 

 of the ring was curved, as shown, to prevent sparking across 

 this surface, and it projected beyond the brass disks to 

 prevent brush discharge and sparking from their edges. 



* Henderson, Trans. Nova Scotian Inst. Sc. xiv. p. 1 (1914-15). 

 t Henderson, Trans. Nova Scotian Inst. Sc. xiv. p. 123 (1916); and. 

 Trans. Rov. Soc. Canada, x. p. 151 (1916). 



