carried by the Ions produced by Rontgen Rays. 533 



deposited by an expansion of T35 is 4*74 x 10~ 6 gram. If N 

 is the number of ions per cub. centim. in the first case when 

 the rays are on, M the number when the rays are off, a the 

 radius of the drops when the rays are on, b the radius when 

 the rays are off, Q the quantity of water deposited : 



The rate of fall varies as the square of the radius of the drops, 

 so that 



a _ y/TJQ 



If dashed letters refer to the second expansion, 



N / fW 3 = M'f7r& /3 =Q / , 

 so that 



Q i?-sJ 



N-M 

 N 7 — Iff 



Q '{a /3 ~~ vA 



4-94 U 9 V19-10 Vioj- 

 "4-74 {14 Vl4-4v/4j- 



= 1*2 approximately. 



Thus the number of the ions produced by the rays which 

 are caught by the larger expansion is slightly greater than 

 that caught by the former. I think that the greater rapidity with 

 which the larger expansions are made, in consequence of the 

 greater time the driving force acts on the piston whose motion 

 produces the expansion, is sufficient to account for this ; for 

 when the expansion is slow the drops first formed can grow 

 before the expansion is completed, and thus rob the others of 

 the water-vapour, so that we should expect to get slightly 

 more drops as we increased the rapidity of the expansion. 



Some experiments made with smaller expansions seemed 

 rather to indicate a considerable increase in the number of 

 ions deposited when the expansion was taken from below 1*3 

 to ahove it. An increase which seemed rather too large to 

 be attributed wholly to the increased velocity of expansion, 

 and to suggest that the ions had not all the same power of 

 acting as nuclei. I hope to make an independent investigation 

 of this point, as it is evidently one which might have con- 

 siderable bearing on the problems of atmospheric electricity ; 

 for if the negative ions, say, were to differ in their power of 

 condensing water around them from the positive, then we 



