170 



C. Barus — Measurement of Fog Particles. 



by the X-rays are not also graded below a certain usually much 

 smaller maximum diameter, and, in general, to ascertain where 

 the nuclei originate. That this maximum diameter will 

 increase with the lapse of time allowed for coherence may be 

 inferred from my experiments with the steam jet : X-ray 

 nuclei will not act upon the steam tube unless a certain time is 

 allowed for growth, as I understand it. The coarse and 

 washed type of coronas obtained with nuclei produced by the 

 X-rays is evidence of graded size, while the fog particles, so 

 far as I have yet caught them, are of varied dimensions. In 

 these cases the X-rays reached the inside of the condensation 

 chamber through its waxed wood walls lined with wet cloth. 

 To obtain a fairly strong and large corona an exposure to the 

 rays lasting 5 to 10 minutes was needed, as the radiation was 

 not very intense. In this interval the original extremely 

 small nuclei are probably undergoing continuous growth, for 

 instance by cohering, so that on exhaustion particles of all 

 sizes* are revealed. In addition to the ragged coronas there is 

 copious rain. 



* Under these circumstances it seems reasonable that the time loss of 

 nuclei must at the outset be proportional to the square but finally to the 

 first power'of the number, assuming that eventually the large nuclei do most 

 of the catching. 



Brown University, Providence, E. I. 



