NEW CONCEPTIONS IN SCIENCE 



cylinder, filled simply with dust-free but not water- 

 free air; when this was suddenly expanded, by 

 means of a piston being pulled out, so that the air 

 would occupy twenty or thirty times its original 

 volume, no effect was to be observed ; but if the 

 chamber was flooded by Rontgen rays, the expan- 

 sion produced a fog. If this fog was allowed to 

 settle and another expansion was produced there 

 often came another fog, and sometimes it took two 

 or three expansions before the fog failed to appear. 

 There could be no possible doubt that in some way 

 the fog was due directly and solely to the Rontgen 

 radiations. Not merely that, but there was a di- 

 rect relation between the time and the intensity of 

 the radiation and the amount of fog produced, so 

 that, for example, it was possible to permit a cer- 

 tain radiation and produce a certain degree of fog. 



All this brought remembrance of a fact noted by 

 the veteran Sir George Stokes, likewise of Cam- 

 bridge, that in the case of a fog there was a certain 

 definite relation between the size of the little drops 

 of water of which it is composed and the time it 

 takes a fog to settle. Knowing the rate of settling, 

 you may readily calculate the size of the drops. 

 Knowing the size of the drops, you may afterwards 

 collect the water deposited, weigh it, and then 

 calculate the number of the drops. 



The only possible explanation of the fog pro- 

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