NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



a small induction coil (4" spark) and 3 to 5 storage cells. Two filters 

 of solidly packed cotton were used, one 7 inches and the other 16 



inches long. 



They were about equally efficient. 



L 



FIG. i. Fog chamber, A B, with appurtenances and X-ray bulb, X. 



3. Vertical radiation at one end of the trough, entering through 

 WOOd. In the preliminary experiments the bulb was placed so as to 

 radiate into the trough in the position shown at R, and kept in action 

 5 minutes. The effect was then observed by condensation at the 

 pressure difference, 8/=iy cm., much below the fog limit of dust-free 

 air (section 6). Two results were noted : In the first place, while the 

 coronas obtained with the X-rays in bulky apparatus are usually of 

 the smaller or normal type, the coronas seen in this shallow apparatus 

 were often enormous, transcending the middle green-blue-purple 

 corona (nucleation, n= 100,000 per cubic centimeter). Even after two 

 or three subsequent exhaustions, filtered air being added prior to each, 

 large coronas were still in evidence. In the second place, the coronas, 

 and hence the nuclei, were observed chiefly on the A side of the 

 apparatus, under the bulb. Fearing that there might be some direct 

 effect due to induced high potentials, the X-ray bulb was raised 10 

 and 20 cm. above the trough, with results naturally smaller in magni- 

 tude, but of the same kind. The following data may be given: 



TABLET. Number of nuclei, n, in thousands per cm. 3 . 5/=i7cm. Temperature 



about 20. Angular aperture 4> = 5/30. 



* Immense, but too diffuse for measurement. 



In all cases the first coronas were accompanied by dense rain and 

 fogs, frequently in horizontal strata, so that sharp measurements of 



