EFFICIENCY OF FOG CHAMBER. 41 



view to which some probability may be assigned. In the case of intense 

 radiation relatively large and possibly persistent nuclei are produced in 

 greater numbers, and these may capture much of the moisture and 

 prevent condensation on the smaller ions, until proportionately larger 

 exhaustions have been applied. Examples of these occurrences will be 

 given in the next paragraph. 



CONNECTING PIPES TWO INCHES IN DIAMETER. 



39. Remarks on the method. The connection between fog cham- 

 ber and vacuum chamber was now further enlarged by using two nipples 

 6 inches long and 2 inches in diameter, on either side of a plug stopcock 

 of 2\ inches bore. The latter was chosen, since much of the resist- 

 ance was heretofore encountered in this place. Usually observations 

 were taken at long intervals of time apart, to allow for the dissipation 

 of water nuclei due to the evaporation of very small fog particles. 

 Moreover, a considerable period (three days) elapsed before definite 

 results could be recorded. Something of a nuclei-producing character is 

 usually present some time after assembling the parts of the apparatus. 

 The coronas under otherwise like circumstance gradually increase to a 

 maximum. Moreover, in the absence of all leakage inward from without, 

 periodicity of coronal diameter is often in evidence in the successive 

 exhaustions and especially active in case of the very large coronas. The 

 best results appear after long waiting before each observation, whence it 

 follows that evaporation from the heavy paraffin oil used for tightening 

 the stopcocks, leakage of nuclei through the filter or through unknown 

 channels is quite ineffective. Waiting was finally made needless by first 

 exhausting the partially exhausted fog chamber before each definite 

 observation. The effective drop of pressure is then small and only the 

 large nuclei are caught and the fog particles do not evaporate appreciably. 

 The filter, moreover, in all tests gives evidence of entire trustworthiness. 

 It was customary to open and close the plug cock quickly, between stops, 

 a spring opening device having been put in action. In no case was there 

 danger of under-saturation, the wet cloth linings being but 8 or 10 cm. 

 apart, above and below. In the highest exhaustions used, dp = 44 cm. 

 about, the volume expansion is fully 2.5, so that 60 per cent of the air 

 must thereafter be readmitted freshly through the filter. Naturally in 

 this slow passage it would be not only denucleated but deionized, if there 

 were not fresh sources of penetrating radiation always in action for the 

 reproduction of both types of nuclei. In general the apparatus seems to 

 have behaved faultlessly. Such irregularities as remain must then in 

 large measure have been introduced by the fact that the drop of pressure 



