EFFICIENCY OF FOG CHAMBER. 23 



The curves, moreover, throw important light on the distance effect 

 D. Thus at dp = 22, the effect of increasing D from 1 to 6 meters is a 

 fall of n from 50,000 to 20,000 about 60 per cent. At dp = 30 the fall 

 is from 130,000 to 40,000 about 69 per cent. Owing to the uncertain- 

 ties of observation these numbers may possibly be the same, in which 

 case the gradation of size of the nuclei would be the same for all inten- 

 sities of ionization. 



The initially slow increase of the upper curves ( = 50 cm.) is possibly 

 due to the formation of persistent nuclei, as stated at the beginning of 

 this section, these capturing much of the moisture. 



Finally, the evidence given by the N curves, fig. 17 (assuming that the 

 nucleation is not restored faster than it can be removed by the exhaus- 

 tions), is much the same as that just detailed. The curves are often 

 straighter than heretofore, and continually rise as the result of the in- 

 creasing volume expansion; but the double inflection remains. In fact, 

 the rise of all the upper curves is eventually at about the same rate as 

 for air. In the latter case the scale is again too small to show the in- 

 crease in the region of ions. 



At dp = 22 the distance effect for 1 and 6 meters is a decrease of 60 

 per cent, at dp = 30 a decrease of 71 per cent, naturally nearly the same 

 as above. 



Both the N and n curves show that while within the limits of observa- 

 tion intense radiation increases the nucleation of dust-free air, this is not 

 the case for weak ionization. Here the curves cross at about dp 29 cm. 

 Above this the presence of radiation must decrease the efficient nuclea- 

 tion of non-energized dust-free air; below this it will increase it. At 

 high pressure differences like dp = 41 cm., as used below, the variation of 

 the efficient nucleation must therefore be a sensitive criterion for the 

 variation of the ionization of air. Similarly the direct experiments of sec- 

 tions 7 and 9 find their explanation in the manner already pointed out. 



17. More rapid exhaustion. Apparatus and data. The preceding 

 set of experiments correspond to the particular exhaustion cock used. 

 This was an inch plug gas cock. As almost the whole resistance to flow 

 was encountered here, the advisability of enlarging it seemed evident. 

 In fact, on opening the cock suddenly to an amount corresponding 

 respectively to about 1/4, 1/2, 3/4, i/i,the nucleations obtained were 

 about NXio~ 3 = 4o, 100, 180, 350 for dp = 4i cm., showing that not 

 even an approach to the limit is reached in the extreme case. There is 

 a further objection in having the large resistance in the stopcock, as the 

 coronas will vary with the degree of opening. 



