NUCLEATION CONSTANTS OF CORONAS. 71 



tive ions v t /v = i .25, positive ions 1.31, cloud i .38). In relation to the 

 other curves of the chart Thomson's graph must be shifted bodily toward 

 the left until the lower and upper steps of the curve correspond with the 

 other cases. In none of the experiments made with my apparatus does 

 the initial step (which should correspond to the branch for negative 

 ions) rise much above the horizontal axis, no matter how intense the 

 ionization. This rise begins at about 0.25 in the chart and continues 

 thereafter in a way to correspond with the ionization. The diagram 

 also shows J. J. Thomson's second group of experiments, in which the 

 initial step (^/v < i .33) lies at an average height of n = 8$X io 3 and the 

 second step at an average height about twice as large. 



Fig. 22, which contains most of the earlier results reduced to the 

 present scale, shows the variation of nucleation obtainable at different 

 times to which reference has already been made. The high position of 

 the X-ray curve is particularly noticeable. All data except C. T. R. 

 Wilson's are given as if the coronas had been observed at 27, for which 

 case the least amount of reduction was needed. The Wilson line should 

 therefore be depressed about 8X2 = 16 per cent in nucleation to be 

 comparable with the others. 



45. Temperature effects. It was demonstrated in Chapter II that 

 the vapor nucleation of dust-free air varies in marked degree with tem- 

 perature, if the relative drop in pressure be computed as x=(dp s [n- 

 7ij])/(p TT). Computed relatively to dp 3 /p, there is a much more mod- 

 erate variation with temperature outstanding, suggesting that the appar- 

 ent variation may be associated with the occurrence of the vapor density 

 TT in x. To throw light upon this subject from a different point of view, 

 the condensation limits of dust-free air and of ionized air were determined 

 at temperatures between 13 and 30 and table 30 contains the results. 

 The notation being as above, it is only necessary to refer to the final 

 column for dp 3 /p and the volume expansion vjv = (p/[p dp 3 ]) lly > 

 computed therefrom. 



The results of table 30 being summarized by giving expansions corre- 

 sponding to the fog limits both for [v 1 /v] = (ix) 11 ^ and v 1 /v=(i- 

 dps/pyif, show clearly that vjv, computed from dp 3 /p, is independent 

 of temperature, whereas the other datum [vjv] varies with temperature 

 in a way referable to the values of n involved. It follows that the fog 

 limits are not changed by temperature in a way found by the nucleation 

 itself in Chapter II. The mean fog limit for dust-free air vjv = i .252 

 agrees with Wilson's data. The fog limit for ionized air is, however, 

 decidedly below this, and thus below Wilson's value. Finally, [vjv] is 

 always less than vjv and under ordinary temperatures from i to 2 per 

 cent less. 



