TIME VARIATIONS OF NUCLEI. 159 



cussed; but the dependence of the nucleation on the fluctuations of the 

 barometer now shows itself even more obtrusively than before. The 

 minima of atmospheric pressure coincide with maxima of colloidal 

 nucleation, and therefore (by inference but not necessarily) with minima 

 of ionization of the dust-free air, both in the daily and in the weekly 

 periods of observation. Maximum pressure, therefore, would correspond 

 to maximum ionization as if the radiant energy originated in the com- 

 pression of the atmosphere, or were dependent on the mass of the atmos- 

 phere bearing on a given place. This would, if finally substantiated, be 

 an important inference, but no more so than the more direct correlative 

 result that minimum pressure and maximum colloidal nucleation of dust- 

 free air go together. 



At the same time, since the change of absolute temperature (r) due 

 to a sudden expansion equivalent to a drop (dp) at a barometric pressure 

 (p) and vapor pressure (it) may be written 



-=(i-op/(p-it) ) 



the correction for the changes of the barometer are in the same sense as 

 the observed changes in nucleation. These corrections are found by 

 varying the numerator of dp I (p- it) and observing the effect on the 

 angular diameter of the corona. While I can see no room for error, it 

 must nevertheless be acknowledged that the present method of small 

 exhaustion, though possibly more sensitive, is not as straightforward 

 as the method mentioned in my address, where no variation could be 

 detected, the terminal corona remaining unchanged. 



At the present stage of investigation, therefore, the need of any 

 external radiation has ceased to be obvious; and the results, if they 

 exceed the barometer correction, are most directly referable to changes 

 of pressure within the gas, the number of colloidal nuclei being greatest 

 when the pressure is least. 



101. Corrections. In table 54 and the following table 55, p is the 

 barometric pressure ; p 3 the final pressure when fog chamber and vacuum 

 chamber are in communication; p 2 the (computed) pressure which would 

 be observed in the fog chamber if it could be instantly separated from 

 the vacuum chamber after exhaustion. Hence the true drop in pressure 

 is dp=p- p 2 while p=p- p 3 is observed. The initial pressure (p) differs 

 from the atmospheric pressure by a few millimeters, due to the leakage 

 of the 4-inch stopcock from fog chamber to vacuum chamber. As it is 

 impossible to retain the same value of p and dp throughout, a correction 

 must be applied, seeing that the adiabatic cooling is given by r 2 /r 1 = 

 (1 -dp/ (p- Tz)Y k ~ c) lk , where it is the vapor pressure of water. By varying 



