C. Bar us — Changes of Colloidal Nucleation. 205 



An example of the results is graphically given in figure 1, 

 the abscissas being the drop Sp=p—p i the ordinates nXlO" 3 . 

 It will be seen at once that the two curves, ([£/>], denoting 

 that the lower limit of pressure, [Sj-?] 7 that the upper limit of 

 pressure is varied, are strikingly distinct and that the variation 

 of the lower pressure limit [Sjp], corresponds, as it should, to 

 a highly increased efficiency of the fog chamber. 



3. Explanation. — It will next be necessary to endeavor to 

 coordinate the two curves* for \S\>\ and [Sp]'. If the abso- 

 lute temperatures of the air w T ithin the fog chamber before 

 and after exhaustion are r and r 1 (adiabatic pressure p^, then 

 apart from the condensation of water vapor at the original 

 vapor pressure it at r, 



(x (k—c)/k 

 P — 7T/ 



With a large vacuum chamber the difference between p x and^> 3 

 is very small relatively to p x and p 3 , so that for the present 

 purposes p—p 3 =p — p^ = Sp (nearly), whence 



T \ p — TT / 



Since the smaller vapor pressure, it — 7r 15 is associated with the 

 smaller Sp, we may write as a limiting case, 



T Jr = (l-8p/p)^- c)/k 



or the degree of sudden cooling from a fixed temperature t to 

 the adibatic temperature r 1 depends primarily on Sp/p. This 

 is at least permissible for comparison where a continuous 

 series of experiments is made all at the same temperature, as 

 in fig. 1. The moisture error is thus a constant throughout. 

 Hence the apertures of coronas, s, and the nucleation, n, will 

 be a function of Sp/p to the degree specified. 



I have therefore arranged the elate for n with reference to 

 the corresponding values of Sp/p, both for the cases where 

 the upper pressure limits of the drop Sp (curve [SpY), and the 

 cases where the lower pressure limits of the drop Sp (curve 

 \Sp\ are varied. This result is also given in the chart, fig. 1. 



As a whole, and with due regard to the subtleties involved, 

 the two sets of data practically belong to the same curve, 

 for the departure of either in the long run is seen to be posi- 

 tive as well as negative. 



The results of the figure, as has been stated, were obtained 

 in a single series of observations, all at the same temperature. 

 If they be compared with observations made at other times 



* Carnegie publication, No. 62, Chaps. II and VI, will show other cases. 



