EQUATIONS OF FOG CHAMBER. 65 



where ?, is the density of the air at t,. The ratio is equal to 



7, _ J< --Q -("', -TTl) + (ftl-""-l) (I-V/V-T'j/Zt) 



ft" (A-T.Mry-r. + VV-rVr,) 



if the vapor pressures 7r are designated like the temperatures and pres- 

 sures with which they are associated. Usually 



7 _ i+v/V-z'Jr, 



ft T i /T l +V/V.T , 1 /V l 



suffices, the term involving the vapor pressures (ft) being a correction of 

 about 1 per cent. The computation, to which I shall return elsewhere, 

 shows that p 2 , computed, is always above p 2 , observed, so that the fog 

 chamber begins to heat itself above the temperature Ti before the cock 

 can be closed again and contains less than its normal allotment of air. 

 Thus, in the example given, p 1 /p l = o.gi; p 2 = 4'j.g, observed; and p 2 = 

 49.9, computed. Hence p and p 3 alone have any definite meaning for 

 the fog chamber. 



49. Observations with 4=inch exhaust pipes. The observations of 

 this paragraph, apart from the exhaustion difficulties already discussed, 

 were made peculiarly difficult by the unavoidable leak from fog chamber 

 to vacuum chamber through the large exhaust cock. It was therefore 

 essential to wait many hours for each observation, since the coronas 

 corresponding to the second, third, etc., of successive exhaustions were 

 smaller than the first. This can not be due to any other cause than the 

 presence of water nuclei from the fog of the first exhaustion. A given 

 corona, moreover, was apt to decrease in aperture as much as one-half 

 during the period of subsidence, showing growth of certain particles at 

 the expense of others, the latter being afterwards detected in the 

 water nuclei specified. This must also be attributed to the continued 

 slow exhaustion due to the leak in question. 



In table 21 and fig. 31 the earlier data with 4-inch pipes are given, 

 chiefly with the object of direct comparison with foregoing results with 

 2-inch pipes. The meaning of the data is clear from the earlier tables, 

 and the dp here mentioned is the isothermal value observed at the fog 

 chamber as heretofore. 



On March 16 to 18 the data are irregular in the way common to 

 observations in a newly adjusted apparatus. The effective nucleation 

 is too small from the presence of interior sources of relatively bulky 

 nuclei. A fairly complete series was undertaken on March 19. The out- 

 going and incoming branches do not quite coincide, and the data, as a 

 whole, still lie below the corresponding results with 2 -inch exhaustion 

 pipes. This is indicated in fig. 31. Of later observations (March 22), 



