13 S C. Bar us — Nuclei and Ions in Dust-free Air. 



after the precipitation of the m grams of alcoholic fog per 

 cubic centimeter. The drop in pressure* is hp from p = 76 cm 

 at 20° C. The data of the last columns will be presently 

 explained. 



fy ti U t mxlO — 6 Pr/p» r x 10 7 



cm °C °C °C grams cm 



10 20° 4- 3-2° +14-8 10-0& 2"42 1-44 



20 20 —15-3 +10-2 18-3 6*21 '75 



30 20 —36*1 + 3-2 22'8 21*4 -48 



These may be compared with the case of water vapor. 



85 20° + 5-8° +14-7° 2-6^ 2*17 1*65 



17 20 — 9-6 + 8'8 4-6 5'68 -77 



22 20 —18-9 -f 4-6 5'5 111 '58 



30 20 —36-1 — 3-5 6*4 69'7 '35 



We may infer from the table that in a perfect apparatus, 

 water fog particles would reach freezing (0° C.) at 8p = 24 cm 

 and alcohol fog particles at hp> — 34*5 cm . Moreover for the 

 same corona there must be on the average about 3*5 times 

 more particles in the alcoholic fog than in the water-fog, which 

 accounts for the opaqueness of the former. 



For the reasons adduced it is not worth while to express the 

 results otherwise than in round numbers, for the data involved 

 are inevitably crude. The assumption of the law of adiabatic 

 cooling as far as — 36° C. is questionable in view of the admix- 

 ture saturated vapor: but as the densities of vapor are for 

 alcohol about 8 per cent that of air and for water vapor about 

 7 per cent, this approximation in a raritied atmosphere, as well 

 as the use of Boyle's law for a wet gas, is probably admissible. 

 It is different, however, with the latent heat of the vapor, 

 which is required at the low temperatures, but is known (as a 

 rule) only at temperatures near the boiling point. From this 

 and similar points of view, measurements of latent heat for the 

 more common vapors at very low temperatures would be desir- 

 able. 



Finally the point at which the drop in pressure ceases to be 

 efficient, because of the increasingly rapid inward radiation of 

 heat from the vessel, is the most serious of the outstanding 

 errors. I have endeavored to diminish it compatibly with the 

 desideratum of a large and easily adjusted fog chamber, by 

 successively increasing the bore of the exhaust pipes and stop- 

 cocks ; and this plan has been in a large measure successful. 

 The extent to which the error is present, as the drop in pres- 

 sure increases more and more, is nevertheless left unanswered. 



* The value of dp here referred to is the experimental value observed 

 under isothermal conditions at the fog chamber. The value computed from 

 the dimensions of fog and vacuum chambers is dp x '775, as will be shown 

 elsewhere. 



