34 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



The dimensions of the three fog chambers were: 



The axes were in all cases horizontal and care was taken to insure the 

 occurrence of saturated air. 



It is interesting to note that immediately after the small fog chamber 

 was put together, there was an internal source of high nucleation active 

 throughout 10 or 20 exhaustions, a result frequently obtained in other 

 similar cases. It is none the less difficult to detect the reason for its 

 occurrence. It vanishes completely in the course of time. It is probably 

 identical with the emanation obtained by Russel* from metals and resins; 

 at all events its eventual evanescence renders it harmless. 



28. Data. The preliminary data are given in table 13, where in the 

 case of small apparatus the color of the corona was chiefly used to deter- 

 mine the nucleation. The ordinary form of goniometer would have been 

 useless, as most of the larger coronas transcend the limits of the appar- 

 atus. The barometer is as usual denoted by p and the sudden drop of 

 pressure after the return of isothermal conditions with both chambers in 

 communication, by dp; n is the estimated nucleation per cubic centi- 

 meter. 



These results are given with sufficient detail graphically in fig. i , and 

 are compared with the curves holding for the normal apparatus, No. i, 

 taken from a preceding report for identical exhaustions. In case of the 

 large vessel, No. 2, it was impracticable to carry the exhaustions very 

 high because of the danger of breaking the vessel. 



The results show that fog and rain limits are not materially changed, 

 no matter whether large or small apparatus is used. But few experi- 

 ments were made. 



The efficiencies of the apparatus, however, differ in marked degree. 

 In case of the large apparatus, No. 2, less than 150,000 vapor nuclei 

 respond, even at the highest exhaustions. The number of nuclei caught 

 increases pretty uniformly with dp/p, even beyond the limits tested, 

 but the efficiency is enormously lower than obtained for No. i , where 

 1,500,000 to 2,000,000 is the maximum number caught. The small 

 apparatus, No. 3, is even in excess of this value, inasmuch as over 

 3,000,000 nuclei per cubic centimeter are precipitated, while for equal 



*Russel: Proc. Roy. Soc. London, LXI, 424, 1897; LXIII, 102, 1898. 



