CHAPTER III. 



THE NUCLEATION CONSTANTS OF CORONAS. 

 RESULTS WITH A SINGLE SOURCE OF LIGHT. 



24. Introduction. At this point it seemed essential to restandardize 

 the coronas in terms of the numbers of nuclei represented by a given 

 angular aperture and type of corona at a given exhaustion and tem- 

 perature. The measurements* carried out for this purpose in my 

 earlier memoirs were made under very different conditions; and though 

 reductions to the present results are feasible in a measure, it will ob- 

 viously be preferable to repeat the work anew. This is particularly 

 the case because the corrections referred to are liable to be large and 

 because the results in the following chapters will essentially depend 

 on the number of fog particles per cubic centimeter. This datum will 

 here as elsewhere be called the nucleation, and in dust-free wet air the 

 types of nuclei present will be the ions and the vapor nuclei only. These 

 will, as a rule, be inefficient in the presence of phosphorus nuclei. 



25. Apparatus and methods. The apparatus used is the same as 

 heretofore described in the Carnegie Institution of Washington Publica- 

 tion No. 62, p. 74, and is shown in fig. n. It consists of a large vacuum 

 chamber V connected with the relatively small fog chamber F, the 

 volume ratio being about v/V = o.o6. The latter was cylindrical in 

 form, with its long axis horizontal, so as to admit of the measurement of 

 coronas of large aperture. This angle may exceed 60 in the extreme 

 cases and there must be some depth (exceeding 5 inches) if the coronas 

 are to be sufficiently intense. The need of large fog chambers is there- 

 fore apparent and the plug-cock fog chamber seems to be the only 

 apparatus adapted to the present purposes. 



The connecting pipe was about 18 inches long, 2 inches in diameter, 

 and the stopcock 2 inches in bore. Phosphorus nuclei w r ere used. To 

 guard against subsidence and undersaturation, the cloth lining of the 

 fog chamber was fitted close to the walls and but two opposite narrow 

 horizontal strips were left open for the observation of coronas. 



The method used was the one previously employed. The highly 

 nucleated medium ($X io 8 phosphorus nuclei per cubic centimeter) was 

 successively expanded by a fixed amount, and the nucleated air removed 

 from the fog chamber was replaced by filtered air. The residual nuclea- 



*Smithsonian Contributions, No. 1373, vol. 29, pp. i to 173, 1903; ibid., No. 1651, 

 vol. 34, pp. i to 226, 1905. 



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