C. Barus — Yelooity and Structttre of the Nucleus. 227 



or acid solute taken. For neutral organic solutes, the number, 

 ?i, is little more than half as large. This seeras then to be in 

 the nature of a class distinction. Pressure decrements of less 

 than 2^°" suffice to precipitate the nuclei; but for the small 

 supersaturations, the greater number re-evaporate to the nuclear 

 stage and many exhaustions are needed to throw them out. 

 For the high degrees of supersaturation all nuclei are precipi- 

 tated at once. The number of nuclei generated depends on 

 the bulk of solution shaken and on the intensity of the agita- 

 tion. Hence they seem to arise in the solution itself and not 

 by friction with the walls of the vessel. The attempt to find 

 a limiting number has not yet been successful. Incidental 

 conditions which have not all been traced to their source seem 

 to have considerable influence on n. 



3. Persistence of nuclei, — A comparison of the data for n 

 and those of h for the same concentration shows that for the 

 same solution n decreases with log c while h increases with log 

 c, the arena of greatest variation being the very dilute solu- 

 tions which merge into water. Usually the growth of Ic con- 

 tinues even after the decrease of n has appreciably subsided. 

 Corresponding to the greater n which characterizes the saline 

 solutes as compared with the neutral organic solutes, the veloci- 

 ties of the nuclei of the latter are greater than those of the 

 former, under otherwise like conditions. ISTuclei with neutral 

 organic solutes in water are less persistent. Extreme per- 

 sistence may be obtained with the volatile hydrocarbon sol- 

 vents with an appropriate solute, like benzol-paraffine, and 

 often the solute seems to be spontaneously nuclei-producing, 

 like benzol-naphthalene. In general persistence for a given 

 nucleus is a question of the mass dissolved per cub. cm. of the 

 solution ; but cases like water-tartaric acid occur in which a 

 low order of nucleation, ?i, is associated with exceptional per- 

 sistence, or low values of h. 



4. Structure of the nucleus. — From experiments like the 

 present I recently came to the conclusion that all condensation 

 nuclei are concentrated solutions ; that the increment of vapor 

 pressure due to increasing convexity is eventually, but slightly 

 before molecular diameters are reached, compensated by a 

 decrement of vapor pressure due either to concentration^ or 

 to electric charge.f The latter case is interesting inasmuch as 

 the nuclei are just about large enough that a single electron 

 spread out over the surface would suffice to equilibrate:}: the 

 increment of vapor pressure due to surface tension. The con- 

 centration hypothesis is more straightforward and requires 



* Science, xv, pp. 913-914, 1902. 



f This Journal (4), xiii, pp. 400-402, 1902 ; ibid., p. 473. 



X This Journal (4), xiii, p. 473, 1902. 



Am. Jour. Sci. — Fourth Series, Vol. XIY, No. 81. — September, 1902. 

 16 



