262 Dr. C. Barus on Spontaneous Nucleation and 



depends upon a knowledge of the law of absorption which we 

 do not at present possess, it is interesting to find that a 

 collection of corpuscles describing circles under forces varying 

 inversely as the square of the distance in the molecules of a 

 substance which shows no selective absorption would, like the 

 ideal u black " body, radiate at a rate proportional to the 

 fourth power of the absolute temperature. 



XXIX. On Spontaneous Nucleation and on Nuclei produced 

 by Shaking Solutions. By C. Barus *. 



Spontaneous Nucleation. 



IN ' Science' (xv. Jan. 1902, p. 178) I communicated some 

 results which seemed to give evidence of the spontaneous 

 production of nuclei from certain organic liquids. Though 

 my own work is rather more concerned with the diffusion of 

 the nuclei with an ulterior view to their velocity, no matter how 

 the nuclei may be localized, it nevertheless seemed interesting 

 to elucidate the subject incidentally. I therefore made a 

 series of experiments in which condensation was produced by 

 the expansion method in case of gasoline, benzine, petroleum^ 

 benzol, carbon bisulphide, and water. 



Hydrocarbons. — The first three hydrocarbon liquids may 

 be dismissed summarily. The air above them, if carefully 

 freed from nuclei by precipitation, remained free from 

 nuclei indefinitely. The test was made by leaving the 

 receiver without interference for fifteen or more hours, all 

 the cocks being shut off, except the one communicating with 

 the atmosphere through a filter of compressed cotton, half a 

 metre long. A perfect filter is essential throughout. In case 

 of petroleum it is exceedingly difficult to remove the nuclei 

 by precipitation alone ; but they vanish in the lapse of time 

 (days), and thereafter the air remains permanently without 

 nucleation. 



In case of benzol I was for a Jong time erroneously of 

 the opinion that nuclei arise spontaneously out of this liquid, 

 and consistent results leading to this inference were obtained 

 in great number. Doubt was cast on this supposition by the 

 behaviour of the hydrocarbons just mentioned. The true 

 explanation was subsequently found : on removing nuclei by 

 precipitation with the object of obtaining dust-free air, a 

 couche of nuclei is apt to remain brooding immediately over 

 the surface of the benzol, where it escapes detection. It is 



* Communicated by the Author. 



