26 i iDr. G. Barus on Spontaneous Nucleation and 



stick to the layers in question, and i£ there is any slight differ ^ 

 ence in temperature between the air inside of the receiver and 

 the inflowing air, couches will result from convection. Con- 

 versely, the entire non-occurrence of semi-coronas &c. in case 

 of water vapour is an indication of relatively rapid diffusion, 

 and therefore of nuclei of minimum size. The nucleus depends 

 for its size, cast, par., on the medium in which it is suspended. 

 Nuclei from the same source will diffuse under otherwise like 

 conditions 100 or 200 times more rapidly in water vapour 

 than in benzol vapour. 



Under these conditions there would be fewer particles as 

 compared with water vapour for like nucleation, &c, and 

 normal coronas may appear at once in volatile hydrocarbons, 

 whereas they are only reached after many exhaustions in 

 water vapour. Experiment * bears this out. Moreover, the 

 particles in the former case being larger require less super- 

 saturation, and are more quickly removed, which is also true 

 of the volatile liquids. 



Carbon Bisulphide. — The case of carbon bisulphide is 

 peculiar, inasmuch as in addition to the occurrence of 

 couches of the kind just described, this liquid emits nuclei 

 .spontaneously. I would not wish to assert this for the fresh 

 and perfectly pure liquid ; but from carbon bisulphide which 

 has been subjected to evaporation and contains sulphur, nuclei 

 are continually evolved at a definite rate. Curiously enough, 

 the addition of nuclei from without, whether coming from 

 sulphur, punk, or atmospheric air (without shaking of the 

 liquid) is almost insignificant. A couche a few centims. high 

 is usually seen on first exhaustion, the remainder of the 

 receiver being clear. 



After shaking the first corona is annular, coarse, and 

 normal, the aperture of its white disk s = 3°*7. The next 

 exhaustion brings out semicircles concave upward or semi- 

 annuli, the succeeding exhaustion quarter-annuli, &c, the 

 colour eventually vanishing in a filmy veil. Left to themselves , 

 these partial nucleation s gradually diffuse upward, and full 

 coronas may be again obtained in the lapse of time. The 

 same result occurs if the air above the liquid has been quite 

 freed of nuclei by repeated precipitation, as the nuclei are 

 now supplied by the liquid itself. These coronas are first 

 partial, then filmy and small but complete. Gradually they 

 become larger and stronger until a limiting aperture and 

 great intensity is reached. In other words, nuclei are emitted 

 by the liquid in the dark, terminating in a state of saturation 



* Cf. American Journ. Sci., (4) xiii. pp. 81-94 (1902). 



