196 CONDENSATION NUCLEI. 



1)V i-cpcatcdl}^ forming ji cloud b}' exi)aiisioii, and alloAvin<»; the drops 

 to fall to tlie bottom of the vessel. Both the facts and the explana- 

 tion lia\(' been lon^- known. The particles which serve as the nuclei 

 of the drops formed, when ordinary atmospheric air is allowed to 

 expand slightly, are conveniently called ''dust" particles; they are 

 generally too small to be themselves visible, and it would be difficult 

 to find a means of determining whether they consist of solid particles 

 or of minute drops of liquid. The number of these dust particles 

 per cubic centimeter of air in different localities and under different 

 weather conditions has been investigated by Aitken and by others 

 with the aid of the ingenious dust-counting apparatus invented by 

 him. 



It is not difficult to understand Avhy nuclei should be necessary for 

 the condensation of water in the form of drops. Lord Kelvin proved 

 that the pressure of aqueous vapor necessary for equilibriinn over a 

 convex or concave surface of water differed from that over a Hat 

 surface, being less over a. concave and greater over a convex surface. 

 He shows how we may calculate the difference. A very snuiU drop of 

 pure water will, if we assume the surface tension to remain the same 

 for very small drops as for large ones, evaporate even Avhen sur- 

 rounded l)y vapor numy times more dense than that in equilibrium 

 at the same temperature over a flat surface. Thus unless the initial 

 stages of the growth of the drops can be, as it were, omitted, owing 

 to the presence of not too minute nuclei, a high degree of supersatu- 

 ration may exist without any condensation in the form of rain or 

 cloud resulting. Lord Kelvin showed that to alter the equilibrium 

 vapor pressure by one part in a thousand the radius of curvature of a 

 s])herical drop must amount to about 10 — * cm. Thus very minute 

 nuclei will enable a cloutl to be formed with a very slight degree of 

 siipersaturation; in other words, as a result of a very slight expansion 

 of the air if this has been initially saturated Avith water vapor. 



Tjord Kelvin refrained from extending his calculations to curva- 

 tui-es of greater amount, as the surface tension can not remain inde- 

 pendent of the radius much beyond that limit. It is convenient, 

 however, to extend the calculations to greater curvatures; for 

 ahliough (he results obtained can not be considered as quantitatively 

 correct, they enable us to form a picture of the mode of action of 

 nuclei of different kinds. Let us imagine an arrangement equivalent 

 to tliat considered by Lord Kelvin; but since we are only here con- 

 cerned with convex surfaces, let the capillary tube be joined as a side 

 tube to the lower part of a tall vessel of water. The capillary must 

 be supposed to have walls of such a nature as not to be wetted by 

 water, and let us suppose the open end of it to be bent round, so that 

 it points vertically upward, and that the height of the vertical por- 

 tion can be adjusted to bring the meniscus to the open end of the tube. 



