PAPER BY PROF. BEZOLD. 273 



, But since the possibility of this has been demonstrated by the inves- 

 tigations of Aitken, Coulier, Mascart, Kiessling, and especially by Rob- 

 ert von Helmholtz,* it has some interest for us to make the precipitation 

 from supersaturated air the object of a special investigation. 



This precipitation, as is well known, occurs when super-saturated air 

 (which can only exist when perfectly free from dust) is suddenly mixed 

 with very fine particles of solid bodies, or possibly, also, when electric 

 discharges take place through such supersaturated air.t We obtain 

 directly from the above-given rules the amount of the precipitation, as 

 also the rise in temperature. 



We have only to omit the parts designated by the indices 1 and 2 in 

 Fig 41, and to consider the condition indicated by the subscript index 

 3 as the starting point, then the ordinate T-^ F-^ = 7/3 gives the quantity of 

 water in the state of supersaturatiou, while y again indicates as before 

 the final remaining moisture ', y^ — y indicates the quantity of moisture 

 precipitated and t — U the consequent rise of temperature. 



This is, therefore, a method ot formation of precipitation, in which 

 one can actually speak of a liberation of latent heat (the latent heat of 

 evaporation), as was formerly done in explaining the formation of pre- 

 cipitation in general. 



In a certain sense this usage is allowable, even in the formation of 

 precipitation by mixture, in so far as the temperature of the mixture 

 comes out higher when water is precipitated than when this, under 

 otherwise similar conditions, is not the case because of the insufficient 

 quantity of water. This rise of temperature is however always a very 

 unimportant one in consideration of the small quantity that can be con- 

 densed by mixture. 



It is otherwise when true super-saturation is present. In such cases 

 the rise of temperature can, according to the degree of super-saturation, 

 be very considerable, as is easily seen from Fig. 41. 



Still more considerable must the precipitation be that is caused by 

 the sudden cessation of the super-saturation, namely : So soon as a sud- 

 den development of heat occurs at any one place in the atmosphere 

 there follows a powerful ascent of the air which then, by adiabatic cool- 

 ing, must always i)roduce new formation of precipitation. 



Under those conditions, when the vertical distribution of temperature 

 approximates even distantly to that of convective equilibrium, then 

 the sudden cessation of the condition of super-saturation causes this 

 equilibrium to become unstable, and thus this cessation then affords 

 the key to the explanation of a series of phenomena. 



I consider it probable that it is in such processes, which indeed 

 deserve a thorough investigation, that we have to seek the reason for 

 the "cloud-bursts" properly so called. Of course, to establish this 



* Wiedemann's Annalen, 1886, xxvii, p. 527. 

 t R. von Helmholtz, Wiedemann's Anvalen, 1887, xxxii, p. 4. 

 80 A 18 



