440 Intelligence and Miscellaneous Articles, 



ON THE CONDENSATION OF FLUIDS ON SOLID BODIES. 

 BY EILHAKD WIEDEMANN. 



Wilhelmy*, from experiments on capillarity, concluded that 

 upon glass surfaces which dip into water the latter is condensed 

 in large quantity. Later experiments, however, by Eontgenf, 

 Schleiermacher J, and Volkmann§, who ascertained the specific 

 gravity of the same body first in whole pieces and then in a state 

 of fine division, showed that the weight of the condensed layer is 

 extremely slight, and not determinable. The latter result is also in 

 perfect accordance with what the newer investigations of Van der 

 Waals || on the magnitude of the molecular forces have taught us, 

 as I intend, in what follows, to show. 



The condensation of the liquid upon the side is owing to this — 

 that the forces emanating from the side, and rapidly diminishing 

 with the distance from it, compress the fluid within a layer the 

 thickness a of which is equal to the sphere of action of the mole- 

 cular forces. "We can conceive the layer in question resolved into 

 thinner ones, each of the thickness dec, within which the molecular 

 force k can be regarded as constant. If we further assume that 

 the coefficient a of compressibility is independent of the pressure, 

 if s is the specific gravity of the fluid, upon the unit of surface 

 a quantity ro 



d= \ latsdoc 

 is condensed. ^ a 



If we put the force k exerted by the side upon the fluid 

 equal to the pull K exerted by a fluid, in consequence of the mole- 

 cular forces, upon its superficial parts, we assume a force at any 

 rate of the same order of magnitude as that which acts between 

 the fluid and the side at the surface of contact. But if in the 

 above expression we substitute K for k, we obtain much too high 

 a value for the amount d condensed, since k diminishes very quickly 

 with increasing distance from the wall. Consequently we have 



d<~Kasa. 

 Tor water, K= 10000 atmospheres, a=about 5 x 10~ 5 , and, accord- 

 ing to Quincke If, a = 0-0000005 central.-; so that d< 10000 x 

 0-00005 . 0-0000005 g. =25 x 10~ 8 g. Accordingly even ILasa, and 

 therefore more certainly d, is a completely inappreciable quantity. 



Volkmann's** value for the thickness of the bounding layer, on 

 the assumption of which the laws of the heights to which fluids 

 ascend in capillary tubes and on solid plane sides are valid, cannot 

 be applied to these reasonings, because the real existence of so thick 

 a boundary layer different in constitution from that of the free 

 fluid has not actually been verified. — Wiedemann's Annalen, vol. xvii. 

 pp. 988-990 (1882). 



* Pogg. Ann. cxix. p. 117 (1863), exxii. p. 1 (1864). 

 t Wied. Ann. iii. p. 321 (1878). 

 \ Wied. Ann. viii. p. 52 (1879). 

 § Wied. Ann. xi. p. 182 (1880). 



|| ! The Continuity of the Gaseous and Liquid States/ German trans- 

 lation by F. Roth. 



U Pog-g. Ann. exxxvii. p. 402 (1869). 

 ** Wied. Ann. xi. p. 117 (1880). 



