PHY8ICS. 343 



of thiu and thick films over a range of thickness extendiuy from 1,350 

 to 12 millionths of a millimeter (Nature, Juue, 1886, xxxiv, 160). Magie 

 has also determined the capillary constant from a formula of Poisson 

 ■which gives its value as equal to the square root of twice the surface 

 tension divided by the specific gravity. The formula contains, besides 

 this constant, the height of the summit of the droj) above the plate, 

 the radius of the greatest section of the drop and the contact angle be- 

 tween the drop and the plate. Since the formula holds for an air-bub- 

 ble formed in a liquid uuder a level plate, the author has made the 

 necessary measurements upon such a bubble, and has obtained for tbe 

 capillary constant of water 15.067 at 20°, absolute alcohol at about 14°, 

 5,764 ; olive oil at 18°, 7.110; and petroleum (sp. gr. at 10°, 0.808), 6.755 

 at 15° (Am. J. Sci., March, 188o, III, xxxi, 18J). Duheni has shown 

 that in order to treat i)roperly the subject of capillarity and to bring 

 the investigations of Thomson on the connection between changes of 

 temperature and changes of the cai)illary surface into accord with the 

 older ones, the ordinary mechanical treatment must be abandoned and 

 general thermo-dynamical methods adopted. He shows, first, that for 

 a system of bodies touching each other the potential is not to be sought 

 at a fixed temperature, but the thermo-dynamical potential, which con- 

 tains the changes of energy for varying temperature. Assuming that 

 the densities and the actions of the molecular forces of bodies vary only 

 in infinitely thin surface layers, this supposition is sufficient to prove 

 that the thermo-dynamic potential consists of two parts, one of which 

 is a linear function of the content of the various bodies, the other a lin- 

 ear function of the surface in contact. From the formulas obtained the 

 laws of Gauss and Laplace for the shape of the surfaces are explic- 

 itly deduced. (Beibltitter d. Phys., x, 330 ; Phil. Mag., August, 1886, V, 

 XXII, 230.) 



3, Of gases. 



Ill an extended experimental memoir on the law of gaseous flow, Hirn 

 has given the results of investigations made to determine whether a gas 

 under a constant pressure flows into a reservoir where the i>ressure also 

 constant is less than its own, Avith a velocity indefinitely increasing as 

 the pressure in the reservoir decreases; or whether there exists alimit- 

 ing velocity which is attained when this second pressure is zero. Ilep- 

 resenting graphicall}* the results of the experiments, it appears that 

 the maximum value to which the volume-eq nations point has no exist- 

 ence, and that so far as the velocity of flow is concerned the limiting 

 value indicated by Weisbach's equation equally has no foundation in 

 fact. Hence it would seem that the trne law of gaseous flow produced 

 by pressure-difference is yet to be discovered. Moreover, the author 

 calls attention to the discrepancy between his results and those pre- 

 dicted from the Kinetic theory of gases. According to this theory, dry 

 air cannot, uuder a constant pressure, flow into a perfect vacuum with 



