Doable- Layer of Solid and Liquid jBodies. 321 



atures, we write the condition of equilibrium in the same 

 form 



P = F, (19) 



and identify P with the internal pressure of the liquid, we 

 sret the following relation between surface-tension o- = W 

 and internal pressure 



P=|- (20) 



Although this relation should strictly hold only for a simple 

 substance, we may, however, expect that, applied to com- 

 pound ones, it should yield for r — the mean radius of the 

 molecule — a value of the right order of magnitude. In 

 fact, we have for ether, alcohol, and water, <r=19*7, 25*8, 



77-0 d ^—, and P = 1400, 2200, 10,700 -^ respectively; the 

 cm. ' cm. 2 l J 



corresponding values of r, calculated by means of (20), are 



3'6 x 10- 8 , 3 X 10" 8 , and 1*7 X 10" 8 cm. Unfortunately, P has 



been determined for very few substances. It would be very 



desirable to calculate the internal pressure of liquid metals 



from their vaporization heat (by means of Stefan's formula 



iY-p)v=Q, 



where Q is the latent heat of vaporization, v the specific 

 volume, p the vapour pressure). j_ 



According to our theory, P must be equal to 28,000 — ^- 



for Na, 72,000 for mercury, 130,000 for Pb, and about 

 800,000 for Pt (see Table II.). 



It must be emphasized that when a neutral atom (or 

 molecule) escapes from the liquid, the energy lost in crossing 

 the surface layer is equal to the work of cohesive forces only, 

 that of the electric ones being obviously equal to zero. The 

 electric double-layer does not, therefore, affect the vapor- 

 ization heat, but permits us to calculate it, provided the 

 above-mentioned connexion between the electric stress and 

 the internal pressure is correct. 



I hope to return to this question in a future paper. 



Summary. 



(1) Assuming the atoms to consist of positive nuclei with 

 electrons rotating about them, it is shown that these electrons 

 constitute over the surface of metals a layer of negative 

 electrification with a corresponding layer of positive electri- 



