C. GENERAL PHYSICS. 121 



sions. If, then, we compare observations made at the same 

 temperature, the activity depends upon the relative humidity 

 of the two portions of air. The study of the influence of the 

 diaphragm, its extent, etc., has enabled him to explain how 

 a small porous vase can, by connecting its interior with a ma- 

 nometer, be utilized in studying the diffusion of vapor in the 

 free atmosphere. Similarly, we may determine the tension of 

 the vapor of water existing in the air, at any moment, by de- 

 termining the difference of the pressures shown by the mo- 

 nometer in the interior, and by the barometer on the exterior. 

 This suggests a new hygrometer, which, simple as it is, and 

 easily applicable, may rapidly find favor among meteorolo- 

 gists so soon as Dufour has exactly developed the laws con- 

 necting the relations of pressure and the hygrometric state 

 of the atmosphere. JBibl. Univers. et Mevue Suisse, 1874, 336. 



OX THE LAWS OF APPARENT ADHESIOX. 



Stefan, who is well known as one of the most industrious 

 investigators in all matters relating to molecular physics, 

 has recently published in full a memoir relating to prelim- 

 inary investigations on the subject of adhesion. The true 

 phenomena of adhesion are easily confounded with what he 

 calls apparent adhesion. That is to say, if two perfectly flat 

 plates be brought quite near together, but not in actual con- 

 tact, and an attempt be made to separate them, a slight 

 force is required, which Stefan states, on further investiga- 

 tion, he is persuaded is simply the result of the resistance 

 which the viscosity of the liquid or gas offers to its imme- 

 diate inflow into the space between the two disks. The ef- 

 fect of this viscosity, of course, would disappear if the ex- 

 periment could be performed in an absolute vacuum. On 

 the other hand, the study of the experiments made by Stefan 

 'in the atmosphere at its ordinary pressure, and of such liquids 

 as alcohol, water, solutions of salt, etc., has enabled him to 

 determine the true co-efficient of viscosity or internal friction 

 for these substances. The values he arrives at are, for water, 

 0.0108, for air, 0.00183; figures which agree exactly with 

 those deduced by Maxwell and Meyer by entirely different 

 methods of experiments. The actual separation by means of 

 a slight force of the disks experimented with by Stefan is, 

 he states, a dynamic, and not a static phenomenon. The time 



F 



