( 389 ) 
If we take 7, = 27; and for v, the value for the greatest cooling 
according to the process %1 = 42, SO vj = 2,26, and for vz a value, 
which corresponds to pp = 20 atm., then even by this one expansion 
the air wouid have been cooled already far below the critical point. 
Lord RAYLeiGH has already pointed out, that the process of LINDE 
might be improved by causing the expanding gas to perform more 
work. It remains therefore desirable to find an arrangement, by 
which the expansion approaches more nearly an isentropic process 
than is the case in the apparatus of LINDE. 
Physics. — Prof. J. D. van per WaALs presents for the proceed- 
ings a communication of Mr. H. Hursnor at Delft, on: 
“The direct deduction of the capillary constant 6 as a surface- 
tension.” 
The amount of the capillary tension and the capillary energy, as 
found by Prof. van DER Waats in his Théorie Thermodynamique 
de la capillarité, may also be determined directly. The existence of 
capillary tension is undoubtedly the consequence of molecular attrac- 
tion. Therefore we shall have to examiae the influence of molecular 
attraction in the capillary layer, i.e. we shall have to determin > 
the value of the molecular pressure for an arbitrary point of the 
capillary layer. The equation of state gives ag? for the value of 
the molecular pressure; the equation of state, however, comprises 
only those cases, in which the distribution of matter is homogeneous. 
As the molecular pressure is the direct consequence of the attraction, 
which the particles exercise on one another and is therefore deter- 
mined in a point by the condition of the surroundings, it may be 
expected that for not homogeneous distribution of matter the molecular 
pressure in different directions will have different values. The 
existence of capillary tension is to be ascribed to the fact, that in the 
capillary layer the molecular pressure in the direction of the surface 
of the liquid is different from that in the direction normal to the 
surface. 
When the matter is homogeneously distributed the molecular 
pressure per surface element do is equal to the force with which 
all the matter on one side of the plane in which do is situated, 
attracts in the direction towards this plane the material cylinder with do 
as base, situated on the other side of the plane. In the capillary 
layer we can also define the molecular pressure in the same way. 
