MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 357 



that of the external air, and it is due to the action of the inner and outer surfaces 

 of the bubble. We may conceive this pressure to arise from the tendency which 

 the bubble has to contract, or in other words, from the surface tension of the bubble. 



If to increase the area of the surface requires the expenditure of work the 

 surface must resist extension, and if the bubble in contracting does work, the sur- 

 face must tend to contract. The surface must, therefore, act like a sheet of India- 

 rubber when extended, both in length and breadth, that is, it must exert surface- 

 tension . 



The intensity of this surface-tension is measured by the stress which it exerts 

 across a line of unit length. When two spherical films meet therefore, the par- 

 tition must also constitute a portion of a sphere, for it falls within the same condi. 

 tions as the other two films ; that is to say, it has, like the latter, for limits the 

 small mass of junction and the water of the vessel. As regards its curvature, this 

 evidently depends on the difference of the action exerted on its two faces by the 

 two portions of imprisoned air. If these two portions of air are equal, the two 

 films will pertain to equal spheres, which will press the two volumes of air with the 

 same intensity, and consequently the partition, exposed on its two faces to equal 

 actions will have no curvature, or in other words, will be plane ; but if the two 

 quantities of air are unequal, in which case the two films will pertain to spheres 

 of different diameter, and will therefore press these two quantities of air unequally, 

 the partition subjected on its two faces to unequal actions will acquire convexity 

 on the side where the elasticity of the air is least, until the effort which it exerts, 

 in virtue of its curvature on the side of its concave face, counterbalances the excess 

 of elasticity of the air which is in contact with that face, which relation as we 



have seen is given by the formula r = > " which gives the radius of the parti- 



p— p 1 



tion when we know those of the two films. 



In Fig. 28 the diameters of the two films are equal, and as a result the par- 

 tition is a plane. In Figs. 29 and 30 they are in the ratio of 2 to 1 and 3 to 1, 



Fig. 28. Fig. 29. Fig. 30. 



and as a result the partitions are curved and the radius of curvature can be de- 

 termined from the formula given above. Thus when the surface is curved, the 

 effect of the surface-tension is to make the pressure on the concave side exceed the 



pressure on the convex side by T \- + — L where T is the intensity of the surface- 



(Ki ix 2 ' 



tension and Rj and R 2 are the radii of any two sections normal to the surface and 



