V] OF SURFACE ENERGY 353 



of all solids, the least surface for a given volume) it accounts for 

 the spherical form of the raindrop*, of the grain of shot, or of the 

 living cell in innumerable simple organisms f. It accounts also, as 

 we shall presently see, for many much more complicated forms, 

 manifested under less simple conditions. 



Let us note in passing that surface-tension is a comparatively 

 small force and is easily measurable: for instance that between 

 water and air is equivalent to but a few grains per linear inch, or 

 a few grammes per metre. But this small tension, when it exists 

 in a curved surface of great curvature, such as that of a minute drop, 

 gives nse to a very great pressure, directed inwards towards the 

 centre of curvature. We may easily calculate this pressure, and so 

 satisfy ourselves that, when the radius of curvature approaches 

 molecular dimensions, the pressure is of the order of thousands of 

 atmospheres — a conclusion which is supported by other physical 

 considerations. 



The contraction of a liquid surface, and the other phenomena of 

 surface-tension, involve the doing of work, and the power to do 

 work is what we call Energy. The whole energy of the system is 

 diffused throughout its molecules, as is obvious in such a simple 

 case as we have just considered; but of the whole stock of energy 

 only the part residing at or very near the surface normally manifests 

 itself in work, and hence we speak (though the term be open to 



* Raindrops must be spherical, or they would not produce a rainbow; and the 

 fact that the upper part of the bow is the brightest and sharpest shews that the 

 higher raindrops are more truly spherical, as well as smaller than the lower ones. 

 So also the smallest dewdrops are found to be more iridescent than the large, shewing 

 that they also are the more truly spherical; cf. T. W. Backhouse, in Monthly 

 Meteorol. Mag. March, 1879. Mercury has a high surface-tension, and its globules 

 are very nearly round. 



t That the offspring of a spherical cell (whether it be raindrop, plant or animal) 

 should be also a spherical cell, would seem to need no other explanation than that 

 both are of identical substance, and each subject to a similar equilibrium of 

 surface-forces; but the biologists have been apt to look for a subtler reason. 

 Giglio-Tos, speaking of a sea-urchin's dividing egg, asks why the daughter-cells 

 are spherical like the mother-cell, and finds the reason in "heredity": "Wenn also 

 die letztere (d. i. die Mutterzelle) eine spharische Form besass, so nehmen auch die 

 Tochterzellen dieselbe ein; ware urspriinglich eine kubische Form vorhanden, 

 so wurden also auch die Tochterzellen dieselbe auch aneignen. Die Ursache warum 

 die Tochterzellen die spharische Form anzunehmen trachten liegt darin, dasa 

 diese die Ur- und Grundform alter Zellen ist, sowohl bet Tieren ivie bei den Pflanzen'' 

 [Arch.f. Entw. Mech. Li, p. 115, 1922). 



