II] OF BULK AND SURFACE 33 



In the case of very small animals, and of individual cells, the 

 principle becomes especially important, in consequence of the 

 molecular forces whose action is strictly limited to the superficial 

 layer. In the cases just mentioned, action is facilitated by increase 

 of surface : diffusion, for instance, of nutrient liquids or respiratory 

 gases is rendered more rapid by the greater area of surface ; but 

 there are other cases in which the ratio of surface to mass may 

 make an essential change in the whole condition of the system. 

 We know, for instance, that iron rusts when exposed to moist 

 air, but that it rusts ever so much faster, and is soon eaten away, 

 if the iron be first reduced to a heap of small filings ; this is a 

 mere difference of degree. But the spherical surface of the rain- 

 drop and the spherical surface of the ocean (though both happen 

 to be aUke in mathematical form) are two totally different pheno- 

 mena, the one due to surface-energy, and the other to that form 

 of mass-energy which we ascribe to gravity. The contrast is still 

 more clearly seen in the case of waves : for the httle ripple, whose 

 form and manner of propagation are governed by surface-tension, 

 is found to travel with a velocity which is inversely as the square 

 root of its length; while the ordinary big waves, controlled by 

 gravitation, have a velocity directly proportional to the square 

 root of their wave-length. In like manner we shall find that the 

 form of all small organisms is largely independent of gravity, and 

 largely if not mainly due to the force of surface-tension : either 

 as the direct result of the continued action of surface tension on 

 the semi-fluid body, or else as the result of its action at a prior 

 stage of development, in bringing about a form which subsequent 

 chemical changes have rendered rigid and lasting. In either case, 

 we shall find a very great tendency in small organisms to assume 

 either the spherical form or other simple forms related to ordinary 

 inanimate surface-tension phenomena ; which forms do not recur 

 in the external morphology of large animals, or if they in part 

 recur it is for other reasons. 



6 cm., the ratio SjV = 1, and such a cube may be taken as our standard, or unit 

 of specific surface. A human blood-corpuscle has, accordingly, a si^ecific surface 

 of somewhere about 14,000 or 15,000. It is found in physical chemistry that 

 surface energy becomes an important factor when the specific surface reaches a 

 value of 10,000 or thereby. 



T. G. 3 



