APPARENT HOMOGENEITY 37 



most distinctly in the photographs of these foams which 

 supplement this work (see Photogr. I.-III.) Although I am 

 not able to give any plausible explanation for this state of 

 things, it seems to me not without significance, especially with 

 regard to similar phenomena in protoplasm. Even in the 

 drops of foam photographed in I. and II., which were 

 adhering to the underside of the cover glass, and hence were 

 drawn out to the smallest possible thinness, this phenomenon 

 shows up distinctly ; in fact, a progressive diminution of 

 size towards the edge can be observed in the alveoli. The 

 most external and thinnest marginal region of this adhering 

 drop at first gives the observer the impression of being made 

 up of quite homogeneous oil, and not of alveoli; and it 

 would of course be conceivable that with such a minimum 

 thickness of the layer of oil, the foam vesicles would be 

 pressed out of it, so that an outermost border of homo- 

 geneous oil would be formed. Very careful observation 

 shows, however, that this apparently homogeneous border 

 really exhibits very faint indications of frothy structure. 

 In the photographs this can be plainly recognised in places. 

 So far as the size of the alveoli in this apparently homo- 

 geneous external margin can be ascertained in the photo- 

 graph, it is about the same as the size of those in the more 

 sharply defined zone bordering on it. 



On what does the fact depend of the alveoli of this 

 external margin being so pale and indistinct that it appears 

 all but homogeneous ? Since the layer of oil becomes 

 thinner and thinner towards the margin until it finally is 

 reduced to a minimum, it may for one thing depend upon 

 the fact that the delicate most external lamellae between 

 the vesicles of the foam become shallower and shallower, and 

 naturally at the same time fainter (see Fig. 4). Moreover, 

 the foam vesicles in the external zone, 

 which runs out quite flat, must them- 

 selves be very strongly compressed 



from above downwards, and hence pressed strongly against one 

 another laterally, which necessarily causes an attenuation of 

 the oil-lamella? between them to a minimum of thickness. 

 Both these factors taken together may explain, as it seems 



