32 PROTOPLASM 



fluid nature of the froths, a fundamentally important fact, by 

 pressing, inclining, or pushing them, by which it is proved 

 that well prepared foams are perfectly fluid, and flow scarcely 

 more slowly than the oil employed in their preparation. 



A phenomenon of especial importance is exhibited by the 

 surface of successfully-prepared foams of even composition. 

 With moderate magnification they appear surrounded by a 

 delicate, somewhat clearer border. To the exterior this 

 border is limited by a sharp and rather dark line ; to the 

 interior, its limit in like manner appears fairly sharp, though 

 less so than externally. Investigation with the highest 

 magnification proves that this border is delicately and 

 minutely striated vertically to the surface (see Fig. 4, 

 Plate V., and the Photographs III. and IV.) The com- 

 parison with the system of alveoli of the neighbouring deeper 

 portions of foam, which are immediately within the border, 

 show in the clearest manner that we are only dealing with 

 the outermost layer of the alveoli, directed radially to the 

 surface, which gives rise to the border. From these results 

 it is also easy to understand that this border can only 

 come into prominence in a froth of composition as regular 

 as possible, for if the meshes are very irregular in size an 

 even border cannot well be formed. 



The thickness of this border, which I have named the 

 alveolar layer, naturally depends on the size of the alveoli of 

 the foam ; if they are of considerable size, the border also 

 will be thicker. In the foams investigated by me the thick- 

 ness of the alveolar layer varied between about "0005 and 

 '005 mm.; alveolar borders so thick, however, were only 

 observed here and there in foams prepared with NaCl ; in 

 the fine ones prepared with K 2 C0 3 they were never thicker 

 than between '0005 and '0007 mm. Eeceiitly, however, I 

 have frequently observed coarser foams of the latter kind 

 which possessed very fine alveolar layers (see Photogr. IV.) 

 of greater thickness. 



The origin of the border is easily explained, since it is 

 only a consequence of the laws which the arrangement of 

 the froth lamellae obey. As has already been pointed out 

 several times, the froth-drops are completely fluid, so that 



