MICROSCOPIC FOAMS 25 



the edge by which they unite can be orientated in very 

 different ways witli regard to the plane of the image. In 

 addition to this point there is the fact that in alveoli of such 

 small diameter, it is, of course, no longer possible to ascer- 

 tain whether the sections of the lamelke or their edges have 

 a slight curvature ; as a rule, it is as much as one can 

 expect if the meshwork can be at all plainly recognised. 



This description shows, on the one hand, that, as has been 

 said, there may occur a great variety of alveoli, a possi- 

 bility which seemed at first doubtful, and on the other that 

 one must by no means expect to find angles of 120 or 

 109 everywhere, so that objections as to the regular frothy 

 nature of the drops described are not to be raised on this 

 account. Should there exist a doubt, however, as to whether 

 the fine meshwork is the image shown by a foam-like struc- 

 ture, it can easily be set aside by the fact that in such froths, 

 which are in one part more coarsely, in another part more 

 finely structured, a transition between the coarser portions 

 of the foam and the finest can be clearly traced. Since, 

 however, the former can be recognised with ease and 

 certainty as froths, this is a proof that the finest portions 

 also, in which only the network is distinguishable, must 

 possess the same structure. Still more decisive in this 

 respect are the results given by the investigation of defective 

 foams, or such as have been again disorganised by long 

 keeping, strong pressure, or the addition of unsuitable fluids. 

 Such foams contain portions of homogeneous oil in which 

 are scattered singly droplets varying in size from the 

 coarsest to the very minutest. In these isolated droplets 

 of froth, even in the finest of not quite 1 p in thickness, 

 it is possible to convince oneself that one is dealing with 

 more feebly retractile and, since they are always spherical, 

 fluid droplets. If the tube of the microscope be lowered 

 a little from the median sharp focus, they increase in 

 lightness, while by slightly raising the tube, on the 

 other hand, they become dark. It is further easy to 

 follow out how, by the gradually increased crowding 

 together of such very minute droplets, the portions 

 with a frothy structure originate, and in these in turn 



