FLUIDITY OF OIL- FOAMS 31 



considerable extent in the foams, and that, there fun ', their 

 structure depends solely on their nature as foam. 



This conclusion receives further confirmation from a very 

 interesting circumstance with regard to the perfectly clear and 

 transparent oil-drops that are obtained in the way described 

 from froths. If water is allowed access afresh to drops of this 

 kind, the smaller and the more minute ones become changed, as 

 if by a magic touch, instantaneously into the most beautiful drops 

 of foam again, with all the characteristic structural relations 

 described earlier. The larger drops also become frothy at once 

 down to a considerable depth, and in a relatively short time have 

 again acquired a frothy consistency throughout. This astonishing 

 behaviour may be explained by the fact, that with the drying 

 up of the froth-drops the soap l contained in the froth vesicles is 

 again taken up by the oil and dissolved, so that in this way a 

 drop of oil is formed richly impregnated with dissolved soap, 

 which, with addition of water, at once passes rapidly back into 

 froth again. If, therefore, we may see in this process, on the 

 one hand, a further confirmation of our views as to what goes on 

 in the formation of the froths, it offers, on the other hand, also 

 one of the finest examples of a so-called clesolution process. From 

 all that has been stated, it can be concluded with perfect certainty, 

 that my view as to the structure of the froth-drops is well 

 grounded, and agrees with all the results of observation. 



In successfully prepared froths the breadth of the finest 

 meshes varies between about *005 mm. and '001 mm. or 

 less. When such drops of froth remain undisturbed for some 

 weeks, the alveoli gradually become arranged in layers 

 corresponding to their relative sizes. The finest froth 

 collects in the uppermost layer, and further below it becomes 

 coarser and coarser. If the drop was defective in froth, so 

 that homogeneous oil was present in addition, the latter 

 gradually assumes the highest position in the drop. This 

 phenomenon naturally depends on the greater specific 

 gravity of the contents of the alveoli in comparison to 

 the oil, but offers, however, a further proof of the complete 

 fluidity of the froths as well as of their frothy structure, 

 since neither net-like nor granular structures could behave in 

 this way. One can, moreover, easily convince oneself of the 



1 Or other unknown substances also, which assist in the formation of froth. 



