38 PROTOPLASM 



to me, why only traces of frothy structure are to be found 

 in the external margin. 



Yet a further point may be touched upon at this oppor- 

 tunity. In the manipulation of these drops of foam under 

 the cover glass, one has occasion to observe, if the drops stick 

 more or less to the glass, that fine lamellae or threads of the 

 frothy mass stretch across between drops squeezed apart. 

 I was able to observe fine lamellae of this kind, which con- 

 sisted of only a single layer of alveoli, in optical section, 

 when it could be seen most distinctly that, as was to be 

 expected, the partitions between the alveoli stand vertically 

 to the two surfaces of the lamella (see Fig. 4, Plate X.) In 

 the finest threads, which stretch out between neighbouring 

 drops and naturally break very soon and become retracted 

 into the contiguous portions of froth, I could not observe 

 any structure, and hence believed formerly that they were 

 only formed of the ground substance of the froth, i.e. oil. 

 But I have become somewhat doubtful on this point in 

 consequence of my experiences with regard to the scarcely 

 recognisable frothy structure of the greatly attenuated mar- 

 ginal portions of drops already described, and now prefer to 

 assume that such finely drawn-out threads also retain a 

 frothy structure, but that their structure can no longer be 

 plainly observed from causes similar to those obtaining in 

 the strongly attenuated marginal portions. In every case 

 where such threads show even the slightest swelling, the 

 latter is always distinctly frothy, even if occasionally only 

 composed of quite a few alveoli. 



2. Some more exact Statements as to the Alterations in Volume 

 of the Drops of Foam under the Influence of the sur- 

 rounding Fluid 



Since it seemed to me of great importance for the correct 

 explanation of the froths, to determine somewhat more 

 accurately the alterations of volume already mentioned, I 

 recently performed some experiments which were directed 

 towards solving this question. For this purpose two narrow 

 glass strips of 0'20 mm. in thickness were fastened with 



