MOVEMENTS OF FOAM-DROPS 47 



foam-like nature of the drops remains unchanged, even when 

 the preparation is left uncovered for a long time. 



6. The Phenomena of Streaming Movement exhibited by the 



Oil-Foams 



If successfully manufactured drops of oil-foam, obtained 

 from oil correctly prepared according to the above prescrip- 

 tion, be carefully washed out with water under the cover- 

 slip an operation effected in the well-known manner, by 

 drawing water through with filter paper, and best done from 

 the two sides alternately, in order to remove any solid 

 particles adhering to the surface of the drop then striking- 

 processes of movement are at once set up in the drops. As 

 long as they remained in the weak solution of K 2 C0 3 they 

 were perfectly quiescent. The movements of the drops of 

 foam, when free from pressure and hence quite opaque, take 

 place in such a manner, that without any striking change of 

 shape, they creep somewhat rapidly backwards and forwards, 

 under the cover glass. At the same time, the direction of 

 movement changes fairly often, though it also happens that a 

 drop may retain for a long time, or permanently, the direction 

 of movement it has once taken up. 



As has been said, the movements of progression were 

 frequently very energetic ; thus, I once observed a drop 

 which in one minute traversed 0'45 mm. ; usually, however, 

 the forward movement was less rapid. When it was re- 

 marked above that these movements go on without particular 

 changes of form, the statement is so far correct, that the 

 changes are not very striking ; still, they are not wanting. 

 Frequently the drops become somewhat elongated in the 

 direction of the forward movement ; bulgings out of the edge 

 also appear here and there, which, for the most part, quickly 

 vanish again. A change of shape is therefore not wanting 

 -only, on the whole, it is slight. In spite of the great 

 opacity of the drops, it can be observed that lively streaming 

 phenomena take place in them. Every bulging out of the 

 edge is accompanied by a stream which starts from the 

 interior, and spreads out on the surface ; the creeping pro- 



