78 PROTOPLASM 



in a drop which is free from pressure (see Fig. 16, A), a 

 marginal centre of stream lines arises by the bursting of some 

 of the alveoli in the region of the equator a, it will gradually 

 be suppressed by the strong polar centres of extension ; the 

 more so, as the soap solution which exudes into the relatively 

 thick surrounding layer of fluid will spread abroad rapidly 

 by diffusion. In the drop which is under pressure (see Fig. 

 16, B) the polar currents become more and more feeble, and 

 since at the same time the surrounding layer of fluid is 

 much thinner, the soap solution dis- 

 charged by the bursting of some of 

 the marginal alveoli will spread 

 abroad much more slowly. This 

 reasoning seems to me to make it 

 to some extent intelligible how it is 

 p . 16 that centres of extension are gradu- 



ally developed at the edge of com- 

 pressed drops, leading to the phenomena already described. 1 

 All phenomena which we observe in the streaming froth- 

 drops, namely the posterior .resting zone, the axial streak, 

 and such like, have already come under our notice in 

 streaming oil-drops, and are to be explained in the same 

 way as in the latter. 



The long continuance of the streaming in the drops of 

 oil-lather is explained by the fact, that they contain a con- 

 siderable store of soap solution in their alveoli, which is very 

 gradually used up during the movement, and as gradually 

 diffuses to the exterior. Since the froths after extinction 

 of their movement show no alteration of appearance, the 

 cessation of the streamings cannot be the result of an altera- 

 tion in structure ; it is rather the diminution of the soap 



1 It would be very important to investigate the movements of froth-drops 

 suspended in the fluid quite free from pressure ; the more so as friction against 

 the slide and cover glass must essentially influence the movements of the 

 strongly pressed drops. Since the drops are specifically lighter than water, 

 such pressure could only be caused by mixing with them finely divided 

 particles of a heavy substance, e.g. sulphate of baryta. I think that such 

 experiments would not be hard \ to carry out and would yield important 

 results. Unfortunately I lacked t*he time for putting the point to the test 

 myself. 



