Il8 T. BRAILSFORD ROBERTSON. 



duce thus methyl alcohol has a lower boiling-point than ethyl 

 alcohol and it also causes a more marked attraction to the edges 

 of the film. 



The fact that in all these cases the attraction to the edges of 

 the film disappears after a certain time may perhaps be due to the 

 volatile substance having become so dilute in the film that the 

 increase in concentration of the salts due to the evaporation of 

 water now takes place more rapidly than the decrease in concen- 

 tration of the volatile substance. While in the case of water-ab- 

 sorbing substances they may have become so dilute that the 

 water-absorption due to their presence in the solution now goes on 

 less rapidly than the loss of water due to evaporation. It seems 

 probable therefore that these phenomena of attraction to or re- 

 pulsion from the edges of the film are in reality special cases of 

 osmotaxis. 



I have alluded to the fact, mentioned by Jensen in the paper 

 to which I have referred, that under ordinary circumstances Para- 

 nicccia under a cover-glass supported at one end tend to collect 

 in the middle of the film but towards the supported end. But when 

 the substances which cause the infusoria to congregate at the 

 edges of the film are added to the culture we obtain precisely the 

 converse effect the infusoria collect at the edges, especially at 

 the edges farthest from the supported end. It thus appears prob- 

 able that this is also an osmotactic phenomenon connected with 

 surface dilution. One would be inclined to fancy that it was due 

 to surface evaporation having more effect upon the concentration 

 of the small bulk of liquid at the shallower end of the film than 

 upon the greater bulk of liquid at the supported end but the 

 ratio of surface to volume is not greater at the shallower end 

 than at the supported end. Dr. Loeb has suggested to me that 

 it may be due to the fact that diffusion and therefore equalization 

 of concentration is less rapid in capillary spaces than in the bulk 

 of the liquid that, in fact, the shallow end of the film may act 

 in a manner analogous to Liebreich's " dead space." * 



1 Zeitschrift Jiir Physikalische C/iewie, Vol. V. (1890), p. 529 and Vol. VIII. 

 (1891), p. 83. 



