Il6 T. BRAILSFORD ROJ5ERTSON. 



different reagents to the culture-medium, in which the infusoria 

 were suspended, upon their distribution under the cover-glass. 



When a substance which evaporates more rapidly than water 

 is dissolved in a watery medium containing salts in solution two 

 contrary effects take place on evaporation the solution becomes 

 more dilute in respect to the more rapidly evaporating substance 

 owing to its evaporation and more concentrated in respect to the 

 salts owing to the evaporation of water. When the decrease in 

 osmotic pressure due to the evaporation of the more rapidly 

 evaporating substance is greater than the increase in osmotic 

 pressure due to the increased concentration of the salts then 

 the total result is a decrease in the osmotic pressure. But if the 

 initial concentration of the more rapidly evaporating substance be 

 less than that necessary to bring about the above result then the 

 increase in concentration of the salts will proceed more rapidly 

 than the decrease in concentration of the volatile substance and 

 the total result will be an increase in osmotic pressure at the sur- 

 face of the fluid. Thus it was to be expected that while under 

 the ordinary conditions of my experiments on Chemotaxis 

 when to a very small amount of culture a comparatively large 

 amount of solution was added --methyl alcohol at a concentra- 

 tion of Ay25 caused a dispersion of the infusoria to the edge of 

 the cover-glass, when I added I c.c. of a solution of methyl alco- 

 hol to 5 c.c. of culture it was found necessary to bring the final 

 concentration of the methyl alcohol up to |A 7 in order to get 

 the first indication of a tendency to seek the edge since in this 

 latter case the salts were correspondingly less dilute. The solu- 

 tion of the volatile or of the water-absorbing substance in the 

 culture medium having been made, drops of the mixture were 

 placed on a slide under a cover-glass which was slightly raised 

 at one end. 



The following are the experimental results : 



Methyl Alcohol.- -\ c.c. of a ^N solution of CH 3 OH was 

 added to 5 c.c. of the culture-medium in which numerous colpodia 

 and paramcecia were suspended. Result at first uniform distribu- 

 tion of the infusoria under the cover-glass, in a few minutes, how- 

 ever, they began to congregate at the edges --especially at the 

 edges farthest away from the supported end. In a short time all 



