TOXICITY OF ACIDS TO CILIATE INFUSORIA 467 



vations under high power, such as Greeley made, of the changes 

 induced in the physical state of the protoplasm might throw- 

 light upon the nature of the effect of various acids upon the 

 tissue of different organisms. 



It is certain that the H ion is exceedingly important in the 

 swelling of muscle (Loeb, '98, no swelling in hypotonic salt 

 solution unless acid is also present) as well as in the swelling of 

 other hydrophilous colloids, such as fibrin, gelatin, etc. (Loeb, 

 '19; Proctor, '16; Fischer, '18). It seems probable that the H 

 ion is also important in the swelling of cilia, perhaps the specific 

 differences observed are due in part to differences in the colloids 

 present. 



Another factor seems to be surface tension. Working with 

 salts, Clowes ('16) found that slight changes in surface tension 

 produce great changes in the physical state of oil-soap emul- 

 sions, and he suggested that the physiological effects of these 

 salts were due to changes induced in the protoplasmic emulsion. 

 There is evidence in other experiments of profound changes in 

 the surface tension of protoplasm induced by acids. Ham- 

 burger ('13) found that ingestion of India ink or charcoal par- 

 ticles by phagocytes was depressed in too high concentrations of 

 fatty acids, but was stimulated in more dilute solutions, and this 

 stimulating action he attributed to changes in surface tension. 

 Koltzoff ('14) used Carchesium (a colonial relative of Yorti- 

 cella) for the same purpose, and found that at some concentra- 

 tions acids increased the ingestion of ink particles, and at 

 slightly higher concentrations produced visible evidence of 

 change in surface tension in the accumulation of ink particles 

 on the cilia. The optimum concentration for this softening 

 effect as well as for stimulation of phagocytosis varied with the 

 acid used. 



Another factor in toxicity is lipoid solubility. Benzoic acid 

 is much more lipoid-soluble than the fatty acids and should 

 therefore penetrate a lipoid rich membrane and attack the cell 

 contents more rapidly. The fact that benzoic is more toxic than 

 valeric to Euplotes but not to Paramoecium suggests the pres- 

 ence of some lipoid in Euplotes which is not present in the same 



