THE BEHAVIOR OF INFUSORIA; PARAMECIUM 63 



2^ per cent ; sodium hydroxide, 5^ per cent ; sulphuric acid, -g^-g per 

 cent of an ordinary laboratory solution ; hydrochloric acid, g ^-g- per cent 

 of the usual solution ; alcohol, i per cent ; chloral hydrate, | per cent. 

 For the inorganic chemicals, many of these solutions are so weak as not 

 to affect at all the sense of taste in man. 



Is the reaction of Paramecium to solutions due to the chemical 

 properties of the dissolved substance, or to its osmotic pressure ? This 

 question may be answered from the data which we possess (partly 

 given above) as to the weakest solutions which cause reactions. If 

 the reactions are due to osmotic pressure, then solutions having equal 

 osmotic pressure must have equal stimulating power. The results of 

 the experiments on the weakest solutions necessary to cause reaction 

 show that this is not true. Thus, if the osmotic pressure of a solution 

 of sodium chloride that will barely cause the reaction is taken as unity, 

 the osmotic pressures of solutions of a number of other substances hav- 

 ing the same stimulating effect are as follows : potassium bromate, ^ ; 

 sodium carbonate, ^2 > copper sulphate, 2^3- ; potassium hydroxide, -^q ; 

 sulphuric acid, -^ ; ethyl alcohol, 8. The stimulating effect is not then 

 proportional to the osmotic pressure, and must be due to the chemical 

 properties of the substances in solution. 



This is further shown by the fact that Paramecia will enter solu- 

 tions of sugar and of glycerine having osmotic pressure many times as 

 great as that of a solution of sodium chloride which they avoid. They 

 swim into a 20 per cent solution of sugar or a 10 per cent solution of 

 glycerine without reaction. The solutions are so concentrated that 

 they cause plasmolysis; the Paramecia shrink into flattened plates. 

 Just as the shrinking becomes evident to the eye of the observer, the 

 Paramecia react in the usual way, by swimming backward and turn- 

 ing towards the aboral side. But this is as a rule too late to save them, 

 and they die in the dense solution. Thus it is evident that osmotic press- 

 ure, acting by itself, produces the same "avoiding reactions" as do 

 other stimuli, but the result is not produced till the Paramecia are 

 already injured beyond help. The reactions to most solutions are then | 

 clearly due to their chemical properties. 



Is the avoiding reaction that is produced by chemicals due directly 

 to the injuriousness of the substance ? This question may be answered 

 by a series of experiments based on a method similar to that used in 

 determining whether the reaction is due to osmotic pressure. If the 

 reaction is due to the injuriousness of the chemicals, then two substances 

 which are equally injurious must have equal powers of inducing reac- 

 tion ; in other words, the repelling powers of any two substances must 

 be proportional to their injurious effects. 



