64 



BEHAVIOR OF THE LOWER ORGANISMS 



An extensive series of experiments has shown that this is not true 

 (Jennings, 1899 c; Barratt, 1905). We may compare, for example, the 

 effects of chromic acid and of potassium bichromate. The weakest 

 solution of the former which kills the Paramecia in one minute is j^ 

 per cent ; the weakest solution of the latter having the same effect is 

 I per cent. Hence the chromic acid is 150 times as injurious as the 

 potassium bichromate. 



On the other hand, the weakest solution of chromic acid that sets 

 in operation the avoiding reaction is still yJtj- per cent, while potassium 

 bichromate has the same effect in a ^q^ per cent solution. The repel- 

 lent power is thus not proportional to the injurious effects ; the potas- 

 sium bichromate is repellent in a strength ^-^ that which is immediately 

 injurious, which chromic acid does not repel until it has reached a 

 strength that is already destructive. Similar relations are found for 

 other pairs of substances. Thus the stimulating power of sodium 

 chloride is ten times that of cane sugar, in proportion to its injuriousness. 



Comparing a large number of chemicals from this point of view, 

 it has been found that they may be divided into two classes. On the 

 one hand are a number of substances which must be classified with 

 potassium bichromate and sodium chloride, because their stimulating 

 power is strong in proportion to their injurious effects. Paramecia 

 avoid these substances markedly ; if a drop of a strong solution of one 

 of them is introduced into a preparation of the infusoria, it remains 

 empty, and none of the infusoria are killed by it. On the other hand, 

 there is a large number of substances which, like chromic acid and 

 sugar, produce stimulation only where they are strong enough to be 

 immediately injurious. When a strong solution of one of these is brought 

 into a preparation of Paramecia, it proves very destructive, for the ani- 

 mals as a rule do not react until they have been injured. The follow- 

 ing table (from Jennings, 1899 c) shows the distribution of various 

 chemicals from this point of view: — 



TABLE 



I . Repellent power strong in proportion 

 to injurious effects ; reaction protective. 



LiCl, NaCl, KCl, CsCl, 



LiBr, NaBr, KBr, RuBr, 



Lil, Nal, KI, Rul, 



Li2C0,„ Na^CO,, KXCj, 



LiNO,,, NaNO^; KNO,, 



NaOH, KOH,'NaF, KF, 



NH4F, NH.,C1, NH.Br, NH.I, 



CaClj, SrClg, BaCl,, 



Ca(N03)2, Sr(N03\, Ba(NO,,),, 

 Potassium bromate, Potassium perman- 

 ganate, Potassium bichromate. Potassium 

 ferricyanide, Ammonium bicliromate. 



2. Repellent power very weak in pro- 

 portion to injurious effects ; reaction not 

 completely protective. 



HF, HCl. HBr, HI, HoSO^, HNO,„ 

 Acetic acid, Tannic acid,' Picric acid, 

 Chromic acid, Ammonia alum, Ammonio- 

 ferric alum, Chrome alum, Potash alum, 

 CuSO,,, CuClj, ZnCl.,, HgClj, A1C1„ Cop- 

 per acetate. Cane sugar, Lactose, Maltose, 

 Dextrose, Mannite, Glycerine, Urea. 



