1 52 THE BO T. 1 M(\ I L MAGAZINE. [Vol. xxi. No. 450, 



sealed, the other end is inserted in to the very dilute solution 

 of acids to be tested. By an exceedingly dilute solution of aeids 

 or by tap-water the spores near the mouth of the tube are not 

 arrested in motion, some moving outside the tube and some 

 proceeding deep into it. However, if the solution of acids is 

 somewhat higher, a reaction immediately takes place on the 

 spores at or near the mouth. We see that some of those, which 

 are previously moving towards the inner extremity of the tube 

 reverse their direction and move backwards. This backward- 

 motion is surely due to a perception of the decrease of the 

 concentration of the acids. Therefore, in order to determine the 

 minimal stimuli we must ascertain the minimal degree of the 

 concentration of given acids, necessary to cause the first re- 

 version of motion. It is approximately as follows : 1} 



Hydrochloric acid 1/10000 mol. 



Sulphuric acid 1/20000 mol. 



Acetic acid 1/1000 mol. 



Malic acid 1/4000-1/6000 mol. 



Tartaric acid 1/8000-1/10000 mol. 2) 



In the preceding I have remarked that a tube containing, for 

 instance, hydrochloric acid less than 1/300 mol does not show 

 a visible attraction of the spores. Consequently, were the 

 usual capillary method applied, we should have concluded that 

 nearly 1/300 mol of hydrochloric acid might be the critical 

 concentration to exert the minimal stimuli, a concentration 

 about thirty times more strong than the actual value 1/10000 

 mol obtained by the method given above. 



In L/3000— 1/4000 mol of sodium hydroxide the swarm-spores 

 shrink in body, though they do not come to rest. In 1/6400 

 mol they arc mostly normal, while in 1/10000 mol they are 



M rimenl wa» done at nearly constant temperature 20°c 



' l> It may be noted thai the concentration of each acid given here is not far 

 from being isohydric. 



