TOXICITY OF ACIDS TO CILIATE INFUSORIA 445 



hand centrifuge. This treatment does not injure them. If 

 they are washed less than five hours, the results are apt to be 

 irregular, but five hours seems to be almost as satisfactory as a 

 longer period. Pond-water was used because ordinary distilled 

 water seems to be somewhat injurious. After being washed and 

 reconcentrated, the organisms were transferred to a watch-glass, 

 in which they could be kept in apparently normal condition for 

 several hours. 



In order to test their- resistance to the acids, the following 

 procedure was adopted. A test-tube was filled with about 15 cc. 

 of the solution to be tested, and less than 0.1 cc. of the concen- 

 trated culture was introduced with a capillary pipette. After 

 being inverted once or twice for mixing, the test-tube was corked 

 and placed in a water-bath, where the temperature could be 

 held at the desired level throughout the experiment. Samples 

 were poured at intervals into watch-glasses and observed under 

 the low power of the compound microscope. It was found 

 desirable to use watch-glasses with curved rather than vertical 

 sides in order to prevent collection of the organisms at the 

 margin where they are difficult to see. It is not enough to note 

 the time at which the organisms cease to move about, for the 

 cilia usually beat for some time afterward. Since there is often 

 considerable variation in individual resistance, the time at which 

 the cilia of just over half cease to beat was adopted as a better 

 measure of average resistance than the time at which all are dead. 

 Every solution was tried five or even ten times over, usually on 

 different days, in order to check deviations due to biological 

 differences, which, however, were relatively slight. In the fol- 

 lowing tables the averages of all the determinations are given. 



DISCUSSION OF RESULTS 



A. Concentration 



In these experiments each acid of the series was used at four 

 of the following concentrations, viz., 0.001, 0.0005, 0.0002, 

 0.0001, 0.00008 N. Full data on the effects of these solutions 

 are given in table 1, but the important points are perhaps 



