OXYGEN CONSUMPTION IN ACIDS. 299 



may be repeated once more that for each hour's test freshly 

 aerated water was employed and this was freshly acidified to the 

 desired degree a few minutes before being used for each hour's 

 determination. Each test lasted for one hour. A determination 

 of the normal rate of oxygen consumption in normal water 

 (pH 8.0) was first run, and this was followed without pause by 

 a determination in the acidified water. In many cases there 

 were two or three trials in the acidified water, usually with 

 intervals between the trials. During such intervals the animals 

 remained of course in the acidified water. As it was found by 

 repeated experiment that the maximum percentage of depression 

 is obtained on the initial exposure to the acid, these later tests 

 in acid were omitted in the latter part of the investigation. 

 Generally the investigation of each acid was begun with the 

 lowest concentration (pH 7.5) and the concentration increased 

 by steps of 0.5 pH at each succeeding experiment. As will be 

 discussed later this procedure gives the most regular results and 

 great irregularities are introduced if the strongest concentrations 

 are employed first. 



There is not the slightest possibility that any differences in 

 the oxygen content of the water either at the start of each test 

 or produced during the test by the withdrawal from the water 

 of oxygen by the animals in any way affects the results. The 

 oxygen consumption of Planaria dorotocephala is entirely inde- 

 pendent of the oxygen content of the water at all concentrations 

 between 8 and 2 cc. per liter. Whether it is affected by concen- 

 trations above or below these limits has not been determined. 

 Suffice it to say that in none of the experiments here recorded 

 nor in any that I have reported in the past with this species 

 has the oxygen content of the water ever reached a value which 

 could have affected to the slightest degree the rate of oxygen 

 consumption of the animals. 



The acids used and the hydrogen ion concentrations at which 

 they were tested have already been stated. Concentrations 

 which might have injured the animals during the exposure were 

 avoided. For this reason very few tests were run at acidities 

 greater than pH 5.0. It w r as desired to keep within physiological 

 concentrations. 



