14 
solutions. For our purposes this choice is most unfortunate, 
as the dissociation of acetic acid is very small, even in dilute 
solutions; indeed, it is only half dissociated when the solution 
contains only about two parts of acetic acid per million.* 
Moreover, the amount of hydrion due to acetic acid is groaL'y 
reduced on its diffusion into a medium containing highly 
ionised salts (as was the case in Dale's experiments), while the 
acetanions are not correspondingly reduced, and the resultant 
proportions of ions depend upon the electrolytes into which it 
is diffusing.! Hence the theoretical effects of acetic acid are 
highly uncertain, and this corresponds with the uncertainty 
of Dale's results. Such sources of ambiguity do not arise 
when we use strong acids in dilute solution and perfectly 
ionised solutions of salts. A number of other investigators 
have tested the effects of various re-agents upon uaicellul.ar 
organisms, but as they did not previously ascertain the nature 
of the ions in the medium in which the organisms were tested 
their results tell us nothing with regard to this theory. I there- 
fore carried out a series of experiments with a view towards 
systematically testing the accuracy of the conclusions put 
forward in this section. The organisms used were the infu- 
soria in the large intestine and rectum of a frog (Ranaodea 
aurea). Four species were found and used in these experi- 
ments, namely: — 1. A species of Spirostomum, closely resem- 
bling, if not identical with, S. anihiguum. 2. A species of 
Opalina, probably Opalina ranarum. 3. A large disc-shaped 
species, more than half the length of Sjih^oHomiiin sp., and 
nearly as wide as it is long, much flattened laterally, endo- 
plasm in front of the mouth, triangular in shape, slightly re- 
curved. 4. A much smaller species, only about half the 
length of Opalina sp., but otherwise resembling the last-men- 
tioned species. The two latter species, in the absence of any 
expert knowledge of the subject, I am unable to name. I will, 
therefore, designate them, respectively, species A and 
species B. 
A cell of wax was made on a glass slide. It measured 
about I in. square, and the walls were about 1 millimetre 
deep. In two opposite walls of the cell were grooves, which 
were the same depth as the walls. A small portion of the 
intestinal or rectal contents was placed in the cell, and a 
large drop of a given solution, the medium, was placed in the 
cell with it. This was left for a varying period, and then a 
cover-glass was placed on the cell, any spaces in the cell being 
Walker: Tnti-oduction to Physical Chemisti-v, third edition, 
page 236. 
t Vide Walker: IntroductioiU to Physical Chemistry, third 
edition, pages 304 and 316. 
