10 
organism, in a given time, than anions; that is, the contact 
P.D. at points such as a (Fig. 4), will be augmented, and 
< 
Figure 4. 
at points such as h unaffected or much less augmented (since 
the concentration of the KCl is as the inverse square). 
Hence the surface tension at h will be greater than that at a, 
and the organism will move fo wards the capillary. 
With a salt like CaCl,, in which many more anions 
would enter the organism, in a given time, than kations, the 
reverse would be the case. 
If the organism were -negative" the above effects would 
be reversed. 
Of course, leaving a 'positive" organism within the 
sphere of influence of CaCl^ for a sufficient time would 
con^•ert its initial repulsion from the CaCl., into attraction, 
for the organism would become "negative" owing to the excess 
of anions entering from the CaCl,. 8hnilarly, a "negative" 
organism, exposed for too lono- a" time to the influence of a 
re-agent in which the kations move faster than the anions 
(e.fjf., KCl, or an acid) would become "positive." 
"Isotactic" organisms— as we may call those organisms 
which are equally loaded with anions and kations— would, of 
course, be attracted by both kinds of re-agents, for an arti- 
ficial P.D. would be established on the side nearest the re- 
agent, and the surface tension therefore decreased at those 
points : but, as this P.D. would be very small except in 
organisms quite close to the capillary, such organisms would 
exhibit no marked re action. 
