564 Williams . — On the Influence of Immersion in certain 
healthy cells. Since the iron salt used was one liable to hydrolyse, further 
study would have been advisable. 
After the results had been published it was found that the formula 
agreed with one already derived by other workers for definite measures of 
change in the case of bacteria (2) and unicellular Algae ( 3 ). The additional 
interest of this similarity led to the decision to repeat and extend the work, 
using a more stable reagent for testing the change in the protoplasm and 
carrying out all immersions at a fixed temperature. 
Choice of Reagent. 
Search was -made for a reagent which would prove a delicate test for 
tannin and at the same time would be stable in dilute solution. Dekker ( 4 ), 
in his work on tannins, recommends a 5 per cent, solution of potassium 
dichromate for staining purposes. 
It was found that in the particular cells studied a solution of o-i per cent, 
was sufficient to produce a definite precipitate in a period of three minutes 
when the cells had previously been rendered permeable. This very dilute 
solution was also suitable for sections studied under the ultra-microscope, as 
the first-formed particles came down with well-marked Brownian move- 
ment. The next step was to find whether this weak solution was stable. 
This was done by a series of measurements of the resistance when a con- 
ductivity cell was filled with the solution and the whole kept in a thermostat 
at 25 0 C. The measurements were made by the ‘ metre bridge 5 method, 
a commutator and galvanometer being employed. The readings were 
taken at different times extending over forty-two days ; for each date the 
mean of three determinations was taken. The values given in Table I 
showed a variation of only 1 per cent., so that the solution was considered 
free from objection. 
All the tests on the plant material necessary in the work given below 
were made with solutions prepared from the same sample of crystals as 
those used in the conductivity tests. 
The influence of various concentrations of the potassium dichromate 
upon the cell had next to be studied. The plant material, treated as for 
the rest of the immersions, was placed in tubes containing 30 c.c. of the 
solution and kept in a thermostat at 25 0 C. In each concentration the time 
was found when a clearly defined precipitate first appeared in the cells. 
The results are shown in Table II, and indicate that this salt has a £ time 
and concentration * connexion of the same type as that already mentioned, 
but that the influence of a o*i per cent, solution, acting upon the cell for 
three minutes, is negligible. 
The effect of changing the temperature of immersion was also studied 
for the same salt, a 3 per cent, solution being chosen. The readings for 
time required and temperature are given in Table III and Fig. 1. Only 
