342 Brown.—Studies in the Physiology of Parasitism. I. 
While it would appear that the retarding action of a chemical substance 
is related to its capacity of ionization, it is still quite possible that some non- 
ionizable substances may exert a specific retarding or even an anti -action 
upon the principle of the fungal extract. In this connexion one may sug¬ 
gest such substances as tannins, alkaloids, latex constituents, &c. In any 
investigation of immunity from the biochemical side, experiment along 
these lines might prove to be of value. As, however, the specialization 
in relation to host is very slight in the case of the fungus under con¬ 
sideration, such a detailed investigation has not been attempted in the 
present case. 
Traces of alcohol, chloroform, acetone, and formaldehyde do not, as far 
as can be seen, have any appreciable effect upon the macerating action 
of the extract. 
Colloidal substances might be expected to influence the activity of the 
extract by disturbing the adsorptive equilibrium within the liquid. In this 
connexion only one colloid has been tested—viz. starch. The investigation 
of this point arose incidentally in the examination of the effects of dilution 
on the activity of the extract. An account of these experiments may 
conveniently be given here. 
Three diluents were used : (i) water, (2) \ per cent, starch solution, 
(3) extract deactivated by heating to 65°. The figures in the table repre¬ 
sent the times required to produce loss of coherence in potato discs. 
Concentration. 
Water. 
Diluent. 
| % Starch Solution. 
Deactivated Extract. 
1 
20 
20 
20 
1 
2 
25 
25 
25 
4 
40 
40 
35 
\ 
65 
65 
45 
From this it appears that the starch has no measurable effect in the 
concentration employed. The greater activity of the extract diluted 
according to the third method is somewhat interesting. It would appear 
to be connected with the maintenance of the original acidity, the acidities 
of the extracts diluted with water or with starch solution being reduced 
very nearly to zero. (See p. 338, on effect of reduction of acidity on 
activity.) 
From the fourth column of the above table we see that the activity of 
the extract is proportional to the square root of the concentration of active 
principle. In the neighbourhood of the full strength of the extract 
(i. e. with ‘standard 5 extract) there is considerable divergence from this 
law. Here the decrease in activity on dilution is less than required by the 
law, or conversely the gain in activity with increase of concentration becomes 
negligible after a certain concentration is reached. This is in agreement 
with what was mentioned in dealing with the technique of extraction, where 
