Brown.—Studies in the Physiology of Parasitism. I. 339 
sharply and at the same point to the addition of alkali as does the macerating 
principle. 1 
When the concentration of alkali is further increased, a macerating 
action again sets in. Control experiments show that this is due to the 
alkali itself. With potato tissue a macerating action sets in when the 
concentration of alkali approaches the value — : with tissue of white 
fl 
Turnip a concentration of alkali can be seen to produce a slow macerating 
effect. It is noteworthy that maceration by dilute alkali is also accom¬ 
panied by death of the cells. 
The active principle is inhibited but not destroyed by the alkali. On 
adding an appropriate amount of acid, the activity of the extract is 
restored, that is, apart from the retarding action of the salt thereby 
produced. 
(ii) Increase of Acidity. In this connexion the following acids have 
been tested— citric, malic , tartaric , sulphuric , and hydrochloric. In the case 
of the two mineral acids and of the dilute concentrations of the organic 
acids, the required concentration of acid in the extract was obtained by 
adding a measured quantity (in drops) from a strong solution of appropriate 
strength. The slight degree of dilution of the principle of the extract which 
is thereby produced is known to be of no importance. The higher con¬ 
centrations of the organic acids were obtained by evaporating solutions of 
the strength desired to dryness on the water bath, and making up to the 
original volume with standard extract. 
In all cases the action of the acid on the activity of the extract is one 
of retardation. The retarding effect in relation to normality is approxi- 
/ M fl \ 
mately equal for all the acids up to a certain point (— to ; above this 
32 4 
point the retarding action for the mineral acids increases very much more 
rapidly than that of the organic acids. In the case of the mineral acids, 
the macerating action of the acid appears at a fairly low concentration, 
so that above this point the action of the acid extract is not distinguishable 
from that of the acid itself. In the case of the organic acids macerating 
effects due to the acid appear at a much greater concentration, so that the 
retarding action of the acid upon the extract can be studied over a 
comparatively wider range. 
1 As the colour reaction is not sharp, it may well be that (6) represents the neutral point more 
closely than does (5). Furthermore, there is an escape of acid juice from the turnip disc, so that 
the acidity of the extract varies with time, especially in the neighbourhood of the neutral point. 
Thus the activity assigned to extract (5) in the preceding table is probably too high, or, more exactly, 
the activity of that extract increases with the gain in acidity due to the escape of acid sap from the 
turnip disc. It is therefore impossible to say whether the activity of the extract disappears when 
the neutral point is reached or whether deactivation is completed when the solution reacts alkaline. 
