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Proceedings of the Royal Society of Edinburgh. [Sess. 
with the previous statements. In view of these facts, it is obviously im- 
perative that any investigation as to enzymatic processes should be carried 
out as far as possible under conditions resembling those existing in the 
natural environment. 
The ideal method would therefore be to follow the chemical changes 
occurring in the various liquids, tissues, or organs of the living animal or 
plant; a method which is, however, full of practical difficulties, and is 
further open to the objection that it is impossible, under such conditions, 
to differentiate between the activity of living protoplasm and that of purely 
catalytic agents. This objection may also be urged when similar observa- 
tions are made on entire organs removed either in vivo or post mortem, 
while, on the other hand, a more or less complete disintegration of the 
tissues and the protoplasmic structure introduces an almost equally great 
difficulty in the probability of the presence of micro-organisms of various 
descriptions. This difficulty is so much the greater in that there is scarcely 
any form of enzymatic activity which is not possessed by some, or all, of 
the more commonly occurring bacteria. Consequently, if the occurrence of a 
reaction is to be taken as a criterion of the presence of an enzyme, it is an 
essential condition that the experiments must be carried through with 
absolute certainty of sterility in the material. This is at once the most 
vital and vulnerable point in experimental enzymology, and it is impossible 
to pay too much attention to it. 
In the case of liquids, sterility may be secured by means of filtration, but 
such cases are comparatively rare, and the method is open to the objection 
that adsorption compounds may be, and very probably are, formed with 
the filtering material (porcelain, etc.). 
Most frequently, therefore, recourse is made to the use of antiseptics, 
although this method is also, at the best, open to objection. 
Leaving out of count, for the present, the fact that the conditions of 
reaction in a disintegrated tissue are not comparable with the natural ones 
where reacting material and reaction products may constantly and gradu- 
ally be added and removed by diffusion, and the fact that it is inconceivable 
that a substance however chemically indifferent can be added to a colloid 
system without causing some change in reaction, conductivity, specific 
surface, etc., there yet remain two essential points to consider, viz. the 
efficiency of the antiseptic as such and its possible action on the enzyme 
itself, the substrate, or the reaction products. 
The ideal antiseptic is, of course, one which combines highest efficiency 
with minimal influence on the other conditions. It is obvious that it is 
almost as undesirable to study enzymatic catalysis in the presence of an 
