1899-im] Dr J. S. M‘Kendrick on Enzymes in Tissues. 75 
4. The change in fibrin when acted on by X in a 1 per cent. 
carbonate of soda solution, and the presence or absence 
of peptones, leucin, or tyrosin, the result of the activity 
of an enzyme similar to trypsin. 
5. The inversion of cane sugar by X into a reducing sugar, and 
this probably by an enzyme similar in its action to 
inversin. 
6. The curdling of milk, and this by an enzyme similar in its 
action to rennin. 
7. Whether the extract itself had any reducing properties. 
Sources of Error in the Experiments, and how these 
WERE AVOIDED. 
1. Length of time for extraction by glycerine — 
We know that enzymes, when present in small amount (and 
they are likely to be so in the tissues), require considerable time 
for their extraction by glycerine. Consequently, little or no 
reaction might be obtained from tissues, although an enzyme was 
present, if the tissue were not a sufficiently long time in glycerine. 
To avoid this source of error, in all cases the tissues were immersed 
in glycerine for six weeks, and in many cases for a longer period. 
2. Length of time required for enzymic action — 
It is of importance to subject the solutions containing the 
supposed enzyme to a temperature of 38° C. for a considerable 
time. W r hile enzymes may exist in the glycerine extract no 
reaction may be obtained, owing to a deficient exposure at the 
proper temperature of the mixed fluid under observation. To 
avoid this cause of error, I allowed the action to go on for a 
period of from eighteen to twenty-four hours. 
3. The purity of the solutions used — 
The solutions of starch and cane sugar must be fresh, and 
possess no reducing properties. Consequently, they must be 
always tested before any observation is made ; and, further, 
these solutions must be tested after remaining in the incubator 
for twenty-four hours. I have found, in regard to this latter 
point, that a pure starch or cane sugar solution, when submitted to 
a temperature not exceeding 40° C. for twenty-four hours (with 
