Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/206

 owing to the dilution of serum so caused. The tissue should never be handled with the fingers.

To the tubes containing the tissue 1 to 1.5 c.c. of serum are now added. A rule should always be made of arranging this experiment first. Afterwards from 1 to 1.5 c.c. of serum are placed in an empty tube (control test). Then the tubes are thoroughly rinsed with distilled water (as described on p. 151), and placed in Erlenmeyer flasks which have previously been filled with about 20 c.c. of sterilized water. Then a large amount of toluol is poured into the tubes and over the liquid outside, care being taken that the part of the tube which projects from the liquid should be soaked with toluol. At this stage of the experiment the following sources of error may arise. First of all, water may get into the tubes while they are being rinsed. If the work is not carried on in a scrupulously accurate manner considerable dilutions may occur. The tube must be completely closed during this operation. I have lately been in a position to observe a second source of error which may arise. Contrary to instructions the flask was filled with 20 c.c. of water and a large quantity of toluol, and only then was the tube and its contents immersed. In this case the liquid in the flask was raised to such a level that it passed from the outside to the inside of the tubes. Besides, the tube came into contact with the neck of the Erlenmever flask