Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/224

 insufficient layer of toluol, changes of temperature in the incubator, and working in places where acid or alkaline vapours are developed.

These sources of error ought, properly, to occur very seldom.

On the other hand, the following point is often overlooked. After the tube has been filled with the organ, and the serum and the toluol have been added, it is absolutely necessary to make sure that the whole of the tissue is covered with the serum and toluol. Should the slightest portion of the tissue project above the toluol, it is then liable to decay in the course of sixteen hours, and so become a source of serious error.

In conclusion, we will add the following supplementary details, which are not at present in general use, because they are not considered as being absolutely necessary. We may, instead of the control with serum, use a control with organ + inactivated serum. The serum is heated for thirty minutes at a temperature of 60° C. This kind of control is capable of indicating an insufficiently prepared organ.

Starting with the idea that a certain limital value must be present, in order to give a colour reaction with ninhydrin, one might conclude that it would not suffice to test the filtered water, in which the organ was boiled, with 1 c.c. of ninhydrin solution. We have therefore produced a solution of silk-peptone,