(b) The Ninhydrin Reaction.—In order to understand the propositions that follow, we must be clear concerning the fact, that blood serum always contains, in varying quantities, substances which are to be found within the peptone group, and therefore react with ninhydrin. After a meal at which albumen has been taken, the quantity of such substances appearing in the serum immediately increases, in consequence of which the blood must be taken during a state of hunger.
A great many experiments have been necessary to determine what quantity of serum, in general, will give off to the dialysate only so much of the substances referred to, as is required for a negative reaction with ninhydrin. An insufficient quantity of serum must not be used, if the decomposition of the organ's albumen is to be as complete as possible. It has been found that, in general, 1.5 c.c. of the serum may be used. It is obvious that, under certain circumstances, an even greater quantity of serum may give off so few substances reacting with ninhydrin that the reaction of the dialysate still remains negative. Conversely it may happen that 1.5 c.c. of serum alone will give a positively reacting dialysate, which is the reason why a control test with serum alone is absolutely essential. The latter test indicates whether the serum in use answers the condition of not giving off, of itself, a sufficiency of substances to react with
