selected. If, in such a case, the sample of organ + serum shows a markedly stronger blue coloration, then the case has to be looked upon as positive in respect to the reaction. Should, however, the difference in the intensity of the coloration be very small, the experiment has to be performed again, using a less quantity, say 1 c.c., of serum. It would then be possible to ascertain whether decomposition had taken place or not, as the serum sample would be negative.
The appearance of the reaction should on no account ever be determined by artificial light. Again, it is not advisable to compare the test-tubes in their stands, but each one should be taken out separately, and examined against white paper by both transmitted and reflected light.
Breaches of this rule are very often committed. Many reactions are declared positive which, when thoroughly investigated, show not the slightest coloration. If a sample is marked as being just perceptibly positive, then a number of other samples should be changed about in the hand, and, only if the same sample can be unhesitatingly picked out as showing a coloration, should one's judgment concerning the reaction be relied on.
Difficulties are only experienced with reddish and yellowish-brown tones, but these have no relation whatever with the ninhydrin reaction. They can
