this case also the dialysis is carried on in an incubator, in order to expose all the tubes to approximately equal conditions.
After some sixteen hours the ninhydrin reaction is applied. As this reaction depends so much upon the degree of concentration, it is advisable to carefully guard against the following sources of error. First of all, the dialysate must not be allowed to evaporate unevenly. To avoid this, an excess of toluol is added, and the Erlenmeyer tube is preferably covered with a watch glass. It is clear that, should the different dialysates evaporate unevenly, the ninhydrin reactions would be of varying intensity. The second source of error lies in the boiling of the separate test-tubes, which is applied in order to produce the formation of the colouring substances. We shall return to this presently.
In the application of the ninhydrin reaction we must never forget the fact, that ninhydrin is a most delicate reacting agent for albuminous substances, peptones, polypeptides, and amino-acids. Perspiration reacts very readily with ninhydrin, as do also the epidermic scales, &c. It is most important to avoid any contact of the dialysing tube with the hand; only sterilized forceps should be employed for holding them, and all the apparatus in use should be absolutely clean and dry. One must never rely upon any rapid drying methods. In the first place, it will not do to
