Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/174

 tube be very dense and allow little or no passage at all of the peptones, we should naturally have in this a considerable source of error.

Numerous dialysing membranes are known, of which very few have any real value for our purpose. The dialysation process requires dialysing tubes which can be used over and over again. The best are those supplied by Schleicher and Schüll, of Düren in Rhineland. The tubes of this firm should in no case be used without a thorough preliminary examination, because tubes are nearly always met with which allow albumen to pass through, while others are found through which peptones diffuse with difficulty, so that careful testing of the tubes is indispensable. Further, the tubes must be short ones. No. 579A is a tube specially prepared for our purpose. If tubes be used, which project too much over the surface of the surrounding fluid towards which the dialysing process acts, this gives rise to a very uneven evaporation of the dialysate. The latter soaks into the tube, is carried upwards, and evaporates. Indeed, as we shall see later, everything depends upon the fact that, in comparative experiments, the concentration of the dialvsates shall not