Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/175

 be affected by unequal evaporation; and every precaution must be taken to avoid this source of error.

The first duty to be undertaken in making use of the dialysation process, is the testing of the tubes, the so-called standardization of the dialysing tubes. This standardization, as we have already emphasized, implies the impermeability of the tubes towards albumen, and a perfectly equal permeability for the products of its decomposition.

(a) Test for Impermeability by Albumen.—A solution of albumen is prepared. The simplest way is to take the white of a new-laid egg. 5 c.c. of perfectly fresh white of egg are diluted with distilled water in a graduated tube to 100 c.c., and thoroughly mixed by shaking. Of the white of egg, which must be absolutely fresh, only the more fluid portion is used, while all flaky matter or bits of skin—in short, all solid parts—are rejected, as otherwise it is impossible to get a good mixture. Instead of the white of an egg, blood serum may be employed.

Now the tubes to be tested are prepared. They are soaked in cold water for about half an hour. The tubes are then placed in small Erlenmeyer flasks (fig. 7) and 2.5 c.c. of the thoroughly mixed solution of white of egg in water are poured into them. The solution is measured by means of a pipette. While filling the tubes the pipette is placed far down in them, and the greatest precautions must be taken