Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/181

 turbid owing to the deposition of copper hydroxide, and the quite colourless liquid below. The slightest trace of a pinkish-violet colour is a proof that the tube from which the dialysate was procured is unsuitable. Often the presence of albumen is shown by the fact that the precipitated copper oxide dissolves after a time—in about half an hour—and a clear violet layer appears which gradually diffuses into the other liquid. With this test it is better to be over-cautious, and the tubes should be rejected each time the biuret reaction gives doubtful results.

(b) Testing of the Dialysing Tubes for equal Permeability to Decomposites of Albumen.—Tubes, which do not allow the passage of albumen, must first of all be thoroughly cleansed. Their contents are poured out, and they are then placed on a sieve and rinsed for about half an hour in clean running water.

For the sake of security they are put in boiling water for not more than half a minute. We may also point out that experience has shown that boiling the tubes is not very good for them, for they easily become too dense. After this, 2.5 c.c. of a 1 per cent. solution of silk-peptone are poured into them; the tubes are again carefully rinsed in cold water, one by one, and are then placed in Erlenmeyer flasks filled with 20 c.c. of sterilized distilled water (compare pp. 150-152). The latter is covered with toluol. In