Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/214

 We have described the performance of the experiment as it is carried out at the present time. Previously it was usual to make use of the biuret reaction for proofs of decomposition of albumen. To 10 c.c. of the dialysate 2.5 c.c. of a 33 per cent. solution of caustic soda were added, and this was then covered with a layer of very dilute copper sulphate solution. (See here p. 155.) If a violet to red ring appeared, the reaction was recorded as positive.

The biuret test has been entirely given up for the ninhydrin test, because the majority of observers have a difficulty in distinguishing with certainty a feeble biuret reaction. Those, however, who are capable of distinguishing a biuret reaction, however slight, should adhere to this test as well under all conditions.

There are many possibilities leading to erroneous results. It is best to consider them from the point of view of utensils employed and manipulations adopted, and to refer again to the sources of error mentioned in the description of the method.

(i) Tubes.—We take it for granted that all tubes are thoroughly and accurately tested before anything else. On the average about 20 to 30 per cent, of the dialysing tubes supplied by the firm Schleicher and Schüll will be useless, because there are nearly