The biuret reaction is unfortunately rather difficult to detect when it is a case of demonstrating slight traces of the reddish-violet coloration. This is due to the fact that the eye is but slightly sensitive to these tints. Again there are great individual differences. If the observer is unable to detect a light biuret reaction then he has to rely on standardized tubes; or else he must make use of the ninhydrin reaction and try, by means of lengthy dialysation, to. increase the quantity of albumen in the dialysate, so far as the tubes are permeable to albumen. Seeing that white of egg, as well as serum, always contains substances which diffuse and react with ninhydrin, we are bound to find out, by means of a standardized tube, what quantity of a given albumen solution we may use without running the risk of the dialysate showing a ninhydrin reaction. How to perform the ninhydrin test we shall describe later, when we give the test for equal permeability to decomposites of albumen.
The biuret reaction is performed as follows: To the mixture of the dialysate with caustic soda about 1 c.c. of a very much diluted copper sulphate solution—e.g., 1 in 500 c.c.—is added. This solution is run down by means of a pipette along the inside of the test-tube, so as to obtain a surface layer. Then we observe by transmitted light the dividing line between the blue layer, which often, however, appears
