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 well as the tubes, in this medium. The rinsing of the dialysing tubes must also be done with distilled water.

Finally, we must bear in mind another source of error, which we have not yet specially referred to. It may sometimes happen, that the substrate added to the serum absorbs some constituents of the latter, and retains them. Such a case would manifest itself in the fact, that the serum alone would react positively, while the dialysate of the experiment, organ + substrate, would give a negative reaction. Further, a reaction might give a negative result, although decomposition had actually taken place. The optical method would easily detect such sources of error.

There is no single point in the rules which lacks a definite foundation. Researches have generally been wrecked owing to trifling details. A glance at the literature, however, shows that at present the method is properly used in many places, and leads to surprisingly beautiful results.

Further sources of error are: The use of vessels that are not dry, and of boiling-sticks that have been touched by the hands, soiling the pipettes with saliva, inaccurate measurement of the ninhydrin solution, the use of infected water, the cultivation of bacteria in the same incubator as is used for experiments on the action of ferments, covering the contents of the tubes, and the outside fluid, with an