In recent years it has been found that in vitro many ferments, etc., lose their power of action at a rate corresponding to the monomolecular reaction, i.e. if the amount of the substance present at the beginning is denoted by unity, and, if after a certain period of time only one half of the substance remains active, then it may be predicted that at the end of an equal period of time, one half of one half, or only one quarter of the original amount, will still retain its power of a.ction. It is not necessary to give examples, they are sufficiently well known.