The scientific striving for exactness has led to two distinct tendencies in materia medica: The first consists in the substitution of the active principles isolated from a drug for the crude drug itself. This is undoubtedly the correct and scientific method, when the isolated principle really represents the full therapeutic value of the drug; but, since this is by no means always the case, the method has its limitations.
The second method consists in the estimation of the strength of a crude or unstable drug by assay processes, i. e., by the determination of the amount of some active constituent contained therein. Perhaps the most notable advance in the new U. S. Pharmacopeia consists in the increased requirements in this respect and in the greatly enlarged list of drugs and preparations for which an assay is directed. With the single exception of diphtheria antitoxin, these assays are all based on