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In many other cases the physiological effect of the compound was distinctly foreseen, and latterly the relation between chemical constitution and physiological action has become the objective of much research. It may be reasonably anticipated that before many years have passed it will be possible to predict the physiological powers of a substance from a knowledge of its structural formula, just as already many of its more noteworthy physical properties may be so foretold. Even at present certain trustworthy rules, affording guidance in this respect, have been formulated. Dujardin-Beaumetz and Bardel, dealing with compounds of the aromatic series, have laid down that (a) those containing hydroxyl (OH) are antiseptic; (b) those containing an amino-group (NH2) or an acid amide are hypnotic; and (c) those containing both an amino-group and an alkyl group (CH3, C2H5, etc.) are analgesic.

In order to show how synthetic remedies have been built up from simple products it will be convenient to take a few typical examples in the order of increasing chemical complexity, rather than with strict regard to chronological progression.

Ethyl (that is, ordinary) alcohol forms a convenient starting point. It has been already stated that the molecule of this substance is represented by the formula C2H5OH but for centuries before its constitution was unravelled it had been prepared in a more or less pure