The whimsical and ill-grounded hypotheses which were framed by the chymists of the sixteenth century, had, unhappily, too much influence, on the medical practice of that and the succeeding age. Among other false theories which the physicians had adopted from them, was that of attributing the origin of most diseases either to an acid or alkaline cause: but the former, being more obvious to the senses, was supposed to be the most frequent parent of diseases. Among others, fevers, even of the putrid kind, were imagined to be occasioned by an acid, and from hence the testaceous medicines acquired so high a degree of reputation, as to be deemed the grand correctors of acrimony, and were almost universally prescribed as alexipharmics; and the most celebrated compositions which are ranged under that head, contain a large proportion of these powders. But a very learned physician, by a series of accurate and ingenious experiments on septics and antiseptics, has proved that chalk and all the testaceous powders accelerate the corruption of animal flesh, and from the result of one he made on bile with crab's eyes, he naturally concluded that all these substances would produce the same effect on that humour, as they had all uniformly proved septic to flesh. Yet, even upon this supposition, he candidly allows, that in some fevers, they may have their uses, even where no offending acid exists; as in order to cure some diseases, it may be requisite to attenuate the humours, and relax the fibres by a degree of putrefaction, and that possibly the crisis of fevers of this kind, may be hastened or perfected by the testaceous powders. In diseases, however, where the disposition to putrescency is already too strong, all medicines, which in the smallest degree increase such tendency should be studiously avoided; and, on this account, the administration of the testacea in putrid, malignant fevers has of late been condemned by some ingenious writers. I was myself fully convinced of the rectitude of this opinion, and, in drawing up an account of the medicinal uses of Magnesia, had therefore suggested the impropriety of prescribing them where a bilious acrimony prevails. But not being able to recollect, that the septic powers of Magnesia had ever been experimentally proved,[r] I thought it would be most satisfactory to determine them by that method; little doubting but that the event would justify my doctrine. My inquiries evince the accuracy with which Sir John Pringle has made his experiments, but as I was induced to carry the investigation of this subject further than he has proceeded, very different practical inferences, from those I at first expected, may, perhaps, be deduced therefrom. [r] I at that time overlooked an experiment of Dr. Macbride's which proves Magnesia to be septic to _animal flesh_; but having met with it just before these papers were going to the press, I take this method of acknowledging it. EXPERIMENT II.