OYSTER-SHELLS, an alkali far more powerful than is generally allowed, and are in all probability much better medicines than many of the more costly and pompous alkalis of the same class. The proof of alkalis is in their solution by acid spirits; and Mr Homberg found, that they dissolved far easier in acids of nitre and sea-salt than either pearls or coral, or indeed than any of the rest. This he supposes to be owing to their containing in the body of the shell a large portion of sal-falsus, which is easily perceived upon the tongue, and which keeps the whole substance of the shell in a sort of half dissolved state. These shells are found to produce very sensible effects on the stomach, when it is injured by acid humours; and Mr Homberg thinks, that this easiness of solution is a great argument for their good effects, and that the quantity of sal-falsus which it contains, contributes not a little towards it; for we are not to look upon that as a salt merely, but as a salt of a peculiar nature, formed of sea-salt by the organs of the animal, and the several fermentations it undergoes in the body of it, in the same manner as the nitrous and other salts of the earth cease to be nitrous, &c. whenever they become blended with the juices of plants, and form with them a salt peculiar to that plant; which is evidently the case as far as respects this salt, it being plainly of a more penetrating taste, and of a different smell, from the salt left by the sea-water between the several external scales or flakes of the shell.
shell. Oyster-shells being thus found by Mr Homberg to be a very valuable medicine, and as one of the common methods of preparing them is by calcination, which, he observes, considerably impairs their virtues, he gives the following method of preparing them for taking inwardly, which he himself always used. Take the hollow shells of the oysters, throwing away the flat ones, as not sufficiently good; make them perfectly clean, and then dry them in the sun; when they appear dry, beat them to pieces in a marble mortar: they will still be found to contain a large quantity of moisture; lay them therefore again in the sun till perfectly dried, and then finish the powdering them, and sift the powder through a fine sieve. Give 20 or 30 grains of this powder every morning, and continue it three weeks or a month. See CHEMISTRY, no 1087.