This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Cancrorum Oculi dicti Pharm. Edinb. Cancrorum lapides. Crabs-eyes so called: stony concretions, found in the head, or rather stomach, of the astacus fluviatilis or river craw fish; generally about the size of peas, or larger; of a roundish shape, flatted on one side; in colour white, sometimes with a reddish, and sometimes with a bluish cast; internally of a leafy texture. They are said to be brought to us chiefly from Holland: perhaps the greatest quantities are the produce of Muscovy, particularly of the river Don, where the craw fish, as I have been informed, are extremely plentiful, and have been commonly laid in heaps to putrefy, after which the stones are picked out.
These stones are said to be sometimes counterfeited with tobacco-pipe clay, or chalk mixed with glutinous materials. Compositions of this kind may be readily distinguished from the genuine crabs-eye, by their texture being uniform and not leafy, and by their sticking to the tongue, and being softened with water. They differ also in their habitude to acids; either not dissolving at all, or dissolving in another manner.
If genuine crabs-eyes be put entire into strong vinegar, or into aqua fortis largely diluted with water; their earthy part is gradually extracted, and there remains a soft transparent gelatinous substance, of the same figure with the original concrete: such as were at first coloured, retain their colour after the action of the acid. The quantity of this gelatinous matter is much less than might be judged from the volume which it occupies; amounting, when the nitrous acid has been used, scarcely to one tenth part of the weight of the crabs-eyes: vinegar leaves a larger quantity, a part of the earth itself seeming to escape the action of this acid.
Crabs-eyes are used as an absorbent of acid humours, and are supposed, when combined with the acid, to be more aperient and resolvent than moil of the other absorbent earths: in this intention they are commended by Hoffman, who looks upon a solution of them in vinegar as capable of resolving both stagnant thick humours and coagulated blood. * They have lately been employed, by an eminent physician at Leyden, with great success, in the cure of the fluor albus. He gives to the quantity of half an ounce in a day; and remarks, that it is particularly ser-viceable in that species of the disease in which the discharge is so acrimonious as to corrode the parts(a). Their earth differs remarkably from that of the preceding article, in not being convertible into quicklime; but the medical differences of their solutions in vinegar, or in other acids of the vegetable or animal kingdom, do not appear to be very great, the solutions of the two earths being in taste nearly alike. The earth of crabs-eyes, in regard to its chemical characters, is of the same nature with that of hartshorn.
(a) Med. comment. vol 1. p. 325.