Blister, in medicine, signi fies either a thin bladder, containing a watery humour raised on the skin, or the application of vesica-tories to different parts of the body. With this intention, Spanish flies are most commonly employed; though we are possessed of a great variety of indigenous plants, which might be effectually substituted. Hence we recommend, from experience, the following : 1. Mustard-seed mixed with vinegar sufficient to convert it into a thick paste, to be spread upon linen;

2. The fresh root of the horse-radish, grated, or in fine shavings;

3. 'The bruised leaves of the different species of the Ranunculus, or crow-foot; 4. The leaves of the Polygonum kydropiper, or water-pepper, growing wild on the banks of; and 5. The most powerful of all indigenous vegetables the Daphne Meaereunt, or spurge olive, every part of which is extremely acrid, but the rind is pre-ferably used for blisters. Whether fresh, or dried, this rind should be previously steeped for a few hours in strong vinegar, and then a piece about one inch broad, and two or three inches long, tied over-night to the part: after it has sufficiently drawn, the blistered place is covered with an ivy leaf; and a similar vesicatory is applied contiguous to the former. In this manner, it is continued, according to particular circumstances, especially in chronic diseases, till the desired effect, is attained. Where no time is to be lost, we advise the use of mustard-seed, as before described, with the addition of a little salt, which greatly increases its efficacy. These cataplasms are often more proper than the blisters prepared with Spanish flies ; because the former operate more speedily, and act with less violence on the fluids than the latter. Hence they are of eminent service to promote critical eruptions} to prevent the small-pox from breaking out on the face, when applied at the commencement of the disease, either to the calves of the legs, or the soles' of the feet; to mitigate the pain arising from internal inflammations, to drive catarrhal and rheumatic humours from the more essential organs of life to the proximate external parts, and to rouse the indolent powers of Nature. In the most acute pains of the head, and the tooth-ach proceeding from a rheumatic cause, as well as in inflammatory affections of the eyes, such plasters may be usefully applied to the neck or the arm ; in inflammations of the chest, to the breast and between the shoulders ; in apoplectic fits, to the temples, etc.

In paralytic diseases, it is of the utmost consequence to place the blister in that direction which corresponds with the situation of the nerves in the part affected; and, in rheumatic disorders, such places should be preferred, as contain nerves connected with the painful part, immediately under the skin. Thus, in the most acute lumbago, or sciatica, it would be of little use to blister the hip or thigh, where the nerves are situated deep in the muscles; but by applying a vesicatory to the sole of the foot on the same side, we may promise almost certain relief.

In all inflammatory, and especially in nervous affections, attended with a small, feeble pulse, and where the powers of Nature are rapidly declining, the use of blisters is very extensive.

Their operation is in a great measure mechanical; as the first action is that of stimulating the vessels of the skin, inducing the blood to flow from the part most affected by inflammation, to the surface; thus exhausting the principle of irritability, and collecting the serum, or watery part of tire vital fluid, under the cuticle.

We shall farther observe, that in acute and dangerous diseases, where it is often necessary. to repeat the application of blisters, the new one should never be delayed till the former is completely healed. But, with respect, to the time they are to be left on the skin, much depends on the degree of irritability in the patient, as well as the relative strength of the plaster, Some constitutions, of an irritable fibre, experience its effects in less than half an hour, while in others it. may remain four, six, or eight hours, without raising the skin. In opening a blister, it is not necessary to cut away the epidermis, or scarf skin, and to cause unnecessary pain and irritation ; as a single longitudinal incision is sufficient to give vent to the collected humour.

Blisters sometimes operate on the urinary canal, and produce a painful strangury, or difficulty of making urine: this effect may be' remedied by the internal use of camphor, assisted by diluent and agglutinating emulsions, such as strong decoctions of barley, lint-seed, solutions of gum arabic, etc.; and to prevent such accidents, the blister itself may be mixed with camphor. If, on the other hand, they will not draw, the skin ought to be previously rubbed with strong vinegar: or, if their action be too violent, a little of the extract of henbane may be added to the composition.

Caution. We think it our duty to warn the reader against the use of blisters, in which the Spanish fly is the principal ingredient. In plethoric persons, or those of a full habit, they increase the circulation of the blood ; and ought to be applied only after the necessary evacuations have been strictly attended to. In diseases of aputrid tendency, such as low fevers, and bilious diarrhoeas, they are pernicious, because they stimulate and spread the contagion over the whole frame. Lastly, when the humours are obviously in a state of dissolution, which is evident from the sallow and lifeless complexion of cachectic persons, blisters are not unfre-' quently productive of incurable mortification. These fatal effects, however, seldom or never take place, from the application of mustard-seed, or horse-radish.