always be made very dry before the fire, when it is used, taking care that it be not heated.

If the jar or battery be charged with positive electricity, and it be required to know the exact time that it becomes difcharged, while you are attempting to charge it negatively, obferve the moment the index comes to the perpendicular station, and at that moment there will not be the least spark left in the jar. If the operation be continued, the index will again advance along the femicircle; and thus fhew the exact quantity of negative electricity which the jar has acquired.

SECT. III.

OF MEDICAL ELECTRICITY.

ELECTRICITY was no fooner brought to any degree of perfection, but it was applied to medical purposes, For by late obfervations, it has been found to poffets the invariable properties of increafing the fenfible perfpiration, quickening the circulation of the blood, and promoting all the glandular fecretions. And among all the variety of cafes in which it has been used, there are none in which it has been found prejudicial, except thofe of pregnancy, and venereal difeafe; and there are a number of cafes, in which it has been applied with confiderable fuccefs. In most disorders where it has been used with perfeverance, it has given at leaft a temporary and partial relief; and in fome cafes it has effected a total cure. Of which, numerous inftances may be seen in

the Philofophical Transactions, and the writings of Meffrs. Lovet, Ferguson, Weftley, Cavallo, &c. &c.

To know what cafes are proper to be electrified, expeperience fhews in general, that all kinds of obftructions, whether of motion, of circulation, or fecretion, are very often removed, and in general alleviated, by electricity. Likewise, nervous diforders have very often been cured; and rheumatic disorders, even of a long standing, are always relieved, and very often quite cured, by only drawing the electric fluid with a point from the affected part, or by drawing fparks from the conductor. It has also been found very beneficial in diseases of a long ftanding; and has not unfrequently been found a powerful remedy in muscular contractions.

There are three instruments generally used for administering medical electricity, befides the electrical machine, viz.An electric jar, with Mr. Lane's electrometer; an insulated chair or ftool, upon which a common chair may be occa. fionally fet; and the directors.

The jar used on this occafion fhould be coated with tin foil, and fhould be about four inches in diameter, and fix in height, which would contain above 72 fquare inches of coated furface. Through the covering of the jar paffes a brafs wire B (fig. 1, plate 21), touching the infide coating of the jar, and having a brass ball F, to which the electrometer FDC is fastened, and terminating at the top in a brafs ball B, which is to touch the prime conductor, and which is fuppofed to ftand before the electrical machine. The electrometer CEDF confifts of a piece of giafs FD, cemented to the two brass caps D and F; from the former of which proceeds a strong perpendicular brafs wire, having at the top an horizontal fpring focket, through which flides the wire CE, having the brafs ball C at one end, and an open ring E at the other end; and fo fixed, that the ball C is exactly the fame height as the ball B, and may be fet at any required distance from the ball B. This distance seldom exceeds half

an

an inch, therefore the electrometer may be made very small. Sometimes there is a scale on the wire CE, which ferves to fet the balls B C to any given distance from each other, with more certainty. When this inftrument is ufed, the jar is fo placed, that the ball B may touch the conductor. Then, suppose the ball C to be fet at one tenth of an inch distance from the ball B, and the electrometer E be connected to the outfide coating of the jar at I by a chain EI. In this cafe, if the electrical machine be put in motion, the jar will be charged; and when the charge is fo high, that the electric fluid accumulated within the jar, can pafs from the ball B to C, which is here fuppofed to be one tenth of an inch; the discharge will take place, the fpark will appear between the balls, and the fhock will pass through the chain EI from E to I; for the part FD being of glass, and generally covered with fealing-wax, is impervious to the electric fluid; therefore, the electric fluid has no way to pafs from the infide to the outside of the jar, but from the ball B to the ball C, and along the wire C E, and from thence along the chain ΕΙ.

When the electrical fhock is to be administered to any part of the human body (as, for example, to the arm), instead of the chain, which must now be taken away, two small pliable wires EL, IL, are to be fastened, one to the ring E, and the other to a hook I, of the stand HI, which communicates with the outfide coating of the jar (if the jar have not the ftand HI, the extremity of the wire I may be put in contact with the outfide coating of the jar in any other convenient manner); the other end of the faid wires is fastened to the brass wires LL of the directors K L. Each director confifts of a knobbed brass wire L, connected to a glass handle K, by means of a brass cap. Then the operator, holding the directors by the extremities of the glass handles, brings their balls into contact with the extremities of that part of the patient's body through which the fhock is to be fent. Then it is evident, from a view of the figure, that

4

that the difcharge of the jar must be made through that

part of the patient's arm, which hes between the two knobs of the directors, as in the former cafe, the discharge was made through the chain EI. Thus, the operator has nothing more to do, but to hold the knobs of the directors to the extremities of that part of the body, through which the fhock is to pafs, while an affiftant keeps the machine in motion. Care must be taken that the two wires EL and IL do not touch each other; otherwife the fhock will not pafs through the patient's body. By thefe means, any number of fhocks, precifely of the fame ftrength, may be given without altering any part of the apparatus. And when it is required to increase or diminish the force of the fhock, it is only neceffary to increase or diminish the distance between the balls B and C, which is done by moving the wire CE through the focket.

It is of little confequence whether the patient ftands upon the ground, upon the infulated stool, or in any other fituation. Neither is it neceffary to remove the clothes from the part that is to be electrified, for the fhocks will readily go through them, except there be too many coverings.

In the application of electricity, the chief difficulty confifts in diftinguishing the proper ftrength of the electric force, that is requifite for a given diforder. For this purpose, it is impoffible to give any general rules, the circumftances being of fo complex a nature, that nothing but long experience, and ftrict attention to every particular phenomenon, can direct the operator. It need hardly be faid, that regard must be had to the fex and age of the patient. The fureft rule that can be given, is to begin with more gentle treatment, at leaft fuch that, confidering the circumftances, may be thought rather weak than strong. If, after a few days trial, this gentle treatment be found ineffectual, then the operator may gradually increase the force of the elec tricity, until he finds the proper degree. But when any limb of the body is deprived of motion, it must be obferved, that

the