and later Dr. Ure of Glasgow, experimented on the bodies of executed criminals, with a success terrible to behold. The facial muscles underwent horrible contortions, and the chest heaved with the contraction of the diaphragm. The small muscles attached to the roots of the hairs of the head appear to be markedly sensitive to electrical conditions from the readiness with which electrification causes the hair to stand on end.

The resistance of the human body to the flow of electric current through it depends mainly on the dryness of the skin. It may vary from 10,000, down to 300 ohms when the skin is moist. From experiments made in America in connexion with the execution of criminals, it was found that the average resistance of the human body is 2500 ohms, and that 3000 (alternating) volts applied between the head and spine caused instantaneous death.

A current of as much as 20 milliamperes produces terrible muscular contractions, whilst a current of 2 amperes traversing a vital part is almost certainly fatal. The effect of the current is twofold; in the first place it acts upon the nerves, causing spasms, secondly it destroys the tissue either by burning or by electrolysis, the blood becoming coagulated. To restore a person who has been rendered insensible by an electric shock, all the same restoratives should be used as for a person drowned.

256. Conditions of Muscular Contraction. - To produce muscular contraction the current must traverse a portion of the nerve longitudinally. In a freshly-prepared frog the current causes a contraction only momentarily when the circuit is made or broken. A rapidly interrupted current will induce a second contraction before the first has had time to pass off, and the muscle may exhibit thus a continuous contraction resembling tetanus. The prepared frog after a short time becomes less sensitive, and a "direct" current (that is to say, one passing along the nerve in the direction from the brain to the muscle) only produces an effect when circuit is made, while an “in

verse" current only produces an effect when the circuit is broken. Matteucci, who observed this, also discovered by experiments on living animals that there is a distinction between the conductivity of sensory and motor nerves, a "direct" current affecting the motor nerves on making the circuit, and the sensory nerves on breaking it; while an "inverse" current produced inverse results. Little is, however, yet known of the conditions of conductivity of the matter of the nerves; they conduct better than muscular tissue, cartilage, or bone; but of all substances in the body the blood conducts best. Powerful currents doubtless electrolyze the blood to some extent, coagulating it and the albumin it contains. The power of contracting under the influence of the current appears to be a distinguishing property of protoplasm wherever it occurs. The amoeba, the most structureless of organisms, suffers contractions. Ritter discovered that the sensitive plant shuts up when electrified, and Burdon Sanderson has shown that this property extends to other vegetables, being exhibited by the carnivorous plant, the Dionæa or Venus' Fly Trap.

257. Animal Electricity. — Although, in his later writings at least, Galvani admitted that the electricity thus operating arose from the metals employed, he insisted on the existence of an animal electricity resident in the muscular and nervous structures. He showed that contractions could be produced without using any metals at all by merely touching a nerve at two different points along its length with a morsel of muscle cut from a living frog; and that a conductor of one metal when joining a nerve to a muscle also sufficed to cause contraction in the latter. Galvani and Aldini regarded these facts as a disproof of Volta's contact-theory. Volta regarded them as proving that the contact between nerve and muscle itself produced (as in the case of two dissimilar metals) opposite electrical conditions. Nobili, later, showed that when the nerve and the muscle of the frog were respec

R

tively connected by a water-contact with the terminals of a delicate galvanometer, a current is produced which lasts several hours: he even arranged a number of frogs' legs in series, like the cells of a battery, and thus increased the current. Matteucci showed that through the muscle alone there may be an electromotive-force. Du Bois Reymond has shown that if the end of a muscle be cut across, the ends of the muscular fibres of the transverse section are negative, and the sides of the muscular fibres are positive, and that this difference of potential will produce a current even while the muscle is at rest. To demonstrate this he employed a fine astatic galvanometer with 20,000 turns of wire in its coils; and to obviate errors arising from the contact of the ends of the wires with the tissues unpolarizable electrodes were used, made by plunging terminal zine points into a saturated solution of sulphate of zinc, contained in a fine glass tube, the end of which was stopped with a porous plug of moistened china clay. Normal muscle at rest shows no current whatever between its parts. Injured muscle at rest shows a current from the injured toward the uninjured part (returning toward the injured part through the galvanometer). Normal muscle when active shows a current from the active part toward the resting part. Du Bois Reymond obtained currents from his own muscles by dipping the tips of his forefingers into two cups of salt water communicating with the galvanometer terminals. A sudden contraction of the muscles of either arm produced a current from the contracted toward the uncontracted muscles. Dewar has shown that when light falls upon the retina of the eye an electric current is set up in the optic nerve. In the skin, and especially in the skin of the common eel, there is an electromotive-force from without inwards.

258. Surgical Applications. Electric currents have been successfully employed as an adjunct in restoring persons rescued from drowning; the contraction of the

diaphragm and chest muscles serving to start respiration. Since the discovery of the Leyden jar many attempts have been made to establish an electrical medical treatment. Discontinuous currents, particularly those furnished by small induction-coils and magneto-electric machines, are employed by practitioners to stimulate the nerves in paralysis and other affections. Electric currents should not be used at all except with great care, and under the direction of regularly-trained surgeons. It is not out of place to enter an earnest caution on this head against the numerous quack doctors who deceive the unwary with magnetic and galvanic "appliances." In many cases these much-advertised shams have done incalculable harm: in the very few cases where some fancied good has accrued the curative agent is probably not magnetism, but flannel!

The usual pathological dose of current is from 2 to 10 milliamperes. Apparatus pretending to cure, and incapable of furnishing such currents, is worthless. Continuous currents appear to produce a sedative effect around the anode, which is of service in neuralgia and painful affections, and an increase in irritability around the kathode, useful in cases of paralysis. The continuous current is also employed electrolytically to disperse tumours. Alternate currents, and rapidly interrupted uni-directional currents stimulate the nerves.

Part Second

CHAPTER IV

ELECTROSTATICS

LESSON XXI. Theory of Potential

259. By the lessons in Chapter I. the student will have obtained some elementary notions upon the existence and measurement of definite quantities of electricity. In the present lesson, which is both one of the hardest and one of the most important to the beginner, and which he must therefore study the more carefully, the laws which concern the magnitude of electrical quantities and their measurement are more fully explained. In no branch of knowledge is it more true than in electricity, that "science is measurement." That part of the science of electricity which deals with the measurement of charges of electricity is called Electrostatics. We shall begin by discussing first the simple laws of electric force, which were brought to light in Chapter I. by simple experimental means.

260. First Law of Electrostatics. Electric charges of similar sign repel one another, but electric charges of opposite signs attract one another. The fundamental facts expressed in this Law were fully explained in Lesson I.