also insulated, and brought into contact with the other, it will receive just half its superabundant fluid.

Exp. The balls being equally good conductors, and in contact, it appears by the two last phenomena that they will contain equal quantities of the fluid, or will be equally deficient, hence half the excess of fluid will pass from the first to the other.

PH. 32. If the balls be unequal, the smaller one will receive more of the fluid than the quantity answering to the ratio of its magnitude. Thus M. Columb found, that when the one ball was fifteen times less than the other, it contained but eleven times less fluid.

Exp. When the balls are in contact, the smaller one forms a prominent part on the other, and hence (prop. 17, and cors. sect. ii.) the ethereal matter will be most protruded on it, and hence it will contain a greater proportionate part. For the same reason it is found that there is an increase of electricity on the projecting parts of electrified bodies. When the balls are negative the same holds good, because in this case, the relations of the forces are the same, but in contrary directions.

PH. 33. If an electrified insulated body be connected with the earth by a good conductor, it is speedily restored to its natural state, if it be a good conductor, but not so if it be a non-conductor.

Exp. The reason of this will be seen at once from a consideration of phenomena 6 and 20; which also will appear from several others.

PH. 34. If two wires, one short and of small diameter, the other long and thick, be employed as conductors; the short and thin wire will transmit a greater portion of the fluid than the thick one, other circumstances being the

same.

Exp. The shorter passage in the small wire affords one reason, it also follows from this, that in the small wire the fluid proceeds in one body, or in a condensed state, while in the thick wire, it is more diffused on the surface, hence, because of its less intensity, it is more resisted by the air, and also on account of the greater quantity of air to which it is exposed in its passage.

PH. 35. A large quantity of electricity will pass through a good conductor of a very great length in an imperceptible portion of time, but a small quantity takes a longer time.

Exp. All bodies in some degree resist the passage of electric fluid of low intensity, as before shewn, and this accounts for the fact.

PH. 36. When a conductor is electrified in a very high degree, a considerable portion of the electricity escapes from its surface.

Exp. For no bodies are perfect non-conductors, and they are the less perfect, as the intensity is greater; hence the air will be a partial conductor, when the body is highly electrified, and hence much fluid will be carried from the electrified body.

PH. 37. When the conductor is electrified in a less degree; if it have long small projecting parts, the fluid will escape at those parts.

Exp. For the small projecting parts will be most charged with the fluid, or most deprived of it, (prop. 17, sect. ii. and ph. 32), and hence by the last, it tends to escape, or if negative to be received at the projecting parts.

Pн. 38. Fine pointed conducting bodies, as metallic wires, projecting to some distance from electrified bodies, or presented towards them at a small distance, readily throw off, or receive the electric fluid, and tend to reduce the body to its natural state,

Exp. This will follow as a natural consequence from phenomena 34 and 37, and from proposition 17 and its cors.; for the resistance to the fluid is taken away from the points by the diffusion, intermixture, and motion of much ethereal matter or electric fluid at those points.

Excitation of Bodies.

PH. 39. When two dissimilar bodies, especially nonconductors, are rubbed together, electrical signs generally appear, one of the bodies being positive and the other negative, and the excitation is in most cases greater, when the bodies are more unlike in their nature.

Exp. The first part of this was explained in ph. 2, and it is evidently to be expected on account of the different degrees of hardness, texture, roughness of surface, and other differences, so that when such bodies are rubbed against each other, the electric fluid on the surface of one will pass over to the other, and the more so as they are the more unlike; this transit of the fluid therefore takes place, when the bodies come into close contact, and separate again, in the act of mutual friction.

Obs. The most general way of exciting common electricity, is by means of an electrical machine. There are various sorts and constructions of this instrument, as may be seen in Treatises on Electricity. The most simple and complete machine consists of a glass cylinder, from 8 to 16 inches in diameter, and from 12 to 24 inches long,

turning by means of a wince, or otherwise, between two upright pillars, fixed on a strong mahogany base. To one side of the cylinder is applied a cushion, formed of leather stuffed with horse hair, or wool, &c. about two inches shorter than the cylinder, and nearly two inches wide; it is attached to a piece of wood, and supported by a strong glass pillar, and may be made to press with any moderate force against the cylinder, by means of a spring. To the cushion a cylindrical conductor may be attached when required. Before the opposite side of the glass cylinder, another insulated conductor is placed called the prime conductor, which is furnished with a row of pointed wires, the distance between the extreme points being about one inch less than the length of the cushion: the points are called collectors, and are to be placed so that the line joining their extremities may be parallel to the axis of the glass cylinder, and at half an inch distance, or less, from its surface.

To the cushion is applied an amalgam, made by melting together one part by weight of tin, and two of zinc, and pouring the melted mass on four parts of mercury, in a wooden box, or iron mortar, and afterwards triturating the compound. Also from the top of the cushion proceeds a silk flap, reaching over the glass to within about one inch of the collectors.

PH. 40. The electrical machine and apparatus being dry and in proper order, and the conductors being removed, on turning the machine the cylinder is electrified positively, and streams of light will appear to pass on the cylinder from the silk flap to the opposite side of the cushion. presenting the knuckle, there will be perceived a hissing noise, and sensible wind, with luminous radiations.

Exp. The glass is strongly excited by the rubber, and rendered positive, while the rubber is left in a negative

state (ph. 3); the excited electric fluid being in abun dance, after leaving the silk flap, darts forward in collected masses to the negative rubber, (ph. 36), because the centers of its atoms are forced far within each other's spherules. When the hand is presented, the fluid flies towards it, the resistance being lessened in that direction, as will be shewn in several of the following phenomena.

PH. 41. The same things being supposed, and now the positive conductor with the collectors being placed before the cylinder, and a chain or wire hung from the cushion, connecting it with the earth, through the medium of the table, floor, &c., the machine being turned, the prime conductor acquires the positive electricity, and it will be found, that, if an uninsulated conducting body be presented to the prime conductor, electricity will be conveyed rapidly along that conducting body.

Exp. The glass cylinder, while passing the cushion in close contact, receives from it a considerable portion of electric fluid, because its atoms attach themselves more strongly to the glass than to the amalgam, (ph. 2 and 3); hence the glass leaving the cushion, has obtained and carries off an accumulation of the fluid; the loss sustained by the cushion is supplied to it immediately, because of its connection with the earth (ph. 6, and 20), and hence it is again in a state fit to supply the successive parts of the glass cylinder as it revolves. Now the pointed collectors readily receive the fluid from the charged revolving surface, (ph. 37 and 38), and the glass thus deprived of its surcharge, again receives a new stock, when it comes in contact with the cushion, and hence a continual current of electric fluid is thus drawn from the earth through the chain to the cushion, and thence conveyed to the earth again through the conducting body, presented to the prime conductor.