points, and in escaping will re-act continually on the electrical part of the atmospherule (prop. 14, cor. 9, 10, and 11. sect. ii.) repelling them in the opposite direction, hence the electrical equilibrium will be restored, and the electrical signs will disappear.

PH. 58. When the temperature of the crystal is raised still higher, the electrical signs appear in a reverse order, viz. the end having the fewest facets is positive.

Exp. The caloric escapes now in greater quantity, and with greater force than as stated in the last; and hence the repulsion or re-action, which necessarily takes place between the caloric which escapes, and the atoms of the electric fluid (prop. 14, cor. 9, 10, and 11. sect. ii.) is sufficient to cause the electrical part of the atmospherules of the atoms of the crystal to extend in the opposite direction; that is, the electrical state of the two ends is reversed, as the crystal cools it passes through the different gradations to its original state, which confirms this explanation.

PH. 59. In the crystal called borate of magnesia, whose form is generally that of a cube, incomplete on its edges, and farther modified by facets, at half of the solid angles alternately placed, the four axes, meeting the opposite angles, find a facet at one extremity, and not at the other, the four extremities where the facets are, become positive, and the other ends or corners are negative.

Exp. This is exactly agreeable to the case of ph. 56, and requires no farther elucidation.

PH. 60. If two polished plates of different sorts of metal, about 4 or 5 inches in diameter, be placed on each other by means of insulating handles, and after a while removed, the metal, which has the greatest affinity for oxygen, is found to be positive, and the other negative.

Exp. Atoms of air, and consequently of oxygen adhere

to the surfaces of metals, and other bodies, more or less closely according to circumstances, (see sect. ii, and vi.) Now the metal which has the greatest affinity for oxygen, will, while they are in contact, cause some of the atoms of oxygen to approach it from the other plate, to which the adhesion is feeble. Hence a portion of the electric fluid will pass with it to this metal, rendering it positive, while it leaves the other negative, and although it is not necessary that combination should take place, yet the effect will be greater or less, according to their different facilities of combining with oxygen, modified perhaps by other circumstances.

Excitement by Induction.

PH. 61. When a body is rendered electrical, the contiguous air is electrified by it to some considerable distance, greater or less, according to the intensity of the action.

Exp. If the body be positive, the accumulated atmospherules of the atoms of that body will cause the atmospherules of the contiguous air to extend outward, or from the body, (prop. 27, sect. ii.); hence the atoms of the air will be in a state fit to receive the electric fluid on the sides facing the electrical body. If the body be negative the reverse takes place, (prop. 28, sect. ii). When the intensity of the electricity on the electrified body is great, a portion of the fluid will pass from it, when it is positive, to the contiguous air, (ph. 6, and 36); since the air is in this case a partial conductor, and hence the neighbouring air will become positively electrified. But if the elec

trified body be intensely negative the contiguous air will yield to it a portion of fluid, and will thus itself become negative.

PH. 62. If an insulated conductor be placed at a small distance from an electrified body, so as not to receive a spark from it, the two extremities of the conductor will be in contrary electrical states; the most distant end of the approximated body will be in the same state as that of the body by which it is affected, and the nearer end will be electrified with the contráry power.

Exp. Let the conductor A, fig. 50, be electrified so as to be in the positive state, and let another conductor, BC, properly insulated, be placed near it, and at such a distance as not to receive a spark. Then, since A is positive, the atmospherules of the atoms of air around it will be extended outward, as in the direction of the short lines, attached to the circles representing the atoms of air, (prop. 27, and ph. 61). Again, since BC is a conductor, the electric fluid moves much more easily along its surface, than through the air, hence the extended atmospherules of the air contiguous to B, will find less resistance towards B than towards other parts in the air, hence the direction of the atmospherules of the air surrounding B, at a little distance from the end, will be changed, and turned towards the conductor B, as shewn by the little arrows, and this will still further tend to protrude the electric fluid from the end B, towards C, till the equilibrium is attained. It is evident that from this effect, the atmospherules of the air near C will extend outward, as before they did at A. Thus the end B of the conductor BC, will be in a condition fit for receiving electric fluid from any other conductor in its natural state, and the end C, for giving out a portion to such a conductor; that is, the end B is negative and C is positive. If A had been negative, the direction

of the atmospherules of the air would have been reversed, and it is seen at a glance that C would have become negative and B positive. This is called induced electricity, and BC is said to be electrified by induction.

PH. 63. If the neighbouring conductor BC, fig. 50, have its extremity connected with the earth by a conductor, it acquires an electrical state opposite to that of the body A; and the more so, if instead of the earth, it be connected with a conducting body in a state contrary to that in which A is electrified.

Exp. Suppose BC connected with the earth, by a wire or chain, as FG is at G, it is manifest that the fluid propelled from B towards C, as shewn in the last explanation, will now readily pass along the chain to the earth, and BC will be negative throughout its whole length, if A be positive, and the contrary if A be negative. If C had been connected with a body in the contrary state to that of A, these effects would manifestly have been increased.

PH. 64. If there be a series of insulated conductors, near each other in a line but not in contact, and the farther extremity of the last of the series communicating with the earth, as BC, DE, and FG, then each of the intermediate insulated bodies will be electrified at the end directed towards the electrified body A, with the contrary power, and at the opposite end with the same power as that of A.

Exp. The effect of A on B will be as shewn in ph. 62, but in a much greater degree, because the resistance is partly removed by the conveyance of a portion of electric fluid to the earth by the chain H, (ph. 63), and since the end C is thus rendered electrical, and in the same state as A, DE will be electrified by C, as BC is by A, and likewise FG will be electrified by the end E, so as to be in a

state opposite to that of E, or A, (ph. 62 and 63), and thus the effect may be propagated through a long series of conductors.

PH. 65. The intensity of the electricity induced on BC or DE, (ph. 62, and 64), decreases from the extremities, and is null somewhere towards the middle.

Exp. This ought to follow from ph. 62 and 64; for since the atmospherules of the air are extended in opposite ways at the two ends, in respect of B and C, there is some part between them, where the natural equilibrium must remain, and evidently the intensity of the electrical state will decrease as we recede from either end towards this part.

PH. 66. The point in BC, which shews no signs of electrical attraction, lies nearer to B, as BC is removed farther from A.

Exp. For the action is more feeble on the fluid at B, as it is more distant, and hence this effect ought to

ensue.

PH. 67. If BC be removed to a considerable distance, it gives no signs of electricity.

Exp. The reason of this is obvious, because the air is insensibly affected at such a distance.

PH. 68. The insulated conductor BC, being removed in its insulated state, is found to have lost scarcely any thing of its electricity, but remains nearly in its natural

state.

Exp. The distance is stated to be such, that the spark does not pass from A to BC, fig. 50, and the electrical atmospherules of the air do not commix with the fluid on the surface of BC, (prop. 14, sect. ii.); and if indeed a small portion of the fluid should pass at one end, nearly an equal quantity would escape at the other, the conditions being much the same; hence all that happens to