Dr Franklin likewise found, that when the glass was gilt, a part of the gilding was torn off, as if the electri city had passed from the glass through the gilding ; and when a thin bottle was broken by the powerful attraction of the negative and positive electricitics on each side, the glass was always broken inwards, while the gilding was broken outwards. Tile theory by which Dr Franklin explained these various phenomena. will be fully ex plained in the third Part of the article.
Other electric substances are e.epable of being char ged in the same mantle' as glass. Ileccaih imparted a considerable charge to a smooth plate of sealing-wax.
made by pouring it in a fluid state upon an oiled marble table. Ile charged also a mixture of pitch and colo phony, sulphur, and pitch, the magnitude of the char ges being in the order in which these substances are named.
In our history of electricity, we have already given a sufficient account of the beautiful experiment in which Wilke and .Epirus succeeded in charging a plate of air, and taking shocks from it as if it were a plate of glass.
We have already seen, that when a jar is discharged by the discharging rod, no effect is transmitted along the glass handle. When the handle, however, is made of brass, or any other metal, a small shock is transmit ted to the person that holds it, NI *. en the quantity of electricity accumulated in the jar is considerable. In like manner, if a small chain is connected with the out side coating of a charged jar, it will appear luminous when the jar is discharged in the dark by any conduct ing circuit, of which this chain does not form a part. This effect has been named the lateral explosion, and may be shewn most conspicuously by the following me thod, which was pointed out by Dr Priestley. Insulate a thick metallic rod, and place it with one of its ends contiguous to the external coating of a charged jar, and at the distance of half an inch from its other extremity place a large body, about 6 or 7 feet long, and a few in ches broad. Let a chain be now placed upon the table, at the distance of 11 inch from the outside coating, and apply one end of the discharging rod to the other extre mity of the chain. As soon as the opposite extremity of the discharging rod is made to communicate with the knob of the jar, the discharge will take place, and a brilliant spark will be seen between the insulated metal lic rod and the large body adjacent to it. This spark
does not change the electrical state of the body ; and it is therefore imagined, that this lateral spark flies from the coating of the jar, and returns to it before an elec t•ometer can be affected. This lateral spark has the same length and brilliancy, whether it is received on flat or smooth surfaces, or on sharp points. In opposition to the result of the preceding experiment, Dr Robison informs us, that he has perceived a delicate electrome ter affected by the lateral explosion.
Tun light which always accompanies the excitation of an electric, was observed about the same time by Ot to Gucricke of Magdeburg, and our countryman Dr Wall, and attracted the particular attention of every sub sequent electrician.
During the excitation of a glass tube in the dark, small flashes of light, accompanied with a crackling noise, are always observed ; and in working an electri fying machine, streams of blue light are constantly flash ing over the surface of the glass cylinder. The electri cal light, however, is more distinctly observed when any round body is brought near the prime conductor of the machine. A brilliant light, called the electric sfiark, is seen between the body and the conducto•, and exhi bits a great variety of appearances under different cir cumstances.
Exp. 1. If we hold a brass ball, about three inches in diameter, nee- the prime conductor of an electrical ma chine, sparks of bright white light will appear between them, attended with a loud snapping noise. When the ball is very near the conductor, the sparks appear Ls quick succession, and are tacit perfectly straight.
Exp. 2. 'laving screwed a brass ball, about 2 inches in diameter, into the prime conductor positively electri fied, so as to stand about three inches from it, holdano ther ball near the first, and long ramified or zigzag sparks will be obtained. This positive spark is represented in Plate CCXLIII. Figr. W. where n a t denotes the ball held in the hand and in a natural state of electricity, while p 0 s is the positive ball connected with the con ductor. When the ball on the conductor is made very small, the spark becomes a faint divided brush of light.