lOO Observations on Mr. Donovan's Reflections, ^c. 



dust, this swims over the water ; but it is very easily swept, by 

 scraping a small flake of soap, taking it on the point of a knife, 

 and endeavouring to let it fall on the middle of the surface of 

 the water with its convexity midermost. As soon as that Hake 

 of soap swims on the water, the dust recedes from it, and ascends 

 to the edge of the water on the side of the basin, where it forms 

 a thin fringe. 



16. Having taken notice of that motion of the surface of 

 water made visible by the dmt, I attempted to imitate the mo- 

 tion of tivo electrified balls by two disks of soap half an inch in 

 diameter, and about i-lOth of an inch thick, in the centre of 

 which I fixed with gum a very thin thread four inches long, 

 and I suspended these disks to a slip of wood at such a distance 

 that they slightly touched each other. When that little a])pa- 

 ratus was prepared, and the two disks made to rest at the same 

 time on the surface of the water, they began to recede from each 

 other, and the motion of the dust on the surface of the water 

 visibly demonstrated that the disks of soap moved both towards 

 the water which had not vet dissolved soap. 



17. Lastly, I have demonstrated by a direct experiment, which 

 I shall relate hereafter, that a perfect vacuum docs not transmit 

 the electric f.uid; which fact will be aperemptory proof of the es- 

 sential interference of air in the motion of apair oi electrified balls. 



18. Another essential part of Volta's system, with which Mr. 

 Donovan is not acquainted, and which when fully understood 

 removes all the difficulties he has found, is that wiiich Volta has 

 cailed electric in/liie7ices. It consists in this effect: when a body 

 positive is brought near one of the extremities of an insulated 

 conductor, it gives more tension to the electric fluid at that end 

 of the conductor, and makes it recede to the further end. Tiiis 

 principle removes all the difficulties that Mr. Donovan has found, 

 and in particular that which seems to arise from his experiment 

 related in p. 338, of a pith-ball suspended to a glass rod, and 

 an excited glass tube brought under it. The excited tube occasions 

 a slow retreat o{ electric matter along the thread. While that 

 matter remains together in the ball, the thread, and at the point 

 of suspension, the ball is repelled l)v the excited tube ; but at last 

 the suspended ball loses some electric matter, and being then 

 negative, it is attracted by the excited tube. 



19. The author attacks with reason the cause assigned by 

 Franklin to the luminous brush which appears at the extremity 

 of a pointed body fixed to the prime conductor of an electric 

 machine in motion. But Volta has also explained this phaeno- 

 menon in a satisfactory manner, as I have explained in pp. 60 and 

 61. Suppose a conductor positively electrified, and to which is 

 presented another conductor in communication with the ground, 



but 



