98 Oheivalions on Mr. Donovan's RefiectlonSy &c. 



electric matter as well as the bodies which it embraces. But I 

 must first relate the Ojiportimitv vvhicir T have liad to learn that 

 system from its author himself. Being at Paris in the year 1782, 

 I made that very interesting acquaintance, and M. Voita was- so 

 good as to explain to nie completely his system. The same 

 year he came to London, and diiected me in the construction of 

 a more extensive set of electrical instruments, with which 1 made 

 all the experiments related in the above mentioned works. 



6. The principle of Volta with respect to electric motions^ be- 

 ing that they have for their true standard the actual electric 

 state of the ambient air, it came into my mind to submit that 

 principle, as the foundation of the whole theory, to an experi- 

 mentum crucis. 



7. This experiment is related from p. 55 to 57 of the first 

 volume of the Traitc elementaire sur le Fluids electrico- 

 galvanique ; where I first mention, as an indispensable condi- 

 tion for the success of that experiment, that the air be very 

 dry; and I had the means, by my hygrometer, to determine the 

 necessary degree of dryness, which is abocrt 43^ of my scale. As 

 these experiments require some time, a greater quantity of 

 aqueous vapour mixed with the air, dissipates too fast the elec- 

 tric Jluid accumulated upon bodies, and transmits too fast some 

 electric fluid to those which have been rendered negative. 



8. I made this series of experiments in two contiguous rooms, 

 separated by a short passage and a door. In one of the rooms 

 I placed a very strong electric machine, by which the air could 

 be modified without affecting that of the other room. I had a 

 pair oi pith lulls with long conducting threads, suspended to a. 

 brass cap at the extremity of a varnished glass rod, and thus 

 thoroughly insulated, as well as the ferrule to which the balls 

 were suspended. Before the electric machine was put in motion 

 in its roon^, the balls had no divergence in either of them. 



9. In one of the experiments, I fixed a point at the extremity 

 of the prime conductor of the machine, therubber being placed 

 in communication with the ground : a luminozis brush appeared 

 at the point, indicating that the electric fluid escaped from it 

 and was communicated to the air of the room. Now, when I 

 brought the pair of balls from the next room, where they did 

 not diverge, they strongly diverged in the room of the maclmie; 

 and by the test of a rubbed stick of sealing-wax they were found 

 to diverge negatively, because the air had acquired some electric 

 fluid by the action of the positive point, and thus was made po- 

 sitive; but when the balls were brought back to the other room, 

 they cca'icd to diverge. 



10. I made the inverse experiment, by fixing a point to the 

 rubber of the machine, its prime conductor being placed in com- 



municaVou 



