1379. Fusinieri took a different view from that of Poisson, Biot, and others, of the reason why rarefaction of air caused easy diffusion of electricity. He considered the effect as due to the removal of the obstacle which the air presented to the expansion of the substances from which the electricity passed[271]. But platina balls show the phenomena in vacuo as well as volatile metals and other substances; besides which, when the rarefaction is very considerable, the electricity passes with scarcely any resistance, and the production of no sensible heat; so that I think Fusinieri's view of the matter is likely to gain but few assents.
1380. I have no need to remark upon the discharging or collecting power of flame or hot air. I believe, with Harris, that the mere heat does nothing (1367.), the rarefaction only being influential. The effect of rarefaction has been already considered generally (1375.); and that caused by the heat of a burning light, with the pointed form of the wick, and the carrying power of the carbonaceous particles which for the time are associated with it, are fully sufficient to account for all the effects.
1381. We have now arrived at the important question, how will the inductive tension requisite for insulation and disruptive discharge be sustained in gases, which, having the same physical state and also the same pressure and the same temperature as air, differ from it in specific gravity, in chemical qualities, and it may be in peculiar relations, which not being as yet recognized, are purely electrical (1361.)?
1382. Into this question I can enter now only as far as is essential for the present argument, namely, that insulation and inductive tension do not depend merely upon the charged conductors employed, but also, and essentially, upon the interposed dielectric, in consequence of the molecular action of its particles (1292.).
1383. A glass vessel a (fig. 127.)[272] was ground at the top and bottom so as to be closed by two ground brass plates, b and c; b carried a stuffing-box, with a sliding rod d terminated by a brass ball s below, and a ring above. The lower plate was connected with a foot, stop-cock, and socket, e, f and g; and also with a brass ball l, which by means of a stem attached to it and entering the socket g, could be fixed at various heights. The metallic parts of this apparatus were not varnished, but the glass was well-covered with a coat of shell-lac previously dissolved in alcohol. On exhausting the vessel at the air-pump it could be filled with any other gas than air, and, in such cases, the gas so passed in was dried whilst entering by fused chloride of calcium.
1384. The other part of the apparatus consisted of two insulating pillars, h and i, to which were fixed two brass balls, and through these passed two sliding rods, k and m, terminated at each end by brass balls; n is the end of an insulated conductor, which could be rendered either positive or negative from an electrical machine; o and p are wires connecting it with the two parts previously described, and q is a wire which, connecting the two opposite sides of the collateral arrangements, also communicates with a good discharging train r (292.).
1385. It is evident that the discharge from the machine electricity may pass either between s and l, or S and L. The regulation adopted in the first experiments was to keep s and l with their distance unchanged, but to introduce first one gas and then another into the vessel a, and then balance the discharge at the one place against that at the other; for by making the interval at a sufficiently small, all the discharge would pass there, or making it sufficiently large it would all occur at the interval v in the receiver. On principle it seemed evident, that in this way the varying interval u might be taken as a measure, or rather indication of the resistance to discharge through the gas at the constant interval v. The following are the constant dimensions.
| Ball s | 0.93 of an inch. |
| Ball S | 0.96 of an inch. |
| Ball l | 2.02 of an inch. |
| Ball L | 0.62 of an inch. |
| Interval v | 0.62 of an inch. |
1386. On proceeding to experiment it was found that when air or any gas was in the receiver a, the interval u was not a fixed one; it might be altered through a certain range of distance, and yet sparks pass either there or at v in the receiver. The extremes were therefore noted, i.e. the greatest distance short of that at which the discharge always took place at v in the gas, and the least distance short of that at which it always took place at u in the air. Thus, with air in the receiver, the extremes at u were 0.56 and 0.79 of an inch, the range of 0.23 between these distances including intervals at which sparks passed occasionally either at one place or the other.
1387. The small balls s and S could be rendered either positive or negative from the machine, and as gases were expected and were found to differ from each other in relation to this change (1399.), the results obtained under these differences of charge were also noted.