Part 38 (1/2)

1280. I endeavoured to find some liquids which would insulate well, and could be obtained in sufficient quant.i.ty for these experiments. Oil of turpentine, native naphtha rectified, and the condensed oil gas fluid, appeared by common experiments to promise best as to insulation. Being left in contact with fused carbonate of pota.s.sa, chloride of lime, and quick lime for some days and then filtered, they were found much injured in insulating power; but after distillation acquired their best state, though even then they proved to be conductors when extensive metallic contact was made with them.

1281. _Oil of turpentine rectified._--I filled the lower half of app. i.

with the fluid: and as it would not hold a charge sufficiently to enable me first to measure and then divide it, I charged app. ii. containing air, and dividing its charge with app. i. by a quick contact, measured that remaining in app. ii.: for, theoretically, if a quick contact would divide up to equal tension between the two apparatus, yet without sensible loss from the conducting power of app. i.; and app. ii. were left charged to a degree of tension above half the original charge, it would indicate that oil of turpentine had less specific inductive capacity than air; or, if left charged below that mean state of tension, it would imply that the fluid had the greater inductive capacity. In an experiment of this kind, app. ii. gave as its charge 390 before division with app. i., and 175 afterwards, which is less than the half of 390. Again, being at 176 before division, it was 79 after, which is also less than half the divided charge. Being at 79, it was a third time divided, and then fell to 36, less than the half of 79. Such are the best results I could obtain; they are not inconsistent with the belief that oil of turpentine has a greater specific capacity than air, but they do not prove the fact, since the disappearance of more than half the charge may be due to the conducting power merely of the fluid.

1282. _Naphtha._--This liquid gave results similar in their nature and direction to those with oil of turpentine.

1283. A most interesting cla.s.s of substances, in relation to specific inductive capacity, now came under review, namely, the gases or aeriform bodies. These are so peculiarly const.i.tuted, and are bound together by so many striking physical and chemical relations, that I expected some remarkable results from them: air in various states was selected for the first experiments.

1284. _Air, rare and dense._--Some experiments of division (1208.) seemed to show that dense and rare air were alike in the property under examination. A simple and better process was to attach one of the apparatus to an air-pump, to charge it, and then examine the tension of the charge when the air within was more or less rarefied. Under these circ.u.mstances it was found, that commencing with a certain charge, that charge did not change in its tension or force as the air was rarefied, until the rarefaction was such that _discharge_ across the s.p.a.ce _o_, _o_ (fig. 104.) occurred. This discharge was proportionate to the rarefaction; but having taken place, and lowered the tension to a certain degree, that degree was not at all affected by restoring the pressure and density of the air to their first quant.i.ties.

inches of mercury.

Thus at a pressure of 30 the charge was 88 Again 30 the charge was 88 Again 30 the charge was 87 Reduced to 11 the charge was 87 Raised again to 30 the charge was 86 Being now reduced to 3.4 the charge fell to 81 Raised again to 30 the charge was still 81

1285. The charges were low in these experiments, first that they might not pa.s.s off at low pressure, and next that little loss by dissipation might occur. I now reduced them still lower, that I might rarefy further, and for this purpose in the following experiment used a measuring interval in the electrometer of only 15 (1185.). The pressure of air within the apparatus being reduced to 1.9 inches of mercury, the charge was found to be 29; then letting in air till the pressure was 30 inches, the charge was still 29.

1286. These experiments were repeated with pure oxygen with the same consequences.

1287. This result of _no variation_ in the electric tension being produced by variation in the density or pressure of the air, agrees perfectly with those obtained by Mr. Harris[A], and described in his beautiful and important investigations contained in the Philosophical Transactions; namely that induction is the same in rare and dense air, and that the divergence of an electrometer under such variations of the air continues the same, provided no electricity pa.s.s away from it. The effect is one entirely independent of that power which dense air has of causing a higher charge to be _retained_ upon the surface of conductors in it than can be retained by the same conductors in rare air; a point I propose considering hereafter.

[A] Philosophical Transactions, 1834, pp. 223, 224, 237, 244.

1288. I then compared _hot and cold air_ together, by raising the temperature of one of the inductive apparatus as high as it could be without injury, and then dividing charges between it and the other apparatus containing cold air. The temperatures were about 50 and 200, Still the power or capacity appeared to be unchanged; and when I endeavoured to vary the experiment, by charging a cold apparatus and then warming it by a spirit lamp, I could obtain no proof that the inductive capacity underwent any alteration.

1289. I compared _damp and dry air_ together, but could find no difference in the results.

1290. _Gases._--A very long series of experiments was then undertaken for the purpose of comparing _different gases_ one with another. They were all found to insulate well, except such as acted on the sh.e.l.l-lac of the supporting stem; these were chlorine, ammonia, and muriatic acid. They were all dried by appropriate means before being introduced into the apparatus.

It would have been sufficient to have compared each with air; but, in consequence of the striking result which came out, namely, that _all had the same power of_ or _capacity for_, sustaining induction through them, (which perhaps might have been expected after it was found that no variation of density or pressure produced any effect,) I was induced to compare them, experimentally, two and two in various ways, that no difference might escape me, and that the sameness of result might stand in full opposition to the contrast of property, composition, and condition which the gases themselves presented.

1291. The experiments were made upon the following pairs of gases.

1. Nitrogen and Oxygen.

2. Oxygen Air.

3. Hydrogen Air.

4. Muriatic acid gas Air.

5. Oxygen Hydrogen.

5. Oxygen Carbonic acid.

7. Oxygen Olefiant gas.

8. Oxygen Nitrous gas.

9. Oxygen Sulphurous acid.

10. Oxygen Ammonia.

11. Hydrogen Carbonic acid.

12 Hydrogen Olefiant gas.

13. Hydrogen Sulphurous acid.

14. Hydrogen Fluo-silicic acid.

15. Hydrogen Ammonia.