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.

16, Hydrogen a.r.s.eniuretted hydrogen.

17. Hydrogen Sulphuretted hydrogen.

18, Nitrogen Olefiant gas.

19. Nitrogen Nitrous gas.

20. Nitrogen Nitrous oxide.

21. Nitrogen Ammonia.

22. Carbonic oxide Carbonic acid.

23. Carbonic oxide Olefiant gas.

24. Nitrous oxide Nitrous gas.

25. Ammonia Sulphurous acid.

1292. Notwithstanding the striking contrasts of all kinds which these gases present of property, of density, whether simple or compound, anions or cations (665.), of high or low pressure (1284. 1286.), hot or cold (1288.), not the least difference in their capacity to favour or admit electrical induction through them could be perceived. Considering the point established, that in all these gases induction takes place by an action of contiguous particles, this is the more important, and adds one to the many striking relations which hold between bodies having the gaseous condition and form. Another equally important electrical relation, which will be examined in the next paper[A], is that which the different gases have to each other at the _same pressure_ of causing the retention of the _same or different degrees of charge_ upon conductors in them. These two results appear to bear importantly upon the subject of electrochemical excitation and decomposition; for as _all_ these phenomena, different as they seem to be, must depend upon the electrical forces of the particles of matter, the very distance at which they seem to stand from each other will do much, if properly considered, to ill.u.s.trate the principle by which they are held in one common bond, and subject, as they must be, to one common law.

[A] See in relation to this point 1382. &c.--_Dec. 1838._

1293. It is just possible that the gases may differ from each other in their specific inductive capacity, and yet by quant.i.ties so small as not to be distinguished in the apparatus I have used. It must be remembered, however, that in the gaseous experiments the gases occupy all the s.p.a.ce _o, o_, (fig. 104.) between the inner and the outer ball, except the small portion filled by the stem; and the results, therefore, are twice as delicate as those with solid dielectrics.

1294. The insulation was good in all the experiments recorded, except Nos.

10, 15, 21, and 25, being those in which ammonia was compared with other gases. When sh.e.l.l-lac is put into ammoniacal gas its surface gradually acquires conducting power, and in this way the lac part of the stem within was so altered, that the ammonia apparatus could not retain a charge with sufficient steadiness to allow of division. In these experiments, therefore, the other apparatus was charged; its charge measured and divided with the ammonia apparatus by a quick contact, and what remained untaken away by the division again measured (1281.). It was so nearly one-half of the original charge, as to authorize, with this reservation, the insertion of ammoniacal gas amongst the other gases, as having equal power with them.

-- vi. _General results as to induction._

1295. Thus _induction_ appears to be essentially an action of contiguous particles, through the intermediation of which the electric force, originating or appearing at a certain place, is propagated to or sustained at a distance, appearing there as a force of the same kind exactly equal in amount, but opposite in its direction and tendencies (1164.). Induction requires no sensible thickness in the conductors which may be used to limit its extent; an uninsulated leaf of gold may be made very highly positive on one surface, and as highly negative on the other, without the least interference of the two states whilst the inductions continue. Nor is it affected by the nature of the limiting conductors, provided time be allowed, in the case of those which conduct slowly, for them to a.s.sume their final state (1170.).

1296. But with regard to the _dielectrics_ or insulating media, matters are very different (1167.). Their thickness has an immediate and important influence on the degree of induction. As to their quality, though all gases and vapours are alike, whatever their state; yet amongst solid bodies, and between them and gases, there are differences which prove the existence of _specific inductive capacities_, these differences being in some cases very great.

1297. The direct inductive force, which may be conceived to be exerted in lines between the two limiting and charged conducting surfaces, is accompanied by a lateral or transverse force equivalent to a dilatation or repulsion of these representative lines (1224.); or the attractive force which exists amongst the particles of the dielectric in the direction of the induction is accompanied by a repulsive or a diverging force in the transverse direction (1304.).

1298. Induction appears to consist in a certain polarized state of the particles, into which they are thrown by the electrified body sustaining the action, the particles a.s.suming positive and negative points or parts, which are symmetrically arranged with respect to each other and the inducting surfaces or particles[A]. The state must be a forced one, for it is originated and sustained only by force, and sinks to the normal or quiescent state when that force is removed. It can be _continued_ only in insulators by the same portion of electricity, because they only can retain this state of the particles (1304).

[A] The theory of induction which I am stating does not pretend to decide whether electricity be a fluid or fluids, or a mere power or condition of recognized matter. That is a question which I may be induced to consider in the next or following series of these researches.

1299. The principle of induction is of the utmost generality in electric action. It const.i.tutes charge in every ordinary case, and probably in every case; it appears to be the cause of all excitement, and to precede every current. The degree to which the particles are affected in this their forced state, before discharge of one kind or another supervenes, appears to const.i.tute what we call _intensity_.

1300. When a Leyden jar is _charged_, the particles of the gla.s.s are forced into this polarized and constrained condition by the electricity of the charging apparatus. _Discharge_ is the return of these particles to their natural state from their state of tension, whenever the two electric forces are allowed to be disposed of in some other direction.

1301. All charge of conductors is on their surface, because being essentially inductive, it is there only that the medium capable of sustaining the necessary inductive state begins. If the conductors are hollow and contain air or any other dielectric, still no _charge_ can appear upon that internal surface, because the dielectric there cannot a.s.sume the polarized state throughout, in consequence of the opposing actions in different directions.

1302. The known influence of _form_ is perfectly consistent with the corpuscular view of induction set forth. An electrified cylinder is more affected by the influence of the surrounding conductors (which complete the condition of charge) at the ends than at the middle, because the ends are exposed to a greater sum of inductive forces than the middle; and a point is brought to a higher condition than a ball, because by relation to the conductors around, more inductive force terminates on its surface than on an equal surface of the ball with which it is compared. Here too, especially, can be perceived the influence of the lateral or transverse force (1297.), which, being a power of the nature of or equivalent to repulsion, causes such a disposition of the lines of inductive force in their course across the dielectric, that they must acc.u.mulate upon the point, the end of the cylinder, or any projecting part.

1303. The influence of _distance_ is also in harmony with the same view.

There is perhaps no distance so great that induction cannot take place through it[A]; but with the same constraining force (1298.) it takes place the more easily, according as the extent of dielectric through which it is exerted is lessened. And as it is a.s.sumed by the theory that the particles of the dielectric, though tending to remain in a normal state, are thrown into a forced condition during the induction; so it would seem to follow that the fewer there are of these intervening particles opposing their tendency to the a.s.sumption of the new state, the greater degree of change will they suffer, i.e. the higher will be the condition they a.s.sume, and the larger the amount of inductive action exerted through them.

[A] I have traced it experimentally from a ball placed in the middle of the large cube formerly described (1173.) to the sides of the cube six feet distant, and also from the same ball placed in the middle of our large lecture-room to the walls of the room at twenty-six feet distance, the charge sustained upon the ball in these cases being solely due to induction through these distances.

1304. I have used the phrases _lines of inductive force_ and _curved lines_ of force (1231. 1297. 1298. 1302.) in a general sense only, just as we speak of the lines of magnetic force. The lines are imaginary, and the force in any part of them is of course the resultant of compound forces, every molecule being related to every other molecule in _all_ directions by the tension and reaction of those which are contiguous. The transverse force is merely this relation considered in a direction oblique to the lines of inductive force, and at present I mean no more than that by the phrase. With respect to the term _polarity_ also, I mean at present only a disposition of force by which the same molecule acquires opposite powers on different parts. The particular way in which this disposition is made will come into consideration hereafter, and probably varies in different bodies, and so produces variety of electrical relation[A]. All I am anxious about at present is, that a more particular meaning should not be attached to the expressions used than I contemplate. Further inquiry, I trust, will enable us by degrees to restrict the sense more and more, and so render the explanation of electrical phenomena day by day more and more definite.

[A] See now 1685. &c.--_Dec. 1838._