Chapter 45

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.)[A] 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.

[A] The drawing is to a scale of 1/6.

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.

1388. The following is a Table of results; the gas named is that in the vessel _a_. The smallest, greatest, and mean interval at _u_ in air is expressed in parts of an inch, the interval _v_ being constantly 0.62 of an inch.

Smallest. Greatest. Mean. _ | Air, _s_ and S, pos. 0.60 0.79 0.695 |_Air, _s_ and S, neg. 0.59 0.68 0.635 _ | Oxygen, _s_ and S, pos. 0.41 0.60 0.505 |_Oxygen, _s_ and S, neg. 0.50 0.52 0.510 _ | Nitrogen, _s_ and S, pos. 0.55 0.68 0.615 |_Nitrogen, _s_ and S, neg. 0.59 0.70 0.645 _ | Hydrogen, _s_ and S, pos. 0.30 0.44 0.370 |_Hydrogen, _s_ and S, neg. 0.25 0.30 0.275 _ | Carbonic acid, _s_ and S, pos. 0.56 0.72 0.640 |_Carbonic acid, _s_ and S, neg. 0.58 0.60 0.590 _ | Olefiant gas, _s_ and S, pos. 0.64 0.86 0.750 |_Olefiant gas, _s_ and S, neg. 0.69 0.77 0.730 _ | Coal gas, _s_ and S, pos. 0.37 0.61 0.490 |_Coal gas, _s_ and S, neg. 0.47 0.58 0.525 _ | Muriatic acid gas, _s_ and S, pos. 0.89 1.32 1.105 |_Muriatic acid gas, _s_ and S, neg. 0.67 0.75 0.710

1389. The above results were all obtained at one time. On other occasions other experiments were made, which gave generally the same results as to order, though not as to numbers. Thus:

Hydrogen, _s_ and S, pos. 0.23 0.57 0.400 Carbonic acid, _s_ and S, pos. 0.51 1.05 0.780 Olefiant gas, _s_ and S, pos. 0.66 1.27 0.965

I did not notice the difference of the barometer on the days of experiment[A].

[A] Similar experiments in different gases are described at 1507. 1508.--_Dec. 1838._

1390. One would have expected only two distances, one for each interval, for which the discharge might happen either at one or the other; and that the least alteration of either would immediately cause one to predominate constantly over the other. But that under common circumstances is not the case. With air in the receiver, the variation amounted to 0.2 of an inch nearly on the smaller interval of 0.6, and with muriatic acid gas, the variation was above 0.4 on the smaller interval of 0.9. Why is it that when a fixed interval (the one in the receiver) will pass a spark that cannot go across 0.6 of air at one time, it will immediately after, and apparently under exactly similar circumstances, not pass a spark that can go across 0.8 of air?

1391. It is probable that part of this variation will be traced to particles of dust in the air drawn into and about the circuit (1568.). I believe also that part depends upon a variable charged condition of the surface of the glass vessel _a_. That the whole of the effect is not traceable to the influence of circumstances in the vessel _a_, may be deduced from the fact, that when sparks occur between balls in free air they frequently are not straight, and often pass otherwise than by the shortest distance. These variations in air itself, and at different parts of the very same balls, show the presence and influence of circumstances which are calculated to produce effects of the kind now under consideration.

1392. When a spark had passed at either interval, then, generally, more tended to appear at the _same_ interval, as if a preparation had been made for the passing of the latter sparks. So also on continuing to work the machine quickly the sparks generally followed at the same place. This effect is probably due in part to the warmth of the air heated by the preceding spark, in part to dust, and I suspect in part, to something unperceived as yet in the circumstances of discharge.

1393. A very remarkable difference, which is _constant_ in its direction, occurs when the electricity communicated to the balls _s_ and S is changed from positive to negative, or in the contrary direction. It is that the range of variation is always greater when the small bulls are positive than when they are negative. This is exhibited in the following Table, drawn from the former experiments.

Pos. Neg. In Air the range was 0.19 0.09 Oxygen 0.19 0.02 Nitrogen 0.18 0.11 Hydrogen 0.14 0.05 Carbonic acid 0.16 0.02 Olefiant gas 0.22 0.08 Coal gas 0.24 0.12 Muriatic acid 0.43 0.08

I have no doubt these numbers require considerable correction, but the general result is striking, and the differences in several cases very great.

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1394. Though, in consequence of the variation of the striking distance (1386.), the interval in air fails to be a measure, as yet, of the insulating or resisting power of the gas in the vessel, yet we may for present purposes take the mean interval as representing in some degree that power. On examining these mean intervals as they are given in the third column (1388.), it will be very evident, that gases, when employed as dielectrics, have peculiar electrical relations to insulation, and therefore to induction, very distinct from such as might be supposed to depend upon their mere physical qualities of specific gravity or pressure.

1395. First, it is clear that at the _same pressure_ they are not alike, the difference being as great as 37 and 110. When the small balls are charged positively, and with the same surfaces and the same pressure, muriatic acid gas has three times the insulating or restraining power (1362.) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air.

1396. Yet it is evident that the difference is not due to specific gravity, for though hydrogen is the lowest, and therefore lower than oxygen, oxygen is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though considerably heavier than olefiant gas or muriatic acid gas, is lower than either. Oxygen as a heavy, and olefiant as a light gas, are in strong contrast with each other; and if we may reason of olefiant gas from Harris's results with air (1365.), then it might be rarefied to two-thirds its usual density, or to a specific gravity of 9.3 (hydrogen being 1), and having neither the same density nor pressure as oxygen, would have equal insulating powers with it, or equal tendency to resist discharge.

1397. Experiments have already been described (1291. 1292.) which show that the gases are sensibly alike in their inductive capacity. This result is not in contradiction with the existence of great differences in their restraining power. The same point has been observed already in regard to dense and rare air (1375.).

1398. Hence arises a new argument proving that it cannot be mere pressure of the atmosphere which prevents or governs discharge (1377. 1378.), but a specific electric quality or relation of the gaseous medium. Hence also additional argument for the theory of molecular inductive action.

1399. Other specific differences amongst the gases may be drawn from the preceding series of experiments, rough and hasty as they are. Thus the positive and negative series of mean intervals do not give the same differences. It has been already noticed that the negative numbers are lower than the positive (1393.), but, besides that, the _order_ of the positive and negative results is not the same. Thus, on comparing the mean numbers (which represent for the present insulating tension,) it appears that in air, hydrogen, carbonic acid, olefiant gas and muriatic acid, the tension rose higher when the smaller ball was made positive than when rendered negative, whilst in oxygen, nitrogen, and coal gas, the reverse was the case. Now though the numbers cannot be trusted as exact, and though air, oxygen, and nitrogen should probably be on the same side, yet some of the results, as, for instance, those with muriatic acid, fully show a peculiar relation and difference amongst gases in this respect. This was further proved by making the interval in air 0.8 of an inch whilst muriatic acid gas was in the vessel _a_; for on charging the small balls _s_ and S positively, _all_ the discharge took place through the _air_; but on charging them negatively, _all_ the discharge took place through the _muriatic acid gas_.

1400. So also, when the conductor _n_ was connected _only_ with the muriatic acid gas apparatus, it was found that the discharge was more facile when the small ball _s_ was negative than when positive; for in the latter case, much of the electricity passed off as brush discharge through the air from the connecting wire _p_ but in the former case, it all seemed to go through the muriatic acid.

1401. The consideration, however, of positive and negative discharge across air and other gases will be resumed in the further part of this, or in the next paper (1465. 1525.).

1402. Here for the present I must leave this part of the subject, which had for its object only to observe how far gases agreed or differed as to their power of retaining a charge on bodies acting by induction through them. All the results conspire to show that Induction is an action of contiguous molecules (1295. &c.); but besides confirming this, the first principle placed for proof in the present inquiry, they greatly assist in developing the specific properties of each gaseous dielectric, at the same time showing that further and extensive experimental investigation is necessary, and holding out the promise of new discovery as the reward of the labour required.

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1403. When we pass from the consideration of dielectrics like the gases to that of bodies having the liquid and solid condition, then our reasonings in the present state of the subject assume much more of the character of mere supposition. Still I do not perceive anything adverse to the theory, in the phenomena which such bodies present. If we take three insulating dielectrics, as air, oil of turpentine, and shell-lac, and use the same balls or conductors at the same intervals in these three substances, increasing the intensity of the induction until discharge take place, we shall find that it must be raised much higher in the fluid than for the gas, and higher still in the solid than for the fluid. Nor is this inconsistent with the theory; for with the liquid, though its molecules are free to move almost as easily as those of the gas, there are many more particles introduced into the given interval; and such is also the case when the solid body is employed. Besides that with the solid, the cohesive force of the body used will produce some effect; for though the production of the polarized states in the particle of a solid may not be obstructed, but, on the contrary, may in some cases be even favoured (1164. 1344.) by its solidity or other circumstances, yet solidity may well exert an influence on the point of final subversion, (just as it prevents discharge in an electrolyte,) and so enable inductive intensity to rise to a much higher degree.

1404. In the cases of solids and liquids too, bodies may, and most probably do, possess specific differences as to their ability of assuming the polarized state, and also as to the extent to which that polarity must rise before discharge occurs. An analogous difference exists in the specific inductive capacities already pointed out in a few substances (1278.) in the last paper. Such a difference might even account for the various degrees of insulating and conducting power possessed by different bodies, and, if it should be found to exist, would add further strength to the argument in favour of the molecular theory of inductive action.

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1405. Having considered these various cases of sustained insulation in non-conducting dielectrics up to the highest point which they can attain, we find that they terminate at last in _disruptive discharge_; the peculiar condition of the molecules of the dielectric which was necessary to the continuous induction, being equally essential to the occurrence of that effect which closes all the phenomena. This discharge is not only in its appearance and condition different to the former modes by which the lowering of the powers was effected (1320. 1343.), but, whilst really the same in principle, varies much from itself in certain characters, and thus presents us with the forms of _spark_, _brush_, and _glow_ (1359.). I will first consider _the spark_, limiting it for the present to the case of discharge between two oppositely electrified conducting surfaces.

_The electric spark or flash._

1406. The _spark_ is consequent upon a discharge or lowering of the polarized inductive state of many dielectric particles, by a particular action of a few of the particles occupying a very small and limited space; all the previously polarized particles returning to their first or normal condition in the inverse order in which they left it, and uniting their powers meanwhile to produce, or rather to continue, (1417.--1436.) the discharge effect in the place where the subversion of force first occurred. My impression is, that the few particles situated where discharge occurs are not merely pushed apart, but assume a peculiar state, a highly exulted condition for the time, i.e. have thrown upon them all the surrounding forces in succession, and rising up to a proportionate intensity of condition, perhaps equal to that of chemically combining atoms, discharge the powers, possibly in the same manner as they do theirs, by some operation at present unknown to us; and so the end of the whole. The ultimate effect is exactly as if a metallic wire had been put into the place of the discharging particles; and it does not seem impossible that the principles of action in both cases, may, hereafter, prove to be the same.

1407. The _path of the spark_, or of the discharge, depends on the degree of tension acquired by the particles in the line of discharge, circumstances, which in every common case are very evident and by the theory easy to understand, rendering it higher in them than in their neighbours, and, by exalting them first to the requisite condition, causing them to determine the course of the discharge. Hence the selection of the path, and the solution of the wonder which Harris has so well described[A] as existing under the old theory. All is prepared amongst the molecules beforehand, by the prior induction, for the path either of the electric spark or of lightning itself.

[A] Nautical Magazine, 1834, p 229.

1408. The same difficulty is expressed as a principle by Nobili for voltaic electricity, almost in Mr. Harris's words, namely[A], "electricity directs itself towards the point where it can most easily discharge itself," and the results of this as a principle he has well wrought out for the case of voltaic currents. But the _solution_ of the difficulty, or the proximate cause of the effects, is the same; induction brings the particles up to or towards a certain degree of tension (1370.); and by those which first attain it, is the discharge first and most efficiently performed.

[A] Bibliothèque Universelle, 1835, lix. 275.

1409. The _moment_ of discharge is probably determined by that molecule of the dielectric which, from the circumstances, has its tension most quickly raised up to the maximum intensity. In all cases where the discharge passes from conductor to conductor this molecule must be on the surface of one of them; but when it passes between a conductor and a nonconductor, it is, perhaps, not always so (1453.). When this particle has acquired its maximum tension, then the whole barrier of resistance is broken down in the line or lines of inductive action originating at it, and disruptive discharge occurs (1370.): and such an inference, drawn as it is from the theory, seems to me in accordance with Mr. Harris's facts and conclusions respecting the resistance of the atmosphere, namely, that it is not really greater at any one discharging distance than another[A].

[A] Philosophical Transactions, 1834, pp. 227, 229.

1410. It seems probable, that the tension of a particle of the same dielectric, as air, which is requisite to produce discharge, is a _constant quantity_, whatever the shape of the part of the conductor with which it is in contact, whether ball or point; whatever the thickness or depth of dielectric throughout which induction is exerted; perhaps, even, whatever the state, as to rarefaction or condensation of the dielectric; and whatever the nature of the conductor, good or bad, with which the particle is for the moment associated. In saying so much, I do not mean to exclude small differences which may be caused by the reaction of neighbouring particles on the deciding particle, and indeed, it is evident that the intensity required in a particle must be related to the condition of those which are contiguous. But if the expectation should be found to approximate to truth, what a generality of character it presents! and, in the definiteness of the power possessed by a particular molecule, may we not hope to find an immediate relation to the force which, being electrical, is equally definite and constitutes chemical affinity?

1411. Theoretically it would seem that, at the moment of discharge by the spark in one line of inductive force, not merely would all the other lines throw their forces into this one (1406.), but the lateral effect, equivalent to a repulsion of these lines (1224. 1297.), would be relieved and, perhaps, followed by a contrary action, amounting to a collapse or attraction of these parts. Having long sought for some transverse force in statical electricity, which should be the equivalent to magnetism or the transverse force of current electricity, and conceiving that it might be connected with the transverse action of the lines of inductive force, already described (1297.), I was desirous, by various experiments, of bringing out the effect of such a force, and making it tell upon the phenomena of electro-magnetism and magneto-electricity[A].

[A] See further investigations of this subject, 1658-1666. 1709-1735.--_Dec. 1838._