the quantity that remains free. Thus, if we call 1 the inductive power of a stratum of air, and x that of a stratum of equal thickness of gum-lac, and remembering that the 113° of charge, in the experiments which were given with the insulating stratum of gum-lac, would have given 176° with an insulating stratum of air, we shall have:-1:x:: 113°: 176°; 176 whence, x=

113

=1.55.

In order to verify these results, Faraday then made the inverse experiment: he began by charging the gum-lac apparatus; he then made it communicate by the inner coatings with the air apparatus. He found, in this case, that the free electricity, with which each of the two apparatus were charged after their separation, was stronger than the half of the initial charge of the former. This result proves that the total quantity of electricity, with which the interior surface of the gum-lac apparatus is charged, is, under the same circumstances, much more considerable than that with which the air apparatus is charged, because the former, in giving to the latter a stronger charge than the half of the charge, which this latter would have acquired had it been charged directly, does not itself lose the half, but merely a more feeble proportion of its proper charge. The experiment made in this manner shows also that gum-lac has a stronger inductive power than air, and it gives the same numerical result for the relation of these two powers. Different di-electric bodies have been submitted to the same proof, always comparing them with air; the experiment upon each of them has been repeated several times, and the following results have been obtained.

Specific inductive powers, that of air being 1.

The result of 2.0 obtained for gum-lac is a little different from that which we found above; this is due to the stratum of gum-lac, in the experiment we have described being not

so thick as that of the air, with which it was compared. Faraday has found that, by supposing the thickness the same, the inductive power of gum-lac ought to be 2, and not

1.50.

Sir W. Harris has completed Faraday's experiments: he gave to each substance the form of a disc of the same thickness, and he covered it on each face with a disc of tinfoil, one-half less in diameter. These were true magic pictures, with insulating substances of different natures. By operating in this manner he has drawn out the following table of the inductive power of these substances:

Faraday has found that, among liquids, those which are the best insulators, such as turpentine and rectified naphtha, when submitted to experiment, have an inductive capacity superior also to that of air; but, in the experiment with liquids, we must operate on those alone which do not possess of themselves any conducting power, a property that would interfere with the results.

The different gases have all been similarly subjected to experiments; they have been compared two and two, by placing one in one apparatus, and the other in the other. In like manner, the effect of air more or less rarefied has been compared with that of air maintained at the normal pressure; also that of air maintained at 0°, with that of air heated from 50° to 200°. As a result of these multiplied experiments, we have succeeded in detecting that all gases have the same power and possess the same inductive capacity, and that the variations in pressure or in temperature, and consequently in density, do not produce any further effect. When the rarefaction of the gas arrives at a certain point, however, the discharge occurs between the two coatings; but, up to this moment, there is no change in its di-electric property. We may even give the apparatus so feeble a charge that the discharge does not occur, and we thus prove that, whatever

be the degree of the rarefactions of the air, its inductive capacity remains the same.

Polarisation of Di-electrics, and molecular Theory of Induction.

It follows, therefore, from the experiments we have related, that the nature of the insulating or coercing* medium through which the induction operates, exercises a marked influence on the phenomenon, and that this new element must be added to those which we have already recognised as having an influence on its intensity, such as the greatness of the electric charge, the distance of the induced body and that of the inducing body. It is an effect that is due to an action proper to bodies; but what is this action? Faraday admits that it consists in the generation of molecular strata, alternately negative and positive, which succeed each other in the dielectric, and which constitute what is called its polarisation. However, some facts that had already been observed by Beccaria and other philosophers, and studied more closely by Faraday himself, by showing that electric charges may penetrate to a certain depth into coercing bodies, seem but little favourable to this hypothesis. Thus, two discs of spermaceti, after having been placed upon each other, were armed with metal plates so as to form a magic picture. The system was electrised and then discharged, and the two discs of spermaceti having been separated, it was found that the one upon which the positive coating was placed, possessed positive electricity, and that the other was negative, like the coating which covered it. This experiment proved, in an evident manner, the fact of penetration. It is true to say that spermaceti is not a perfect insulator, and that this circumstance may explain to a certain point how the two electricities, being strongly attracted towards each other, have penetrated it in part.

We have already seen, in the fact of the possibility or

*We shall frequently employ this word, which expresses the idea of a body acting in a certain manner upon electricity better than the word “insulating," which indicates only a passive or negative state.

removing the two coatings of a Leyden jar without discharging it, the proof of the adherence or of the penetration into glass of the electricities possessed by bodies in contact with it: we may draw the same conclusion, as far as resin is concerned, from the experiment of the Lichtenburg figures. However, these facts, while demonstrating that electricity accumulated upon the surface of an insulating body may penetrate it to a certain depth, greater or less according to the nature of this body, are not conclusive against the possibility of its polarisation.

The following experiment of M. Matteucci seems, on the other hand, to prove in a positive manner the polarisation of coercing bodies. Many very thin plates of mica are superposed and strongly compressed; upon the two opposite faces of this kind of pile are applied two tin-foil coatings, so as to constitute a magic picture. After having charged it, the coatings are taken away with an insulating handle, and the different plates of mica are then successively detached one from the other. It is found that each of them has one of its faces positive, and the other negative; the face in contact with the positive coating being positive, and that which had been in contact with the negative coating being negative. The intensity of the electric charges goes on diminishing from the extreme plates, where they are at their maximum, to the middle plates, where they are at their mininum: this difference is probably due to the surfaces of the plates being found greater at the extremities than at the centre of the system. Care, however, must be taken, in separating the plates from each other, to avoid all friction; we discover, by direct means, that the simple fact of detaching them does not constitute them in an electric state. The polarisation of insulating or coercing bodies seems to be proved by the experiment we have just related; and it may equally occur, even though their insulating power is not sufficiently good to prevent their being penetrated to a certain depth by the electricity of a coating applied upon their surface. Setting out from the principle, that all insulating bodies are susceptible of being polarised, Mr. Faraday deduced the consequence, that it is by

their intervention that all the phenomena of induction are brought about, and that there is no action at a distance, or at least at a distance greater than that which separates two molecules that immediately follow each other. Most commonly it is the air whose polarised particles, like the plates of mica in the preceding experiment, transmit from one electrised body to another, which is not so, the inductive effect of the former. It may, in like manner, be any other gas or insulating body; but then the effect is transmitted in a better or worse degree, according to the inductive power of the body. This manner of regarding the phenomena differs essentially from that which we had hitherto adopted: in this latter, the insulating or coercing bodies only play a passive part, consisting simply in preventing the reunion of the opposite electricities, without themselves experiencing any modification in the electric state of their mass.

To render the molecular polarisation of the contiguous particles of a di-electric body visible, which cannot be the case when we are acting upon air, we employ essence of turpentine, in which are placed filaments of silk about a tenth of an inch in length. The bottom of the vessel, in which the essence is placed, is of metal, and communicates with the charged conductor of an electrical machine: no effect is manifested until we bring near to the surface of the liquid a conductor held in the hand; but at the moment when this approach takes place, we see the filaments of silk arrange themselves in every direction, and unite end to end, so as to form an uninterrupted chain between the metal body and the exterior conductor, towards which they tend without touching it.

These particles adhere strongly to each other, as we may discover by touching them with a glass tube; but immediately the conductor of the machine is discharged, they fall and go to the bottom of the vessel. The filaments of silk represent the particles of the essence in which they float; and the polar state that they assume is similar to that of conductors insulated from each other, and placed under the same circumstances. In place of the filaments of silk we may put powdered sugar into the vessel, the particles of which in a