N. B. The solutions of the neutral salts were all of such strength that the acid in them was equiv. to that in salt in 29.

The f. alk. also was equiv. to that in salt in 29, but the acids were equiv. to that in salt in 59.

Salt in 20,000 conducts about 7 times better than distilled water.

696] Therefore the resistance of water with different quantities of salt in [it] are as follows †:

NOTES

BY THE EDITOR.

NOTE 1, ARTS. 5 AND 67.

On the theory of the Electric Fluid.

The theory of One Electric Fluid is here stated very completely by Cavendish. The fluid, as imagined by him, is not a purely hypothetical substance, which has no properties except those which are attributed to it for the purpose of explaining phenomena. He calls it an elastic fluid, and supposes that its particles and those of other matter have certain properties of mutual repulsion or of attraction, just as he supposes that the particles of air are indued with a property of mutual repulsion, but according to a different law. See Art. 97 and Note 6. But in addition to these properties, which are all that are necessary for the theory, he supposes that the electric fluid possesses the general properties of other kinds of matter. In Art. 5 he speaks of the weight of the electric fluid, and of one grain of electric fluid, which implies that a certain quantity of the electric fluid would be dynamically equivalent to one grain, that is to say, in the language of Boscovich and modern writers, it would be equal in mass to one grain.

We must not suppose that the word weight is here used in the modern sense of the force with which a body is attracted by the earth, for in the case of the electric fluid this force depends entirely on the electrical condition of the earth, and would act upward if the earth were overcharged and downward if the earth were undercharged.

Cavendish also supposes that there is a limit to the quantity of the electric fluid which can be collected in a given space. He speaks (Art. 20) of the electric fluid being pressed close together so that its particles shall touch each other. This implies that when the centres of the particles approach to within a certain distance, the repulsion, which up to that point varied as the nth power of the distance, now varies much more rapidly, so that for an exceedingly small diminution of distance the mutual repulsion increases to such a degree that no force which we can bring to bear on the particles is able to overcome it.

We may consider this departure from the simplicity of the law of

For an earlier form of Cavendish's theory of electricity, see "Thoughts concerning electricity" (Arts. 195-216), and Note 18.

force as introduced in order to extend the property of "impenetrability" to the particles of the electric fluid. It leads to the conclusion that there is a certain maximum density beyond which the fluid cannot be accumulated, and that therefore the stratum of the electric fluid collected at the surface of electrified bodies has a finite thickness.

No experimental evidence, however, has as yet been obtained of any limit to the quantity of electricity which can be collected within a given volume, or any measure of the thickness of the electric stratum on the surface of conductors, so that if we wish to maintain the doctrine of a maximum density, we must suppose this density to be exceedingly great compared with the density of the electric fluid in saturated bodies.

A difficulty of far greater magnitude arises in the case of undercharged bodies. It is a consequence of the theory that there is a stratum near the surface of an undercharged body which is entirely deprived of electricity, the rest of the body being saturated. Hence the electric phenomena of an undercharged body depend entirely upon the matter forming this stratum. Now, though on account of our ignorance of the electric fluid we are at liberty to suppose a very large quantity of it to be collected within a small space, we cannot make any such supposition with respect to ordinary matter, the density of which is known.

It is found,

In the first place, it is manifestly impossible to deprive any body of a greater quantity of the electric fluid than it contains. indeed, that there is a limit to the negative charge which can be given to a body, but this limit depends not on the quantity of matter in the body but on the area of its surface, and on the dielectric medium which surrounds it. Thus it appears from the experiments of Sir W. Thomson and those of Mr Macfarlane, that in air at the ordinary pressure and temperature a charge of more than 5 units of electricity, either positive or negative, can exist on the surface of an electrified body without producing a discharge. In other media the maximum charge is different. In paraffin oil, and in turpentine, for instance, it is much greater than in air*. In air of a few millimetres pressure it is much less, but in the most perfect vacuum hitherto made, the charge which may be accumulated before discharge occurs is probably very great indeed.

Now this charge, or undercharge, whatever be its magnitude, can be accumulated on the surface of the thinnest gold leaf as well as on the most massive conductors. Suppose that there is a deficiency of five units of electricity for each square centimetre of the surface on both sides of a sheet of gold leaf whose thickness is the hundred thousandth part of a centimetre. We have no reason to believe the gold leaf to be entirely deprived of electricity, but even if it were, we must admit that every cubic centimetre of gold requires more than a million units of electricity to saturate it.

By Messrs Macfarlane and Playfair's experiments the maximum electromotive intensity is 364 for paraffin oil and 338 for turpentine. For air it is 73, between disks one centimetre apart. (Trans. R. S. Ed. 1878.) They have since found that the electric strength of the vapour of a certain liquid paraffin at 50 mm. pressure is 1.7 times that of air at the same pressure, and that the electric strength of a solid paraffin which melts at 2207 C. is 2.5 when liquid and 5 when solid, that of air being 1.

But we have by no means reached the limit of our experimental evidence. For Cavendish shows in Art. 49 that if in any portion of a bent canal the repulsion of overcharged bodies is so great as to drive all the fluid out of that portion, then the canal will no longer allow the fluid to run freely from one end to the other, any more than a siphon will equalize the pressure of water in two vessels, when the water does not rise to the bend of the siphon.

Hence if we could make the canal narrow enough, and the electric repulsion of bodies near the bend of the canal strong enough, we might have two conductors connected by a conducting canal but not reduced to the same potential, and this might be tested by afterwards connecting them by means of a conductor which does not pass close to any overcharged body, for this conductor will immediately reduce the two bodies to the same potential.

Such an experiment, if successful, would determine at once which kind of electricity ought to be reckoned positive, for, as Cavendish remarks in Art. 50, the presence of an undercharged body near the bend of the canal would not prevent the flow of electricity.

But even if the electric fluid were not all driven out of the canal, but only out of a stratum near the surface, the effective conducting channel would thereby be narrowed, and the resistance of the canal to an electric current increased.

Now we may construct the canal of a strip of the thinnest gold leaf, and we may measure its electric resistance to within one part in ten thousand, so that if the presence of an overcharged body near the gold leaf were to drive the electric fluid out of a stratum of it amounting to the ten thousandth part of its thickness, the alteration might be detected. Hence we must admit either that the one-fluid theory is wrong, or that every cubic centimetre of gold contains more than ten thousand million units of electricity.

The statement which Cavendish gives of the action between portions of the electric fluid and between the electric fluid and ordinary matter is nearly, but not quite, as general as it can be made.

Since the mode in which the force varies with the distance is the same in all cases, we may suppose the distance unity. Two equal portions of the electric fluid which at this distance repel each other with a force unity are defined to be each one unit of electricity.

Let the attraction between a unit of the electric fluid and a gramme of matter be a. Since we may suppose this force different for different kinds of matter, we shall distinguish the attraction due to different kinds of matter by different suffixes, as a, and a.. Let the repulsion between two grammes of matter entirely deprived of electricity be r, these two portions of matter being of the kinds corresponding to the suffixes 1 and 2.

129

Now consider a body containing M grammes of matter and F units of the electric fluid. The repulsion between this body and a unit of the electric fluid at distance unity is F-Ma.

(1)

If this expression is zero, the body will neither repel nor attract the electric fluid. In this case the body is said to be saturated with the electric fluid, and the condition of saturation is that every gramme of matter contains a units of the electric fluid. From what we have already said, it is plain that a must be a number reckoned by thousands of millions at least. The definition of saturation as given by Cavendish is somewhat different from this, although on his own hypothesis it leads to identical results. He makes the condition of saturation to be (in Art. 6) "that the attraction of the electric fluid in any small part of the body on a given particle of matter shall be equal to the repulsion of the matter in the same small part on the same particle." Hence this condition is expressed by the equation

But as the essential property of a saturated body is that it does not disturb the distribution of electricity in neighbouring conductors, we must consider the true definition of saturation to be that there is no action on the electric fluid.

Now consider two bodies of different kinds of matter M, and M,, let each of them be saturated.

and

The quantity of electric fluid in the first will be

Now we know that the action between two saturated bodies is an

for every two kinds of matter, being the same for all kinds of

matter.

According to Baily's repetition of Cavendish's experiment for determining the mean density of the earth*,

k=6·506 × 10-8
X

(centimetre)3

gramme. second

(9)

This number is exceedingly small compared to the product aa,,

Baily's adopted mean for the earth's density is 5.6604, which, with the values of the earth's dimensions and of the intensity of gravity at the earth's surface used by Baily himself, gives the above value of k as the direct result of his experiments.