685] A shock is diminished very nearly the same, but if anything rather more, by passing through 9 tubes, 37 inches of which hold 3373 grains of, than through one tube, 37 inches of which hold 3480 grains of t.

686] A shock is as much diminished in passing through 6.8 inches of a tube, 37 inches of which hold 567 grains, as through 441 of one 37 inches of which hold 3480. So that resistance should seem as 1.03 power of velocity +.

687] If resistance is as iron wire is

sea salt.

(437000 1607000

(1.03

1.08 power of velocity, the resistance of

times less than that of saturated solution of

688] Resistance of sat. sol. S. S in 99 of distilled water is 39 times greater than that of the sat. sol.

Resistance of distilled water is 18 times greater than that of sat. sol. in 99 of distilled water§.

689] Experiments in 1776 and 1777.

N. B. It is not said what water the solutions were made with. By the comparison of salt in 999 with salt in 20,000, it should seem either that they were not made with distilled water, or that some mistake was made in the experiment.

690] In Jan. 1777, salt in 2999 conducted about 70 or 90 times better than some water distilled in the preceding summer, or about 25 or 50 times better than the distilled water used in the year 1776 *.

Salt in 2999 conducted about 25 times better than salt in 150,000.

691] Salt in 69 conducts 1.97 times better in heat of 105° than in that of 58°1 t.

The proportion of the resistance of sat. sol. and salt in 999 to each other seems not much altered by varying heat from 50° to 95° ‡.

692] Salt in 150,000 seemed to conduct rather better than the same water deprived of air by boiling in the same vial in which it was kept, and cooled quick in water to prevent its absorbing much air. But the difference was not more than might arise from error of experiment§.

693] Distilled water impregnated with fixed air from oil of vitriol and marble conducted 2 times better than the same water deprived of its air by boiling||.

694] Conducting power of other saline solutions compared with that of salt in 29 of water T.

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.

In the first place, it is manifestly impossible to deprive any body of a greater quantity of the electric fluid than it contains. It is found, 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 hody 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 22°-7 C. is 2.5 when liquid and 5 when solid, that of air being 1.