tricity will therefore flow more towards the edges of the outer disks, and, as this will raise the potential near the edge of the middle disk, the charge of the middle disk will be less than on our assumption. not attempted to estimate the distribution more approximately.

I have

Cavendish found the charge of the middle disk and of what it would have been without the outer disks. This is much less than the first approximation here given, but much greater than Cavendish's own estimate, founded on the assumption that the distribution of electricity follows the same law in the three plates.

NOTE 24, ARTS. 338, 652.

On the Capacity of a Conductor placed at a finite distance from other Conductors.

Cavendish has not given any demonstration of the very remarkable formula given in Art. 338 for the capacity of a conductor at a finite distance from other conductors. We may obtain it, however, in the following manner.

If the distance of all other conductors is considerable compared with the dimensions of the positively charged conductor, C, whose capacity is to be tried, the negative charge induced on any one of the other conductors will depend only on the charge of the conductor C and not on its shape. This induced charge will produce a negative potential in all parts of the field; let us suppose that the potential thus produced at the centre of the conductor C is where E is the charge of C and x is a quantity of the dimensions of a line.

E

x

E

If L is the capacity of C when no other conductor is in the field, then the potential due to the charge E will be, and the potential, which arises from the negative charge induced on other conductors, E will be so that the actual potential will be E

Dividing the charge by the potential we obtain for the actual capacity

or the capacity is increased in the ratio of x to x - L.

The idea of applying this result to determining the value of x by comparing the charges of bodies, the ratio of whose capacities is known, is entirely peculiar to Cavendish, and no one up to the present time seems to have attempted anything of the kind.

The height of the centre of the circles above the floor seems to have been about 45 inches. If we neglect the undercharge of other conductors and consider only the floor, x would be about 90 inches in modern measure, but as a capacity x is reckoned by Cavendish as 2x "inches of electricity," the value of x in "inches of electricity" would be 180.

If we could take into account the undercharged surfaces of the other conductors, such as the walls and ceiling, the "machine," &c., the value of x would be diminished, and it is probable that the value obtained from his experiments by Cavendish, 166, is not far from the truth.

NOTE 25, ARTS. 360, 539, 666.

Capacities of the large tin Cylinder and Wires.

The dimensions of the cylinder are given more accurately in Art. 539. It was 14 feet 8.7 inches long, and 17.1 inches circumference. Its capacity when not near any conductor would be, by the formula in Note 12, 22.85 inches, and when its axis was 47 inches from the floor it would be 31.3 inches, or in Cavendish's language 62.6 inches of electricity. Cavendish makes its computed charge 48.4, and its real charge 73.6. See Art. 666. Now the charge of either of the plates D and E was by Art. 671, 26.3 inches of electricity, so that

The capacities of the different wires mentioned in Arts. 360 and 539 are, by calculation,

The ratio of the charge of the first of these wires to that of the second is 1.37.

NOTE 26, ART. 369.

Action of Heat on Dielectrics.

The effect of heat in rendering glass a conductor of electricity is described in a letter from Kinnersley to Franklin* dated 12th March, 1761. He found that when he put boiling water into a Florence

* Franklin's Works, edited by Sparks (1856), Vol. v., p. 367.

flask he could not charge the flask, and that the charge of a three pint bottle went freely through without injuring the flask in the least.

Franklin in his reply describes some experiments of Canton's on thin glass bulbs, charged and hermetically sealed and kept under water, showing "that when the glass is cold, though extremely thin, the electric fluid is well retained by it."

He then describes an experiment by Lord Charles Cavendish, showing that a thick tube of glass required to be heated to 400° F. to render it permeable to the common current.

A portion of a glass tube near the middle of its length was made solid, and wires were thrust into the tube from each end reaching to the solid part. The middle portion of the tube was bent, so that a portion, including the solid part, could be placed in an iron pot filled with ironfilings. A thermometer was put into the filings; a lamp was placed under the pot; and the whole was supported upon glass.

The wire which entered one end of the tube was electrified by a machine, a cork ball electrometer was hung on the other, and a small wire, reaching to the floor, was tied round the tube between the pot and the electrometer, in order to carry off any electricity that might run along upon the tube.

"Before the heat was applied, when the machine was worked, the cork balls separated at first upon the principle of the Leyden phial. But after the middle part of the tube was heated to 600, the corks continued to separate, though you discharged the electricity by touching the wire, the electrical machine continuing in motion. Upon letting the whole cool, the effect remained till the thermometer was sunk to 400."

Experiments on the conductivity of glass at different temperatures have been made by Buff*, Perry †, and Hopkinson‡

Hopkinson finds that if B is the specific conductivity divided by the specific inductive capacity and multiplied by 47, then for

glass No. 2, log B = 1·35+0·04150,

where is the temperature centigrade.

Glass No. 2 is of a deep blue colour; it is composed of silica, soda, and lime.

Glass No. 7 is "optical light flint," density 3-2, composed of silica, potash, and lead; almost colourless, the surface neither "sweats" nor

* Annalen der Chemie und Pharmacie, xc. (1854), p. 257.

+ Proc. R. S. 1875, p. 468.

Phil. Trans. 167 (1877), p. 599.

tarnishes in the slightest degree. This glass at ordinary temperatures is sensibly a perfect insulator.

The conductivity of glass when heated makes it very difficult to determine its capacity as a dielectric. It appears from the experiments of Hopkinson on glasses of known composition, that the glasses made with soda and lime conduct more, and are also more subject to "electric polarization" and "residual charge" than those made with potash and lead.

Both the conductivity and the susceptibility to residual charge increase as the temperature rises, and this makes it very doubtful whether the apparent increase of dielectric capacity, which was observed by Cavendish and also by recent experimenters, is a real increase of the specific inductive capacity, or merely an effect of increased conductivity.

The experiments of Messrs Ayrton and Perry* on wax at different temperatures would seem to indicate a real increase of dielectric capacity, as well as of conductivity, as the temperature rises up to the melting point. During the process of melting the capacity decreases and at higher temperatures begins to increase again, but the conductivity continues to increase as the temperature rises.

The rule by which Cavendish computed the charge of a condenser consisting of two cylindrical surfaces having the same axis is given at Art. 313.

If R is the external and r the internal radius, and the length of the cylinders, then Cavendish's expression for the "computed charge"

is

1 R+r7

1.

The true expression for the capacity is

1

2 log R-logr

when the logarithms are Naperian.

We may express log R-logr in the form of the series

and we thus find as an approximate value of the capacity

The first term agrees with Cavendish's rule, for the "capacity" is half the "inches of electricity," but the other terms show that Cavendish's rule gives too large a value for the computed charge.

The following table gives the charge as computed by Cavendish compared with that given by the correct formula.

The fishes which are known to possess the power of giving electric shocks belong to two genera of Teleostean Fishes and one of Elasmobranch Fishes, and the position and relations of the electric organs are different in each.

In every instance, however, the electric organ may be roughly described as being divided in the first place into parallel prisms or columns by septa, which we may call (with reference to the organ, not the fish) longitudinal septa, and in the second place each column is divided transversely by diaphragms, the structure of which is different in the different families, but in every case the terminations of the nerves