in the article on Electricity in this work). The outside of the thermo-electric pile and its attached cone was wrapped round with wadding and cloth, so as to be entirely unaffected by currents of air.

"During these experiments the disc of aluminium was rotated rapidly for half a minute, and a heating effect was, in consequence of the rotation, recorded by the thermo-electric pile (an instrument described fully in the article on Electricity).

"To obviate the objection that the electric currents which take place in a revolving metallic disc might alter the zero of the galvanometer, the position of the line of light was read before the motion began, and immediately after it ceased, the difference being taken to denote the heating effect produced by the rotation.

"The thermometric value of the indications given by the galvanometer was found in this way:-The disc was removed from its attachment and laid upon a mercury bath of known temperature. It was then attached to its spindle again, being in this position exposed to the pile, and having a temperature higher than that of the pile by a known amount. The deflection produced by this exposure being divided by the number of degrees by which the disc was hotter than the pile, gives at once the value in terms of the galvanometric scale of a heating of the disc equal to 1° on Fahrenheit's scale.

"The disc of aluminium being blackened with a coating of lampblack, applied by negative photographic varnish, and rock salt inserted in the cone, the following results were obtained:

"To ascertain whether the radiant heat recorded was derived from the rock salt, or from heated air, or from the surface of the disc, the next series of experiments were tried.

"With certain modifications of the above experiments it was satisfactorily proved that the effect was not due to heating of the rock salt, or to radiation from heated air; it must therefore be due to the disc of aluminium, which seemed to have rubbed against some matter which remained in the receiver after the air was removed. The question being, was this ether?"

The authors further state that,

"I.-It may be due to the air which cannot be entirely got rid of.

"2.-It is possible that visible motion becomes dissipated by an etherial medium in the same manner and possibly to nearly the same extent as molecular motion, or that motion which constitutes heat.

“ 3.—Or, the effect may be due partly to air and partly to ether. "Not to leave the matter wholly undecided, it was suggested by Professors

Maxwell and Graham that there is another effect of air, viz., fluid friction, the coefficient for which they believe to be independent of the tension.

"It would appear, however, that the fluid friction of hydrogen is much less than that of atmospheric air, so that were the heating effect due to fluid friction it ought to be less in a hydrogen vacuum. An experiment proved that the heating effect due to rotation in a hydrogen vacuum was 225, while in an air vacuum it was 23'5, and the authors are inclined to consider these numbers as sensibly the same, and that the experiment indicates that the effect is not due to fluid friction; at the same time they do not suppose that their experiments have yet conclusively decided the origin of this heating effect, but they hope to elicit the opinions of those interested in the subject, which may serve to direct their future research."

These experiments are more satisfactory than any previously tried, and, taken in conjunction with other facts, such as the temporary phosphorescence of certain bodies by what is termed insolation or irradiation, or the action of light in reducing certain salts to their metallic state, or the elaborate and beautiful effects obtainable from thin films of solid, fluid, and gaseous bodies, or the action of crystallized bodies on polarized light, they do altogether impress the reasoning faculties with a conviction that a vibrating motion accompanies the production of all light, which can only be propagated by the communication of these vibrations or tremblings to a medium, itself as subtile, rare, and exquisite as the delicate mechanism that sets it in motion. Starting with the proposition that all sources of light and luminous bodies, like musical instruments, must first vibrate, it is not difficult to understand by analogy how these vibrations may travel at the rate of 182,000 miles per second, in straight lines, called rays.

B

tat

FIG. 3.

A, tuning-fork struck on the leaden cone B, capped with leather, and applied to the end of the rod c, whilst the other end is held against the sounding-board D.

A tuning-fork emitting sound might by analogy represent a source of light like the sun, whilst a long rod communicating with it would stand in the place of the theoretical ether, propagating the undulations from the sun through a space of 92 millions of miles, and if the other end of the rod communicates with the sounding-board of a guitar, the audible sound obtained might compare with the light falling on the earth and becoming apparent by radiation.

The conversion of a continued series of mechanical impulses into waves is beautifully shown by taking hold of the end of a long vulcanized india rubber tube filled with sand, and having attached one end to the ceiling or other convenient place, it is easy by a jerk to produce the appearance of a wave, which travels distinctly from the hand to the ceiling; at the same time it demonstrates the progressive nature of the wave or undulation, and as the portion held by the operator cannot move from his hand to the ceiling, it shows how the eye is deceived whilst looking at the motion of waves of water. Every wave in water is propagated by the rising and falling of that which has preceded it, and not because the volume of water representing the wave travels bodily from the spot where it is first noticed to the shore where it breaks.

FIG. 4.-The Vulcanized Tube attached to the ceiling, and thrown
into protuberance or waves by the hand of the operator.

Dr. Tyndall has shown, by a modification of Dr. Young's experiments with vibrating strings upon which light is thrown, a number of very beautiful effects. A silvered cord attached to the iron arm of a curved spring band, one end of which is made to vibrate by an electro-magnet, displays the divisions of the cords into wave-like figures most perfectly when the cord is illuminated by the lime or, better still, the electric light. (Figs. 5 and 6, p. 7.)

Using the brilliant light as before, a still more perfect and admirable experiment may be conducted by attaching one end of a bright silvered chain to a hook screwed into a vertical whirling table, and the other to a proper stand. The chain being horizontal and the wheel vertical, it may be swung into one long wave, or, by a still more rapid rotation, can be divided into three, four, or more. The links of the chain flash in the light, and produce the most pleasing effects. It must be remembered that if cords, chains, water, air, &c., can assume a wave-like motion, the wonderful tension and elasticity of the hypothetical ether

would permit the latter to adapt itself to the most complicated movements almost with the rapidity of thought. The very spiral, spindle-like, or corkscrew motion observable in the chain and cord affords a good idea of the mechanism of the propagation of light, as the movement of each molecule of ether is always perpendicular to the path of the ray or wave of light.

The astonishing rapidity of the periodic movements of the non-gravitating molecules of ether becomes apparent, when it is stated that to produce white light five hundred millions of millions of vibrations of the ether, 1,000,000,000,000 X 500 must occur in every second of time.

Ór, taking the coloured rays at the extremities of the solar spectrum, viz., the red ray and the violet, the former demands the recurrence of four hundred and fifty-eight millions of millions, 1,000,000,000,000 × 458; and the latter, the violet, a still larger number, and greater rapidity of vibration, six hundred and ninety-nine millions of millions, 1,000,000,000,000 X 699 per second.

The coloured rays of light are supposed, according to the undulatory theory, to be distinguished from each by the breadths of the different waves, just as the sound of a stringed instrument may vary according to the diameter and thickness of the strings. A tightly-stretched thin cord vibrating would be the parallel to violet light. It is an axiom that, “ The rapidity of vibration is inversely proportional to the length and diameter of the string, and proportional to the square root of the tension." A thicker cord not so tightly stretched would be the parallel to red light.

SOURCES OF LIGHT.

At the various instrument-makers cases containing four or five tubes, filled with white powders and hermetically sealed, are to be obtained. When the tubes are observed in a dark room (and, of course, before exposure to light), they are invisible; if, however, a piece of magnesium wire is now burnt close to the tubes, they will be found to shine in the dark and to emit various coloured rays of faint light. To this curious effect is given the name of phosphorescence; and when the same result is obtained by exposing the tubes to the light of the sun, the resulting phenomenon is denominated phosphorescence after insolation, i.e., after exposure to the sun. The chemical substances which possess the property of developing light after exposure to light are called phosphori, and the best are the diamond, Bolognian phosphorus, or Bologna stone, made from sulphate of baryta, which occurs in nature as a mineral, and is called heavy spar or barytine. It is prepared by heating this mineral with charcoal to a dull red heat, or by the process of Margraf, in which the mineral is powdered, mixed with flour, and made red hot; or more amusingly by the process of Daguerre, who uses a marrow-bone for his crucible, and, after it is freed from fat and thoroughly dried, fills it with heavy spar, powdered in any non-metallic mortar. The bone is now closed with a clay lute, and inclosed in an iron tube, which is surrounded with fine clay, and the whole exposed for three hours to a red heat in a furnace. The substance which produces the effect is a sulphuret of barium. In the same manner strontian phosphorus is obtained from cœlestin.

Canton's phosphorus is prepared by exposing a mixture of three parts of sifted and calcined oyster-shells and one part of flowers of sulphur to a strong