FIG. 66.

a compact arrangement, they are soldered together, and combined as in Fig 67, A representing one of the faces of the pile. When both faces are equaliy heated, there is no current. If the face, A, is warmed, there is a current in one direction, due to the difference of temperatures between the two faces. If the opposite face is warmed, or, what is the same thing, if

D

FIG. 67.

B

Thermo-electric Pair.

Arrangement of the Bars.

the face, A, is cooled, there is a reverse current.

148. In Fig. 68, A B represent the thermo-electric pile as mounted for lecture-room use. A shows one of

FIG. 69.

W

B

Thermo-electric Pile as mounted for Use.

the faces; w w are wires connecting it with the galvanometer. The needle m n is suspended by a fibre of un

spun silk, s s, and protected from currents of air by the glass shade G. To one end of the needle is fixed a piece of red paper, and to the other a piece of blue. If the face of the pile is merely breathed upon, the warmth swings the needle round to 90°, or at right angles to the cur rent-the pieces of paper making the movement visible throughout the room. This important instrument detects heat radiation from sources much lower than the human body, and announces the heat emitted from the bodies of insects. Lately it has been used to detect the heat from the fixed stars.

148a. Animal Electricity.-Certain fish possess the rcmarkable power of giving electrical shocks. They have internal organs for this purpose, which play the part of batteries, and the discharges from them produce all the effects of ordinary electricity. In the torpedo the electrical organs are situated on each side of the head, and consist of a mass of cells filled with a dense fluid consisting of water, albumen, and common salt. These organs give rise to electricity just as the muscles do to mechanical motion. A dense mass of nerves links them with the brain, which controls the electrical discharges in the same way as it does the mechanical movements.

It has been proved that currents of electricity circulate in the frames of all animals, and that different parts or sections of the muscles are in different electrical states. The smallest shreds of muscular tissue have been proved by Prof. Du Bois-Raymond to manifest currents, the longitudinal section being always positive to the transverse section. By arranging a series of half-thighs of frogs, alternately connecting the exterior and interior surfaces, he obtained an electrical current that decomposed potassic iodide, deflected a magnetic needle 90°, and caused the gold leaves of an electroscope to diverge. Many have supposed that the nervous force is electrical, but this is disproved by the comparatively slow rate of its motion. It is, however, probably an analogous polar force.

CHAPTER V.

LIGHT,

81. Motion of the Radiant Forces.

149. Their Motion.-Light is that agent which, acting on the eye, produces vision. Other forces are generally associated with it that obey the same laws of motion; and, as that motion is in rays, they are known as radiant forces; their laws of movement are the same. Light moves in straight lines and in all directions from the point of emission-diminishing in intensity in inverse ratio to the square of the distance. Its velocity is about 190,000 miles per second. When light falls upon bodies, some reflect it, others absorb it, and others transmit it. The laws of these motions are explained in books upon optics.

FIG. 69.

Viale

150. The Analysis of Light.-By the prism-a triangu lar piece of glass, or other transparent substance—the white ray is decomposed into a series of colors. A beam of solar light passing through such a prism, Fig. 69, is refracted by it, and produces an oblong colored image called the solar spectrum. It is usuGreen ally considered to comprise Yellow the seven colors enumerated in the accompanying diagram. White light is, therefore, held to be a comcolored lights, which are only

Decomposition of Light.

Indige

Blue

Orange
Red

pound consisting of these

separated by the prism. Each color has its own peculiar

refrangibility, or degree of divergence from the original source, the red being least refracted, and the violet most. Certain of the rays accompanying light produce heating effects, and others chemical effects. These rays are governed by the same laws of motion as light itself, but by the prism the heat is mainly distributed through the red end of the spectrum, and the chemical force through the violet extremity. (162.)

151. The Wave Hypothesis.-The motion of the radiant forces is explained by what is called the wavetheory. It is known that sound is propagated through the air by means of the vibration of its particles; and it is supposed that light moves by a similar mode of action. There is a great amount of evidence to show that the radiant forces are propagated by some kind of undulatory movement, but the hypothesis implies a medium that is capable of this kind of motion. This medium is assumed to be an infinitely rare and elastic ether that fills all space and pervades all matter.

152. Cause of Colors.-According to this hypothesis, light is transmitted by ethereal undulations just as sound is by those of the atmosphere; with only this difference, that, while the air-particles move backward and forward in the same direction as the advancing wave (longitudinal vibrations), the ethereal particles move across the course of the wave (transverse vibrations). Thus the spectrum is to the eye what the gamut is to the ear. As the pitch of sound depends upon the length of the air-wave, so the color of light depends upon the length of the ethereal wave; and as loudness of sound depends upon the extent of the swing of air-particles, so the brightness or intensity of color results from the extent of the excursions of the ethereal particles.

By several refined methods which cannot be detailed here, the lengths of the ethereal waves upon which colors depend have been estimated. The motions which produce

red are slower, and the undulations longer than those which produce violet. It is found that 40,000 waves of red light would measure an inch, while 60,000 waves of violet light would fill the same space. The other colors are intermediate, their number of waves increasing gradually from red to violet. As light moves 190,000 miles per second, that length of ray streams into the eye each second. If this distance be reduced to inches, and the product be multiplied by 40,000, we shall have the number of waves which beat against the retina each second, when look upon a red color. If the same product is multiplied by 60,000, we get the number of pulses per second which strike the retina when looking upon a violet color.

we

153. Transmission of Radiant Motion.-In this view it is necessary to distinguish between vibrations and undulations. In the case of sound, the vibrations of a sonorous body, as a bell, produce undulations in the air which, when striking against distant bodies, may set them also into vibration. The vibrations of the bell, transmitted as airwaves, are taken up by the tympanum of the ear, which, when set to vibrating, gives rise to the sensation of sound. So the vibration of atoms in a flame produces undulations in the ether; these are transmitted to the nerve of vision, and, breaking against it, throw its atoms into the vibra tions which produce sight. In the same way the particles of a heated body are supposed to be in a state of vibration, which are transmitted by ethereal undulations, and these, falling upon other bodies, set their particles into vibration, and raise their temperature. This is the explanation afforded of radiant heat.

§ 2. Interference and Polarization.

154. Interference of Waves.-When two sets of waterwaves are made to flow together, if they coincide, that is, if ridge corresponds to ridge, their height will be increased;