The light and heat of the electric spark are intense though instantaneous; but a powerful induction apparatus like Ruhmkorff’s gives so rapid a succession of sparks that the light and heat are sensibly continuous and of great intensity. The light and heat, powerful as lightning itself, are produced by the combined currents of two batteries, each consisting of fifty Bunsen elements of moderate size. This formidable united current passes through a circuit of thick copper wire coated with silk thread, with an intensity of perpetually renewed heat that no substance can resist. When the copper conducting wires are fitted with charcoal terminals and brought near to one another, the dazzling lights emanating from each pole combine in one blaze of insupportable brilliancy. The most refractory substances, silica, alumina, iron and platinum, when placed between the poles, immediately melt like wax, and volatilize. Charcoal is so good a conductor of electricity that when the terminals are in contact they complete the circuit, and neither light nor heat appear. Air and glass are non-conductors, yet the spark has passed through several inches of air and perforated a mass of glass two inches thick. A long electric spark combines or decomposes a greater quantity of gas or vapour than a short one, and for a given induction apparatus and induction current, M. Perrot has shown that there exists a length of spark corresponding to a maximum chemical action.
Professor Seebeck of Berlin discovered that electric currents are produced by the partial application of heat to a circuit formed of two solid conducting substances as antimony and bismuth soldered together,—another proof of the correlation of heat and electricity.
There cannot be a doubt that the atoms of a conducting wire are in motion, and that they successively take definite and momentary positions during the passage of an electric current, after which they return successively to their normal state. When electricity is invariably sent from the same pole of an inductive apparatus through the wire of a telegraph, in a very short time the wire is torn or divided into small sections, which destroy its continuity; but when the electricity is sent from each pole alternately, the conducting wire is not injured. As each atom of the wire has its own electricity, this seems to indicate that during the successive transits of the same kind of electricity, the pole of each individual atom is attracted more and more in the same direction, till at last they no longer return to their normal state, the cohesive force is overcome, and a rupture takes place, the more readily if there be any imperfection in the wire. Since the electricity from the other pole of the machine would have the same effect, but in the contrary direction, an alternate motion in the atoms must maintain the continuity of the wire.
A closed current of electricity or magnetism is accompanied by a simultaneous current of the opposite force in the tangential direction equal in quality and intensity. Thus the electric and magnetic currents, which are merely transmissions of energy, differ by moving at right angles to one another; their effects are alike, yet they are not identical.
The amount of the chemical action of light has been determined by Professor Roscoe to be directly proportional to the intensity of the light; and when the light is constant the amount of action is exactly proportional to the time of exposure. It appears that equal volumes of chlorine and hydrogen explode in sunshine, but combine slowly in shade; and as the combined gases are absorbed by water as soon as combined, the gradual diminution of the volume of the mixed gases during the time of absorption is a measure of the amount of action exerted by the light.
Professor Wm. Thomson has computed, by the aid of Poullet’s data of solar radiations and Mr. Joule’s mechanical equivalent of heat, that the mechanical value of the whole energy, active and potential, of the disturbances kept up on the ethereal medium by the vibrations of the solar light in a cubic mile of our atmosphere, is equal to 12,050 times the unit of mechanical force: that is to say, twelve thousand and fifty times the force that would raise a pound weight of matter to the height of one foot. The sensible height of the atmosphere is about forty miles, whence some idea may be formed of the vast amount of force exerted by the sun’s light within the limits of the terrestrial atmosphere. The green mantle which clothes the earth proves under a beautiful form the influence of light on the organic world.
It has been proved that at any given fixed temperature the amount of light and heat absorbed and that which is emitted remains constant for all bodies. The greater the amount absorbed, the greater the amount radiated. The molecules or atoms of the bodies in consequence of the law of resonance emit those ethereal undulatory motions which have been previously impressed upon them, as a musical instrument resounds in answer to the note impressed upon it. The whole is referable to molecular or atomic motion, for in absorption the vibrations of the ether are communicated to the atoms, and in radiation, the vibrations are returned again to the ether. This principle is known as the law of exchange.[[5]]
Matter has a decomposing and an elective power with regard to both radiant light and heat; most coloured bodies, such as flowers, green leaves, dyed cloth, &c., though seen by reflection, owe their colour to absorption. The light by which they are seen is reflected, but it is not in reflection that the selection of the rays is made which causes the objects to appear coloured. When light falls upon red cloth, a small portion is reflected at the outer surfaces of the fibres, and this portion, if it could be observed alone, would be found to be colourless. The greater portion of the light penetrates into the fibres, when it immediately begins to suffer absorption on the part of the colouring matter. On arriving at the second surface of the fibre, a portion is reflected and a portion passes on, to be afterwards reflected from, or absorbed by, fibres lying more deeply. At each reflection the various kinds of light are reflected in as nearly as possible the same proportion, but in passing across the fibres while going and returning they suffer very unequal absorption on the part of the colouring matter; so that in the aggregation of the light perceived the different components of white light are present in proportions widely different from those they bear to each other in white light itself, and the result is a vivid colouring.
In certain substances however, as gold and copper, the different components of white light are reflected with different degrees of intensity, and the light becomes coloured by these reflections. Gold is yellow by reflection; red cloth is red by absorption. In the same sense, physically speaking, in which the red cloth is red, gold is not yellow but blue or green; such is in fact the colour of gold by transmission through gold leaf, and therefore gold is greenish blue by absorption. In this case we see that while the substance copiously reflects and intensely absorbs rays of all kinds, it more copiously reflects the less refrangible rays with respect to which it is more intensely opaque. In general absorption and radiation are independent of colour.
There is a vast diversity in the property which substances possess with regard to the transmission of radiant light and heat; glass, for instance, transmits light abundantly, but is impervious to heat from non-luminous sources; while other substances, which are altogether opaque to light, transmit heat copiously, as the bisulphide of carbon, which of all liquids is the most diathermic, while water in all its forms is almost impervious to heat.