560 Other Sources of Light. Phosphorescence. product of combustion. Fixed substances like iron and charcɔal emit a great amount of light in proportion to the heat. In all combustions in which oxygen takes a share, the greater the amount of this element consumed within a given time, the greater the quantity of light emitted. The Bude light owes its great brightness to the current of oxygen introduced into the centre of the coal-gas flame. 786. Other Sources of Light-The Electric Light.-In the production of the electric light in vacuo there is no combustion, the heat of the electric current is converted into light, manifested by the vivid incandescence of the particles of charcoal at the two poles. The intensity of the electric light thus emitted is considered to be equal to one-fifth, or even to one-fourth, of that of the sun. Phosphorescence.-There are other sources of light unattended with any perceptible degree of heat, as in the luminosity of fluor. spar and other mineral substances when moderately heated, and to this, the term phosphorescence has been somewhat loosely applied. It is a condition in which invisible heat has been converted into visible light. Light from Animals.-As with heat, so with light; it may be produced in the animal body as a result of vital force. This source of light is seen in the bodies of the glowworm, the firefly, and other luminous insects met with in warm countries, as also in the minute rhizopods and jelly-fish (Acalephæ), which often give a splendid light to large surfaces of the sea.* In describing the powers which these minute rhizopods or foraminifera possess of secreting light, Mr. Rymer Jones observes, " Few visitors at the seaside can have failed to observe that often in the summer-time, the waves are luminous and shine with phospho *The wingless female glowworm (Lampyris) is highly luminous. It emits the light from the hinder part of the abdomen, and the intensity of it can be varied at pleasure. Unless the insect is physically injured, it continues to emit light when completely immersed in water, thus showing that this does not depend on any phosphoric matter. The light has the brilliancy of the diamond, and sometimes appears coloured like the sapphire, emerald, or topaz. No sensible heat accompanies this intense production of light. It is entirely under the control of the animal, for any disturbing cause will at once render the insect non-luminous. The Cucujo (Elater noctiluens), found in South America, has upon cach side of its chest, a round spot from which there issues at night, a light so brilliant that, when several are put together in a glass vessel, it is said to be easy to read by it the smallest print. Light. Diminution of Intensity. 561 rescent splendour. The ripples, as they totter towards the beach, sparkle with scintillations, and the crested waves blaze with a pale but brilliant light. The fisherman, who from his boat surveys the lambent flames that play around him, seems to float in fire. The mariner can trace his path by the long wake of light that streams behind like the train of some vast sky-rocket; or, looking from the prow, he sees his vessel, as she breasts the waves, dash from her bows broad sheets of liquid splendour. As morning dawns, the fairy vision vanishes, nor can the keenest eye perceive, in the translucent element, the tiny lamps that caused the grand illumination.” The cause of this phenomenon in every part of the world has been traced to the presence of myriads of living animalcula. It shows that light may be manifested in an intense form without any corresponding degree of heat, or it might be said of these bodies that the heat emitted from them has been entirely converted into light. 787. Light is sometimes emitted by substances of an organic nature, before they have reached a certain stage of decay. Certain kinds of fish, such as the whiting, herring, and mackerel, have been observed to emit a pale light in the dark; and this phenomenon has been even observed in the dying and recently dead human body. The appearance thus presented is of a phophorescent kind, and it probably depends on chemical changes in the solids, although, as no sensible heat is emitted, it cannot be regarded as dependent on combustion. It is arrested by putrefaction. Light is in some cases emitted by crystalline substances. In the sudden crystallization of sulphate of soda (Art. 671, HEAT), light has been observed to issue when the experiment was performed in a dark room. Common loaf-sugar rubbed in the dark in warm dry weather emits a pale phosphorescent light. The heat produced by molecular changes in the substance or by friction, may explain these facts. Some have ascribed it to an electrical effect. "Light becomes less intense as it spreads.” 788. Light, like any other emanation from a central point, in spreading through wider space, becomes thinner or less intense in proportion as it spreads. Thus, if a taper be placed in the centre of a cubical box every side of which is a foot square, the light falling on the sides of the box will have a certain intensity there: if the taper be then placed in a similar box with sides of two feet square, there will be only the same quantity of light, but it will be spread 562 Separation of Light and Heat. over four times as much surface-for a square having two feet in the edge, is made up of four squares of one foot-and will, therefore, on any part of that surface, be only one-fourth part as strong or intense as in the £rst box; and so for any other size of box or space, the intensity will diminish as the square of the distance increases. (See Art. 19, fig. 1, page 9.) Hence, if the earth were at twice its present distance from the sun, i.e., beyond the planet Mars, it would receive only one-fourth of the light and heat which it now receives, just as a man placed four yards from a fire receives only one-fourth of the heat which falls on a man at two yards. At three times its present distance from the sun, the earth would receive only one-ninth of the light now received, and conversely, if it were only one-third of its present distance, the heat and light would be increased ninefold. A reference to fig. 1, page 9, will enable the reader to follow this statement of the law of increase and diminution. Separation of Light and Heat. Calorescence. 789. Light and Heat are generally found associated. This is observed in the sun's rays as well as in the phenomena of combustion. They admit, however, of separation. A stream of concentrated light from the carbon-points of a powerful battery received upon a prism, is resolved, like solar light, as will be more particularly described in a future article, into the seven primitive colours of which white light is constituted. When a similar beam was passed through a layer of rock-salt it was found that heat, as well as iight, traversed the salt, and produced all its usual effects. Having thus the two forces at command, Professor Tyndall adopted the following ingenious plan in order to separate them. He placed in front of the concentrated beam of electric light a rock-salt cell, containing a strong solution of iodine in sulphide of carbon. This was found to have the property of entirely arresting all the rays of light, although these were concentrated by a powerful concave mirror, the focus of rays being so adjusted as to fall at a point on the darkened side of this liquid screen. When the solution was removed, the rays of heat and light were demonstrated to exist in an intense form at the visible focal point, about six inches distant from the electric light. Substances placed in the focus were illuminated and burnt. On replacing the screen not the slightest portion of light could be seen ; the rays of light were quite cut off by the solution, but the dark rays of heat traversed the opaque liquid as if no screen were inter The focus of dark rays posed, and produced all their usual effects. was easily found in the impenetrable darkness, by bringing into it a variety of combustible substances. Black paper was immediately inflamed wood, zinc, and magnesium wire instantly took fire and burnt with their usual splendour. The most interesting result was witnessed on placing thin platinum foil, or platinized platinum, in the focus of dark rays. It was speedily rendered white-hot, and emitted a strong light by its incandescence. Other substances had undergone combustion, and had emitted light and heat from chemical changes. The platinum underwent no change; it became luminous by simply absorbing the dark rays of heat, and at a certain temperature throwing them off as light. This incandescent platinum would have readily furnished a spectrum of seven colours like that obtained from the sun or any self-luminous body. To these phenomena Tyndall has given the name of Calorescence, by which term is to be understood the conversion of obscure radiant heat into light. This experiment clearly proves that, at a certain temperature, black heat may be converted into a bright light, provided there is some substance which can receive and retain the dark heat-rays for a certain period of time. 790. It has been found that glass, coloured with carbon, by stirring a stick in it while in a melted state, has the property of cutting off the rays of light entirely, and of partially quenching those of heat. On the other hand, Melloni discovered by a series of numerous experiments that light, whether derived from the sun or from ordinary combustion, might be deprived of all heat-rays by passing it successively through water and a stratum of glass coloured green with oxide of copper. He found that the rays, on issuing from these media, did not retain sufficient heat to affect the most delicate thermometer. Light is thus separated from heat, but only as coloured light, and no process has yet been devised whereby this light can be converted into heat. By a series of ingenious experiments, Melloni was enabled to prove that the light reflected from the moon is not accompanied by heat-rays. By concentrating the light of the moon through a lens of forty inches in diameter, he obtained a strong focus of light of four-tenths of an inch in diameter. When this was directed upon a very sensitive thermo-pile (see Electricity), there was only a feeble indication of heat. 791. There is another fact of some importance in reference to the convertibility of these two forces. Although heat-rays and lightrays come to us from the sun, they are very unequally distributed 564 Non-luminous bodies through the spectrum. As it will be fully explained hereafter, the rays of heat are more associated with the least refrangible rays, i.e., with the red, and even with those beyond the red. When a rock-salt prism, which is highly diathermanous (¿.e., transmits heat), is used for the decomposition of light, it is found that the maximum intensity of the rays of heat is considerably beyond the red ray, in fact, they are much less refrangible than the least refrangible of the rays of light. This clearly shows a remarkable difference between the two forces. In the annexed illustration (fig. 185) the rays of light are shown by the corresponding letters, V, I, B, G, Y, O, R. The heat-rays are indicated by the dark space beyond the red, R, and the maximum of heat corresponds to the parts marked by arrows. It was at a point in the direction of the arrows that the focus was obtained, in which the experiments above mentioned were performed. "Light falling on non-luminous bodies, makes them visible.” 792. If a beam of the sun be admitted into a dark apartment so as to fall upon some object, that object becomes bright, and affects the eye almost as if it were itself luminous. It returns a part of the light which falls upon it, and it is visible in all directions, proving that it scatters the received light all around. This scattered light falling on other objects, and reflected again and again among them until absorbed—like an echo repeated many times and lost between perpendicular rocks,—may make all of them also visible, although in weaker degrees, and the whole apartment is then said to be lighted. If the direct ray be made to fall upon a surface which reflects much of the light, as a sheet of white paper or a mirror, the apartment will be more ghted :—if, on the contrary, it be received |