127 v.] ABSORPTION OF CHEMICAL RAYS. 279 producing the combination of chlorine with hydrogen, was found to be reduced to one-tenth when transmitted through a column of chlorine 6.811 inches (173mm.) in depth; if the chlorine were diluted with an equal volume of air, the length of the column required to produce a similar absorption was exactly double, or 13622 inches (346mm.). But when a mixture of equal volumes of chlorine and hydrogen was used, the depth of the mixture which was required to reduce the chemical effect of the light to one-tenth of its original intensity was only 9'212 inches (234mm.); hence it appears that a certain quantity of the active rays are absorbed during the production of a given chemical effect. With light from different sources, analogous results were obtained, but the amount absorbed was found to vary with the source of the light. For instance, the diffused zenith light of a cloudless sky in the morning was reduced to one-tenth of its intensity by transmission through 1795 inch (45-6mm. of chlorine, and through 2.894 inches (73.5mm.) of a mixture of chlorine and hydrogen; the absorbent action of chlorine upon the chemical rays of diffused daylight being much more energetic than on those emitted by burning coal-gas. Observations made with evening light showed that a depth of 0.776 inch (19.7mm.) of chlorine was sufficient to reduce the chemical power to one-tenth of that possessed by the incident light. The relative thickness of the stratum of chlorine required to produce an equal reduction in the chemical power of the incident light was therefore the following: (127) It was stated more than 60 years ago by Ritter, and the observation was confirmed and extended by Sir J. Herschel, that the two ends of the spectrum produce opposite chemical effects, though the violet extremity appears greatly to predominate in power. If, for example, paper soaked in argentic nitrate be partially blackened by exposure to diffused daylight, and then submitted to the action of the solar spectrum, the portion upon which the violet end falls speedily becomes much darker, while the portion beneath the red rays assumes a brick-red hue. If the spectrum be thrown upon white paper coated with argentic nitrate, and diffused daylight be allowed at the same time to fall upon it, the spot where the red rays fall retains its whiteness, while the rest of the paper speedily darkens. It thus appears, that by combining the influence of two rays of different refran 280 ABSORPTION OF CHEMICAL RAYS. [127 0. gibilities, effects are producible which cannot be obtained by either ray separately. Paper soaked with argentic nitrate and blackened by the action of light, if washed over with a solution of potassic iodide, becomes gradually bleached when exposed to diffused daylight. If the solar spectrum be allowed to fall upon paper thus prepared, whilst moist, and before it has become bleached, the part beneath the violet end is quickly bleached; but this effect is bounded by a sharp border in the yellow, while the paper under the red end becomes darker. The phenomena of phosphorescence also exhibit similar opposition in the effects produced by the opposed extremities of the spectrum (112). Claudet (Phil. Trans. 1847, 256) found that an iodized Daguerreotype plate, when submitted in the focus of a camera to the red image of the sun as seen through a London fog, became subsequently whitened on exposure to the vapour of mercury, in all parts except in the track traversed by the image of the sun-this portion continued to be perfectly black. In another experiment, a plate was covered with black lace, and exposed to diffused daylight: after a few minutes' exposure, the lace was removed, one-half of the plate was then covered with an opaque screen, the other half with a red glass, and the exposure was continued for a short time: in the mercury box the red half continued to be black, whilst on the other portion the image of the lace was distinctly traced. The photographic effect at first produced over the whole plate had in fact been neutralized by the red glass. A pleasing variation of the last experiment was made by exposing an iodized plate to diffused daylight, then covering it with a piece of black lace, and screening it with red glass; a negative picture was now developed in the mercury box, the red glass having destroyed all photographic action except on those parts screened by the lace. Orange and yellow glasses give similar results. After exposing a plate to daylight, and then submitting it to the action of light passing through red glass, it again becomes sensitive to light, so that, as Claudet observes, it is no longer needful to prepare the plates in a dark chamber, since, if placed beneath a covering of red glass, they are always ready for immediate use-even though subsequently to their preparation they may have been for some time exposed to solar light. But though the red and yellow glass have the power of completely counteracting the effect of the radiation of the more refrangible rays, they have a peculiar effect of their own. The neutralizing power of the red ray is exerted more slowly than the photographic effect of the white light, nearly in the proportion of 100 to 1; that of the yellow ray was found to be about that of IO to I. From the foregoing remarks, it is evident that the colour of objects must exert a material influence upon the nature of the photographic image produced. Reds and yellows, from the want of chemical energy in the less refrangible portion of the spectrum, will be characterized by absence of photographic action in the image, and will be represented by black spots, which often produce singular disfigurement in portraits. Yellow freckles, for instance, on the skin of the face are accurately copied, but are depicted in the portraits as black spots. Much judgment and knowledge are therefore required in selecting a dress of a colour which is adapted to produce a suitable depth and contrast of tint in the photograph. * It must be borne in mind that all results obtained by coloured media are liable to ambiguity, for it seldom happens that the light transmitted through them is homogeneous (105); the effects are liable to become complicated from the intermixture of results produced by rays from different parts of the spectrum. 128] GENERAL EFFECTS OF HEAT. 281 (127 w) Action of the Solar Spectrum on Vegetable Colours.— This subject has been particularly examined by Herschel (Phil. Trans. 1842, 188). White paper coloured with various vegetable juices was subjected by him to the influence of the prismatic spectrum, and in some cases these papers were washed over with solutions of metallic salts. The following are the most important general conclusions which may be drawn from these experiments: -1. That the action of light is in almost all cases of a nature to obliterate the colour; or if it does not entirely bleach it, a faint residual tint is left, upon which it has little further action. The older the paper or the tincture, the more decided is this residual tint, which is probably the result of an oxidizing action upon the colouring material, independent of the action of light. 2. The action is confined to luminous rays of the spectrum-offering in this respect a marked difference between these actions and those produced upon the metallic compounds. 3. The rays which are most effective in destroying a given tint are in many cases those which are complementary (105) to the tint destroyed. Orangeyellows, for instance, are bleached most powerfully by the blue rays: blues by the red, orange, and yellow rays; and purples and pinks by the yellow and green rays. CHAPTER V. НЕАТ. § I. Expansion.—Measurement of Temperature.-§ II. Means of maintaining Equilibrium of Temperature.—§ III. Specific Heat, Latent Heat.—§ IV. Heat of Combination. (128) General Effects of Heat.-Upon the due understanding of the principles and applications of heat, much of the successful prosecution of chemical research depends. There is scarcely a chemical operation in which heat is not either emitted, absorbed, or purposely applied to produce the required result. Heat in one mode of its manifestation presents the closest analogy with light, which it very generally accompanies. In this condition it is known as radiant heat: and it is in this form that the supply of heat is transmitted from the sun to the surface of the earth. It is, however, after heat has fallen upon the surface of an object and has become absorbed, that its most important effects are manifested. It is only then that the sensation of warmth is 282 GENERAL EFFECTS OF HEAT. [128. experienced; then it is that expansion takes place in the heated body; and it is then only that the phenomena of liquefaction or of evaporation may ensue. Heat may also, after its absorption, be again transmitted from the heated body, by secondary radiation, to other objects around, or it can be propagated more slowly by conduction from particle to particle through the mass. Other most important effects of heat are seen in the change of state in bodies from the solid to the liquid and from the liquid to the gaseous condition. Whenever a solid becomes liquid, or a liquid becomes converted into vapour, the change is attended with the disappearance of a quantity of heat, which is perfectly definite; for instance, a pound of ice in undergoing liquefaction always requires the same quantity of heat to produce this effect; the water obtained appears to be no warmer than the ice; but the heat, though it for a time ceases to affect the senses, is not lost, for it reappears when the water passes back into the state of ice. The heat which disappears in liquefaction is said to have become latent; and it again becomes sensible as the solid condition is resumed. Finally, it is found that the quantity of heat produced by the chemical actions of definite amounts of matter upon each other is definite, whether the chemical action occur rapidly or slowly. In considering the relations of heat, the subject may therefore naturally be subdivided into four sections: The first of these embraces the phenomena of expansion, and their application to the measurement of temperature, including the principle of the thermometer and the pyrometer: the second refers to the modes in which the equilibrium of temperature is sustained or restored-viz., by conduction, by convection, and by radiation as well as the phenomena of specific heat the third relates to the latent heat of liquefaction and vaporization, including the processes of congelation and liquefaction, and those of ebullition and evaporation: whilst the fourth embraces heat of combination, or the quantitative estimation of the heat evolved by chemical action. Before passing to the immediate consideration of these subjects, it will be advantageous to review briefly the principal means at our command for procuring a supply of heat by artificial means. (129) Sources of Heat.-1. The sun obviously affords the main supply of warmth to the globe. It may furnish some aid towards a conception of the enormous amount of heat continually emanating from the sun, when we state that, calculating from the 129.] PRODUCTION OF HEAT BY MOTION. 283 mean distance of the earth from the sun, and from the area which the earth exposes to the solar rays, the quantity of heat which reaches the earth is not at any given moment more than the two thousand three hundred and eighty millionth part of that emanating from the sun. 2. There are, however, many other sources whence heat may be procured. Friction is one of them. Some savage nations employ the friction of two pieces of dry wood as a means of obtaining fire; and it is known among ourselves that the axles of wheels and other parts of machinery sliding in contact sometimes become so much heated as to char or ignite the woodwork in their immediate vicinity. In order to obtain some idea of the amount of heat produced by friction, the following experiments were instituted by Count Rumford (Phil. Trans. 1798,80): -A brass cannon, weighing 113 lb., was made to revolve horizontally with a pressure of about 10,000 lb. against a blunt steel borer, at the rate of 32 revoIntions per minute; in half an hour the temperature of the metal had risen from 15° C. to 55°; this heat would have been sufficient to have raised 5 lb. of water from 0 to 100° C. The experiment was subsequently varied by placing o° the cannon in a vessel of water, and friction was again applied; in this case, 183 lb. of water at 15° C. were actually made to boil in two hours and a half. The heat thus obtained was calculated by Count Rumford to be somewhat greater than that given out during the same period by the burning of nine wax candles each inch in diameter. One of the most remarkable proofs of the generation of heat by friction was afforded in an experiment by Davy, in which two pieces of ice, made to rub against each other in vacuo, at a temperature below o° C., were melted by the heat developed at the surfaces of contact. The experiments of Joule (Phil. Trans. 1850, 61) appear to show that the actual quantity of heat developed by friction is dependent simply upon the amount of work expended, without regard to the nature of the substances rubbed together. He found, as a mean of forty closely concordant experiments, that when water was agitated by means of a horizontal brass paddle-wheel, made to revolve by the descent of a known weight, the temperature of 1 lb. of water was raised 1° F. by the expenditure of an amount of work sufficient to raise 7727 lb. 1 foot high. foot high. When castiron was rubbed against iron, the work required to raise 1 lb. of water 1° F. was found, as a mean of twenty experiments, to be about 775 foot pounds; and by the agitation of mercury by means of an iron paddle-wheel it was found to be 774 foot pounds. I о The conclusion drawn from these experiments was--that the quantity of heat capable of raising the temperature of 1 lb. of |