table substances in a state of decay, and by exposure of many substances to sources of light. If calcined oystershells be placed for a short time in sunshine, and then withdrawn into darkness, they will continue to glow for some time, while other bodies, as the diamond and chlorophane, after exposure, remain for a long time luminous. Recent investigations have shown that the same property exists in a much lower degree in a great number of bodies, their phosphorescence continuing in most cases for the briefest moment-sometimes only for the ten-thousandth of a second. It would seem that, in cases where the luminosity continues, the molecules of matter are set in motion by the ethereal undulations, and continue to move after the withdrawal of the exciting cause. Fluorescence is a kind of phosphorescence, in which the highly-refrangible dark rays of the solar spectrum, next to be considered, are turned to light when falling upon certain substances, as fluor-spar, or sulphate of quinine solution.

162. A Third Radiant Force. Besides the heat-rays,

which take effect upon all kinds of matter, and the luminous rays, which act only upon special forms of nervesubstance producing the sensation of vision, there is a third class of rays which act upon certain chemical bodies, producing combination and decomposition. These have been called actinic rays, and the agency actinism; but they are better known as chemical rays. They accompany the light of the sun and stars, and are produced in arti

ficial illumination; but they are, nevertheless, distinct from light. The art of photography depends upon them, and has given a great stimulus to their recent investigation. Like light and heat, the chemical radiations are measurable in their effects, and have given rise to an independent branch of scientific inquiry.

163. Refrangibility of the Invisible Radiations. The heat-rays and the chemical rays are reflected and refracted like light, and like the colored rays they exhibit marked differences in their degrees of refrangibility. When the sunbeam is passed

through a prism

(Fig. 87) not only is there an oblong visible image thrown upon the screen, but there is also an invisible heat-image, and an invisible chemical image, which are revealed

in different ways. The position and varying intensity of the heat-spectrum may be traced out by a

delicate galvanom

FIG. 87.

eter, and it is found that it begins down in the neighborhood of a, and runs up into the luminous region. A large portion of the heat-rays are hence of a lower refrangibility than the red, and are dark radiations. If, now, a solution of argentic nitrate is washed over a large sheet of paper, which is then placed upon the screen so as to receive the visible spectrum and extend through the space

above it, a chemical change takes place upon its surface, producing a blackening, which defines the outline of the chemical spectrum. It is now found that the chemical rays are more refrangible than the luminous; and that, while the blackening takes place in the colored spectrum, it extends also through the dark space up to b. That the heat of the spectrum is greatest in the red, and that there are dark thermal rays of still lower refrangibility, was shown by Sir William Herschel, in the year 1800. That the chemical rays of the luminous spectrum are most active in the violet region was pointed out by Scheele, in 1777; while their extension into the dark space beyond, was discovered by Ritter, in 1801.

164. Distribution of the Forces.-The forces of the spectrum are thus very unequally distributed, as is illustrated in Fig. 88, where they appear to rise like the peaks of mountains. The middle curve shows the varying intensity

Varying Intensities of the Spectrum Forces.

of the luminous force. The maximum is at B in the ycllow space, and from this point the intensity of the light rapidly declines each way; its extent being shown by the space shaded with oblique lines. The curve A, with the vertical lines, represents the position and varying force of the heat; and the curve C, horizontally shaded, exhibits the distribution and unequal energy of the chemical force. The three maxima are widely separated, as if there were some antagonism among them; and it is noticeable that where the light is strongest the chemical force seems quite

neutralized. Different kinds of prisms (180) give somewhat different effects, but do not change their order.' The mode of action of all these radiations is unquestionably the same. Heat-rays, light-rays, and chemical rays, differ from each other only as yellow differs from green, that is, by wave-length and intensity of vibration. They all exhibit the effects of interference and polarization which proves the mode of ray-action to be alike in all.

165. Actinometry.-It has been stated that the chemical rays are measurable in their force, and for this important step of research we are indebted to Dr. J. W. Draper. If hydrogen and chlorine gases be mixed in equal proportions in a glass vessel, and kept in the dark, they will not conbine; but, if exposed to the light, they unite with each other, forming a compound. Upon such a mixture, however, the red rays produce no effect, while the violet rays cause the gases to combine explosively. It is the chemical rays that are here active, and Dr. Draper employed a mixture of these gases to test, by the rate of combination, the varying intensity of the force. Instruments for this purpose have been called actinometers. Roscoe and Bunsen afterward employed papers, made sensitive by silver nitrate, which were blackened in given times to certain shades as standard tests of the varying force of the chemical rays.

166. Variation of Chemical Rays in England.-Observations were made at the Kew Observatory, near London, to determine the changes of chemical activity in the solar rays at different hours of the day, and different seasons of the year. The diagram (Fig. 89) represents the results graphically. The experiments were made from 6 A. M. to 6 P. M. throughout the year 1866. The figures below give the hour of the day, and each curve represents the daily change in intensity for the average of a month-the hori zontal lines marking the scale of effects. The maximum effect occurs at twelve o'clock, and the forenoon rise and

1 See note on this subject in the Appendix.

afternoon decline are very nearly equal. But by comparing the highest and lowest curves it will be seen that the chem

in July as in December.

167. Effects at the Equator. As we go south, though the light increases in brilliancy, the chemical action is impeded or interfered with, so that it is said to take ten or twenty times longer to get a

FIG. 90.

picture under the blaze of the Mexican sun than in New York. Yet the effect seems not due to lack of intensity of the chemical rays, but, perhaps, to some obscure cause of irregular action. Fig. 90 represents the effects obtained at Pará, in North Brazil, situated nearly under the equator. The zigzag lines show the sudden chan

ges of intensity from hour to hour, which were accompanied by heavy showers. The dotted line below represents the chemic alresults at Kew at the same time.

168. Relation to Vegetation.-Of the effects produced