Royal Observatory of Paris. It brings a very striking accession of evidence to the undulatory theory.

It has been long known to chemists, that the chloride of silver becomes instantly black in the sunbeam, but retains its snow white colour for any length of time when kept in a dark place. On causing to fall on newly-prepared chloride of silver, the fringes produced by the interference of two beams reflected from the above described slightly inclined mirrors, they were observed to trace on it equidistant black lines, separated by white intervals.

This proves, that the chemical influence of the luminous rays is modified by their interference, like their optical properties, and that it varies in intensity according to the differences of the paths percurred. When this difference is equal to an entire number of undulations, the two systems of waves are in perfect accordance, and their vibrations have the greatest energy possible. In this case, their chemical effects ought to reach their maximum ; and on the other hand, in the points where the difference of the paths percurred is an odd number of semi-undulations, the discordance being complete, the chemical effects ought to be null, like the sensation of light which the same points produce on the eye. These interesting positions have been confirmed by experiment. M. Arago further proved, that the unequal action of the light at the different points of the space where the two beams are united, depends on their mutual influence, for on withdrawing one of the beams, the chloride of silver assumes an uniform dark tint in the very same

M. ARAGO'S BEAUTIFUL EXPERIMENT.

49

space, in which lines alternately black and white were formed, when the two sunbeams arrived there simultaneously. In the same way it was demonstrated, that in the points corresponding to the differences of paths percurred equal to an odd number of semi-undulations, the chemical action of light is insensible when the two reflected beams arrive there together, whilst it re-appears when one of the beams is intercepted. This fact, independently of all theory, overturns the hypothesis adopted by several philosophers, according to which the chemical effects of light result from the combination of its matter with bodies; for if it were so, the effect produced would be greater the more considerable the quantity of luminous particles, and in no case assuredly could the chemical effect of light be increased by withdrawing a portion of the incident rays.

When the two beams are reflected by two mirrors, forming a sensible angle with one another, it happens that the same rays which lose in one point their luminous and chemical properties, by their complete discordance with the rays which they there encounter, find themselves a little farther on in dif ferent circumstances, and recover these properties. This shows, as M. Arago justly remarks, that they did not mutually destroy each other, but were merely neutralised for the moment, in the spot where their movements towards opposite quarters counterbalanced their vibrations. This play of interferences will be understood by inspecting the previous figure, p. 44.

H

As

The facts now detailed are amply sufficient to prove, that not only mere space, but that even the dense forms of matter are pervaded by a luminiferous medium, by whose undulatory movements the phenomena of light are produced. To the creation of this marvellous essence, the Divine mandate, Let there be Light, seems to refer. Its preexistence was necessary to the luciferous functions of the sun, and the other foci of vibration. we know that its undulations may be excited by many causes independent of the sun, we can find no difficulty in conceiving that alternations of light and darkness, constituting the evening and the morning of the first three days of creation, might have taken place. A far more vivid excitation of the luminiferous ether no doubt commenced when the solar globes were invested on the fourth day with their phosphoric atmospheres, to which most gratuitously a state of igneous combustion has been ascribed. This is a process of waste and change, unlike the frugal economy observed in the domains of Nature. What brilliant radiations may be produced by transmitting the influence of a voltaic battery through a bit of charcoal, placed in vacuo, yet the carbonaceous matter is not consumed! This light vies with the sun, but is certainly not borrowed from his beams. How, therefore, should purblind sciolists dare to cavil at the Hebrew prophet for recording in the sublimest language, that light, the first-born offspring of heaven, enlivened the wilderness of space before certain ponderous and inert spheroids were ordained

DECREASING DENSITY WITH INCREASING HEIGHTS. 51

to modify its operations! As justly might they assert that the electric power, whether substance or quality, did not exist till philosophy mounted its cylinder, to excite luminous phenomena.

CHAP. III.-THE ATMOSPHERE.

THE rare elastic medium which envelopes the earth, extending to an unknown, but not unlimited distance from its surface, is called the Atmosphere. As it exercises no observable action on the rays of light, at a greater elevation above the earth, than 45 miles, this height has been regarded practically as the atmospheric boundary. Experiment joined to calculation shows, that the density of the air diminishes in a geometrical progression; such as 2, 4, 8, 16, &c. while the distance from the earth increases in an arithmetical progression, such as 1, 2, 3, 4, &c. Thus it becomes known that the atmosphere, at an altitude of 7 miles, is 4 times rarer than at the surface of the earth, or contains only one-fourth of ponderable matter in the same bulk; and that it quadruples its rarity at every additional recedure of 7 miles; being 16 times rarer at 14 miles, 64 at 21, and so forth. Hence a cubic inch of air, such as we respire, would expand at the height of 45 miles into 9000 cubic inches; an attenuation 8 times greater than the best air pumps can produce.

The preceding computations are founded on the property which air, and other gaseous matter possesses, of accommodating its density to the incumbent pressure, and conversely of expanding its

volume by an innate elastic force, in proportion as the pressure is lightened. It is therefore the mutual resilience of the aerial particles themselves which causes them to escape from the receiver of an air pump, as we relieve them from atmospheric pressure by raising the piston; and not any active sucking force, which, by this instrument, we can exercise.

This curious constitution, scarcely recognisable in water and other liquids, fits the air for its numerous atmospheric functions. It becomes thus a springy yielding medium, though which the feeblest insect and largest bird may wing their way, with well adjusted buoyancy and speed; in which every form of pulmonary organ may find its appropriate exercise. As the elastic vehicle of sound, it is the medium of speech and music; thus enabling innumerable orders of animal beings to maintain the social intercourse prompted by their mutual wants and inclinations.

The dogmatists of Greece, and their scholastic followers, pronounced air, water, earth, and fire, to be the four elements, out of which every form of material being is composed. Experimental research has dissipated these illusions. Fire alone retains with the chemist, any pretensions to elementary rank, and even these are untenable. Air is by no means simple in constitution, but complex, in subserviency to its manifold functions. It consists, in the first place, of two main constituent principles, azote and oxygen, which are blended in the proportion of 79 measures of the first, and 21 of the