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SoHnd and Light--Nhen the medium CT substance through which sound or light passes IS homogeneous, or of the same density, and IS uninterrupted by cracks or openings, the light and the sound will be transmitted with the least loss, and with the greatest distinct ness; but if the medium has different densi- ties, or consists of different bodies imperfect ly mixed, or is interrupted by empty spaces, the light and sound will be either greatly di- unfinished or entirely destroyed. When we add syrup to water, or brandy to water and look through the glass at candle before they have combined, the candle will appear like a cloud, as if we had viewed it through piece of ground glass. When the light passes from portion of the water to the brandy, or from the brandy to the water, a part of it suffers reflection, and, as the separating surface Can seldom be perpendicular to the direction of the ray, part of the light will also suffer re- fraction Now, as this must take place many hundred times while the light is passing thro a large glass of these imperfectly blendid li- quids, it is not difficult to understand how we are unable to sec objects distinctly through the mixture. The very same effect is pro duced if we transmit light through piece of glass full of cracks. QuQr4eriv Repiegv- HerscheH's Treatise on 8ound.

 M. Chladni has Illustrated the effect of mixed medium by an elegant experiment of easy repetition. If we pour sparkling cham pagne into tall glass, till it is half full, the glass cannot be made to ring by a stroke upon its edge, but admits dull, dsagreeable, and puffy sound. This effect continues as long as the ehervescence lasts, and while the wine is filled with air bubbles. But as the efferve- scence subsidies, the sound becomes clearer and clearer, till, at last, the glass rings as usual, when the air bubbles have disappear ed But reproducing the effervescence, the sound is deadened as before. The s..-.e =. pertinent may be made with effervescing malt liquors, and with still more effect by p.ttng piece of sponge, or little wool or tow into a tumbler of water. The cause of the result obtained by M. Chladni is, that the glass and the contained liquid, in order to give musi- cal tone, must vibrate regularly in unison as q system: and if any considerable part of system is unsusceptible of regular vibration, the whole must be so. Baron Humboldt has employed this interesting experiment to illus trate and explain the well known phenome- non of distant sound being more distinctly heard during the night than during the day. This fact, which had been observed by the anccnts, and in large cities, or their vicinity was commonly ascribed to the repose of ani- mated beings. When Humboldt first heard the noise of the great Cataracts of the Ori- noco, in the plain which surrounds the Mis- sion of the Apures his attention was particu- larly directed to this curious fact, and he was of opinion that the noise was three times lou der at night than during the day. As the humming of insects was much greater at night than during the day, and as the breeze which might have agitated the leaves of the trees never rose till after sunset, this eminent tra- weller was compelled to seek for another cause of fthe phenomenon. In not day, when warm currents ofair ascends from the heated ground, and mixed with the cold air above of differ ent destiny, the transparency of the air is so much affected, that every object seen through it appears to be in motion, just as when we looked at any object over fire or the flame of a candle. The air is, therefore, during the day, mixed medium, in which the sounds gre reflected and scattered in passing through streams of air of different densites, as in the experiment of mixing atmospheric air and and hydrogen. At midnight, on the contrary, when the air is transparent and of uniform density, as may be seen by the briliancy and number of stars, the slightest sound reaches the ear without interruption.

 7ne Valley of La Plata.-The great val

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