Page:Popular Science Monthly Volume 2.djvu/266

252 must prove highly serviceable to moist, mossy hills, and also to meadows that are not too wet. The north side of a hill is sometimes greatly benefited by plaster, when upon a southern exposure it produces no perceptible effect. It may be used with confidence on pastures and fields which are strong enough, and moist enough, to support deciduous trees. A hill-side, where moss will grow so as to crowd out good grasses, is, usually, promptly benefited by plaster, white-clover quickly following its application.

Facts about Glass.—Common greenish glass is found to change color under the influence of the sun's light, becoming first yellow, then rose-colored, and finally violet. And even the purest white glass is found sometimes to undergo the same changes. Thus M. E. Siegwart, from whose monograph on "Glass Manufacture," we gather these items, found the abductor tube of a chlorohydric gas apparatus deeply tinged with violet at the parts exposed to sunlight. When fused anew, the original color returns to the glass. It is commonly supposed that glass is not corroded by the atmosphere, nor even by strong acids. But this is an error; for Siegwart found, on actual experiment, that the exposed surface of glass combines with the constituents of the air. If glass is suffered to cool very gradually, it undergoes a transformation which is in most cases visible to the eye. At first there appear specks, which soon dot the entire surface. The glass now becomes cloudy; and finally changes into an opaque body like porcelain, and called Réaumur porcelain. This devitrification is the most remarkable phenomenon in the manufacture of glass, and explains several of the faults found in that material.

War and Insanity.—It was supposed that the disasters attending the late war had had the effect of increasing the number of insane persons in France, but statistics, so far from confirming this conclusion, show rather that the number of insane patients was smaller during the year ending July 1, 1871, than during the preceding year. Dr. Lunier, who has studied this subject, shows that in 1869-'70 there were admitted into the asylums 11,165 patients; whereas for the year of the war and the insurrection the number was 10,243. Of these 10,243 patients, 1,322 became insane by reason of the calamities produced by the war. During the first half of the year after the war, 400 patients were admitted who had lost their reason from the same cause. The sum total, therefore, of such cases is between 1,700 and 1,800; and the asylums now contain 3,000 less patients than in 1869.

Parliamentary Ventilation.—After spending immense sums of money, and trying numerous methods, the ventilation of the English houses of Parliament is still exceedingly imperfect. The system now in operation is one of exhaustion, or in other words one of suction, the air within the building being sucked out, and, to supply its place, the surrounding external air is sucked in, no matter how impure or how unfit for breathing purposes it may be. This plan is condemned by a writer in the Journal of the Society of Arts on the following grounds: Exhaustion creates a partial tendency to a vacuum, when, to maintain the atmospheric equilibrium, the surrounding air rushes in from every quarter. Impure sources are thus as likely to be drawn upon as any other, and the air introduced is scarcely an improvement on that withdrawn. The tendency to a vacuum also favors the occurrence of draughts. Every chink about a door or window, and every crack in the wood-work, becomes an opening for the admission of cold damp air in the shape of a sensible current. A system of suction also perceptibly affects the acoustic properties of rooms to which it is applied. The reason for this is that, whenever there is a partial vacuum, in the same ratio as that has been reached, has the power of the air to transmit pulsations of sound been impaired.

Preparations for observing the Transit of Venus.—The French are making active preparations for observing the approaching transit of Venus, the Assembly having voted $20,000 for the construction of instruments, with the promise of $40,000 more during the coming year. Nine