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THE POPULAR SCIENCE MONTHLY. 



plaster 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 sup- 

 port 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 in- 

 fluence 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 mono- 

 graph 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 experi- 

 ment, 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 Reaumur 

 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 num- 

 ber of insane persons in France, but statis- 

 tics, so far from confirming this conclusion, 

 show rather that the number of insane pa- 

 tients 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 af- 

 ter 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 spend- 

 ing immense sums of money, and trying 

 numerous methods, the ventilation of the 

 English houses of Parliament is still exceed- 

 ingly imperfect. The system now in op- 

 eration 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 follow- 

 ing 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 with- 

 drawn. The tendency to a vacuum also fa- 

 vors the occurrence of draughts. Every 

 chink about a door or window, and every 

 crack in the wood-work, becomes an open- 

 ing for the admission of cold damp air in 

 the shape of a sensible current. A sys- 

 tem of suction also perceptibly affects the 

 acoustic properties of rooms to which it is 

 applied. The reason for this is that, when- 

 ever 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 in- 

 struments, with the promise of $40,000 

 more during the coming year. Nine sta- 



