Page:Popular Science Monthly Volume 43.djvu/150

140 and softness and the less its porosity and corrodibility; softness or liability to abrasion does not indicate inferiority, but is an indication of strength and good weathering qualities. The strongest slate stands highest in weathering qualities, so that a flexural test affords an excellent index of all its properties, particularly if the ultimate deflection and the manner of rupture be noted. The strongest and best slate has the highest percentage of silicates of iron and aluminum, but is not necessarily the lowest in carbonates of lime and magnesia. Chemical analyses give only imperfect conclusions regarding the weathering qualities of slate, and they do not satisfactorily explain the physical properties. The soft roofing slates weigh about one hundred and seventy-three pounds per cubic foot, and the best qualities have a modulus of rupture of from seven thousand to ten thousand pounds per square inch. The test of a slate by balancing it, striking it, and observing its ring is a good one, but is not susceptible of quantitative expression.

Pasteur's Seventieth Birthday.—The seventieth birthday of Louis Pasteur was imposingly celebrated December 27th, in the presence of eminent men of science and statesmen of different countries. The first address was made by the French Minister of Public Instruction, who spoke of the occasion as the "festival of France and of mankind." Addressing M. Pasteur, he said that while his work could be analyzed only by the scientific, the ignorant and the learned alike knew that he had accomplished something great. All his success was due to his unswerving "apostle's faith" in science. Had he devoted himself to pure science, the topmost place would have been his. Happily for himself and for mankind, he deserted that path and henceforth passed his days in inventing antidotes for diseases that had for centuries decimated the animal and human populations. Prof. Joseph Lister acknowledged the obligations of the professors of the healing art to M. Pasteur. Numerous testimonials and offerings of different kinds were presented to M. Pasteur, with a splendid gold medal, the product of an international subscription.

Origin of the Asteroids.—A paper on Groups of Asteroids, by Prof. Daniel Kirkwood, illustrates the theory that these bodies were formed by the resolution of nebulous asteroids. When the number of telescopic planets had grown to hundreds, and when the perihelion distance of some of them had become greater by many millions of miles than the aphelion of others, the theory of explosion was necessarily abandoned. But the doctrine of similarity of origin, the author holds, was not so easily disposed of. The original dimensions of nebulous asteroids were probably many times greater than those of the present bodies. The disrupting tendency of the great bodies of the system, especially when resisted only by the slight central attraction of nebulous asteroids, is easily imagined. Such separation, in short, has no improbability whatever. The dismemberment of comets, as is well known, has actually occurred under our own eyes. Why not also the pulling asunder of nebulous planets? The fact that in many cases the motions of asteroids indicate a common origin, affords strong presumptive evidence in favor of the nebular hypothesis. Possibly, indeed, its true form may have differed from that proposed by Laplace. How many primitive, separate nebulæ were contained in our system, and how many of these primitive masses suffered dismemberment while Mars and the then future earth were yet floating in the solar atmosphere, can not now be told. An indefinite number may, however, undoubtedly be traced. "May not similar processes be also indicated in the slow evolution of binary and multiple stars in the sidereal heavens?"

Early Fans.—The extreme antiquity of fans is attested by their appearance in ancient Egyptian and Assyrian sculptures, where they have the shape of a semicircle with a long handle attached at the center. They were probably used in worship to protect the offerings and sacred objects against contamination by dust and flies. They were known also in India, where they were perhaps introduced from China. The story of their origin in the latter country runs that the daughter of a powerful mandarin was obliged, on account of the heat, to take off her mask during the feast of lanterns, in violation of the law and convention. She shook it rapidly in front of her face, both to give herself air and by the quick motion to veil her identity as fully as