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THE POPULAR SCIENCE MONTHLY. 



ness and softness and the less its porosity 

 and corrodibility ; softness or liability to abra- 

 sion does not indicate inferiority, but is an 

 indication of strength and good weathering 

 qualities. The strongest slate stands high- 

 est in weathering qualities, so that a flexural 

 test affords an excellent index of all its prop- 

 erties, particularly if the ultimate deflection 

 and the manner of rupture be noted. The 

 strongest and best slate has the highest per- 

 centage 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 hun- 

 dred and seventy -three pounds per cubic foot, 

 and the best qualities have a modulus of rup- 

 ture 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 im- 

 posingly celebrated December 2*7th, in the 

 presence of eminent men of science and states- 

 men 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." Ad- 

 dressing 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 "apos- 

 tle's faith" in science. Had he devoted him- 

 self to pure science, the topmost place would 

 have been his. Happily for himself and for 

 mankind, he deserted that path and hence- 

 forth 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. Pas- 

 teur. Numerous testimonials and offerings 

 of different kinds were presented to H. Pas- 

 teur, with a splendid gold medal, the product 

 of an international subscription. 



Origin of the Asteroids. — A paper on 

 Groups of Asteroids, by Prof. Daniel Kirk- 



wood, illustrates the theory that these bodies 

 were formed by the resolution of nebulous as- 

 teroids. When the number of telescopic plan- 

 ets had grown to hundreds, and when the peri- 

 helion 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 doc- 

 trine of similarity of origin, the author holds, 

 was not so easily disposed of. The original 

 dimensions of nebulous asteroids were proba- 

 bly many times greater than those of the pres- 

 ent 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 what- 

 ever. 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 com- 

 mon origin, affords strong presumptive evi- 

 dence in favor of the nebular hypothesis. 

 Possibly, indeed, its true form may have dif- 

 fered from that proposed by Laplace. How 

 many primitive, separate nebulae were con- 

 tained 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, how- 

 ever, undoubtedly be traced. " May not simi- 

 lar 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 offer- 

 ings 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 che 

 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 



