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THE POPULAR SCIENCE MONTHLY. 



the laws of chemical change. Still the ten- 

 dency to resolve matter into force continues 

 with many, who of course abandon the con- 

 ception of atoms, and the theory that im- 

 plies them. 



Dr. Tillman shows that the facts of in- 

 variable weight which experiment estab- 

 lishes, of equal or multiple gaseous volumes, 

 of specific capacities for heat, of the equi- 

 librium of chemical forces, of isomeric trans- 

 formations, and of uniformity, homogeneity, 

 and constancy of structure, in the constitu- 

 tion of material substances, are all expli- 

 cable only by the conception of indivisible 

 and indestructible atoms. 



WHAT BECOMES OF COMETS t 



Prof. G. B. Donati, whose name is al- 

 ready famous in connection with comets, has 

 just issued a paper on this subject from the 

 Royal Observatory of Florence, which ap- 

 peared in the Evening Post in full, and of 

 which the following is the substance : He 

 denies that any astronomer has recently 

 seen a large comet, and affirms that none at 

 all are visible at the present time. As for 

 new comets, no astronomer can tell whether 

 they will or will not appear. As to periodi- 

 cal comets, that of Biela, which completes 

 its circuit round the sun in six years and 

 nine month?, is due next August. The 

 earth and the comet of Biela travel different 

 ways, and their paths cross each other at a 

 certain point. A collision is, therefore, not 

 impossible. The comet and the earth have, 

 however, hitherto passed the point in ques- 

 tion at very different periods. Should Biela's 

 comet arrive, it would traverse the terres- 

 trial orbit on August 26th ; but on that day 

 the earth would be distant from the comet 

 almost half as far again as it is distant 

 from the sun. As to whether any other 

 comet may encounter the earth, Prof. Do- 

 nati holds it to be possible, but infinite- 

 y improbable. " The comets," he says, 

 "have masses so small that, if one of them 

 were to approach to within even a short 

 distance from the earth, the latter perhaps 

 would have nothing to fear, and, in all 

 probability, in such an event, the comet 

 would become a satellite of the earth." As 

 for the comet of Biela, he says there is 

 great probability that it no longer exists ; 

 although it should appear every six years 



and nine months, it has not been see* 

 since 1852. From 1826, the time of its dis- 

 covery, to 1852, it appeared regularly. In 

 1846 the comet presented a most extraor- 

 dinary appearance. Instead of appearing 

 single, as was the case on all preceding oc- 

 casions, it appeared double that is, com- 

 posed of two parts, separated by a distance 

 of more than 100,000 miles. When it ap- 

 peared in 1852, the separation was still 

 more complete, the interval amounting to 

 1,200,000 miles. Its non-appearance in 1858 

 was attributed by some to its immersion in 

 the solar rays ; but in 1866 it should have 

 reappeared in a position so far from the 

 sun, that it would have been visible at 

 night ; but it was impossible to discover it. 

 Nor is the comet of Biela the only one 

 which has failed to appear when due. That 

 discovered by Prof. De Vico at Rome in 

 1814, and which should return every five 

 and a half years, has never since been seen. 

 What becomes of them ? Kepler held that 

 tbey may be dissipated, and said that as 

 the silk-worm consumes itself while spinning 

 its cocoon, so comets may consume them- 

 selves, and die while generating their long, 

 interminable tails ; and Newton thought 

 that they might fall directly into the sun. 

 But Donati conjectures that the material of 

 Biela's comet has already fallen in part, 

 and is still falling upon our planet. Prof. 

 Schiaparelli, who has just got the gold 

 medal of the London Astronomical Society 

 for his study of comets, maintains that they 

 are resolved into shooting-stars meteors 

 which traverse the earth's atmosphere. It 

 is well known that on August 10th and No- 

 vember 13th many such falling stars are 

 seen ; and this is explained by supposing 

 that the earth passes through two great 

 belts or bracelets of meteoric matter, and 

 draws some of the fragments or corpuscles 

 toward itself. Prof. Schiaparelli has de- 

 "monstrated that there are several comets 

 which move round the sun, in the direction 

 of these meteoric bracelets ; and Prof. 

 D'Arest has noticed that every year, on 

 December 5th, shooting-stars are seen that 

 irradiate from that part of the celestial 

 dome in which the comet of Biela would 

 appear, if it came at all. It seems probable 

 that the comet of Biela forms part of a 

 bracelet of cometic corpuscles which move 



