the laws of chemical change. Still the tendency to resolve matter into force continues with many, who of course abandon the conception of atoms, and the theory that implies them.
Dr. Tillman shows that the facts of invariable weight which experiment establishes, of equal or multiple gaseous volumes, of specific capacities for heat, of the equilibrium of chemical forces, of isomeric transformations, and of uniformity, homogeneity, and constancy of structure, in the constitution of material substances, are all explicable only by the conception of indivisible and indestructible atoms.
WHAT BECOMES OF COMETS!
Prof. G. B. Donati, whose name is already famous in connection with comets, has just issued a paper on this subject from the Royal Observatory of Florence, which appeared 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 periodical comets, that of Biela, which completes its circuit round the sun in six years and nine months?, 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 question at very different periods. Should Biela's comet arrive, it would traverse the terrestrial 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. Donati holds it to be possible, but infinitely 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 seen since 1852. From 1826, the time of its discovery, to 1852, it appeared regularly. In 1846 the comet presented a most extraordinary appearance. Instead of appearing single, as was the case on all preceding occasions, it appeared double—that is, composed of two parts, separated by a distance of more than 100,000 miles. When it appeared 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 they may be dissipated, and said that as the silk-worm consumes itself while spinning its cocoon, so comets may consume themselves, 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 November 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 demonstrated 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
