3. On the 7th of September, 1871, Professor Young, of Dartmouth College,[29] witnessed an extraordinary explosion on the sun's surface. The observer, with his telescope, followed the expelled matter to an elevation of over 200,000 miles. The mean velocity between the altitudes of 100,000 and 200,000 miles was 166 miles per second. This rate of motion _in vacuo_ would indicate an initial velocity of about 260 miles per second. But the sun is surrounded by an extensive atmosphere, whose resistance must have greatly retarded the velocity of the outrush before reaching the height of 100,000 miles. The original velocity of these hydrogen clouds was therefore sufficient, in all probability, to have carried them, if unresisted, beyond the solar domain. Solid or dense matter propelled with equal force would doubtless have been driven off never to return.[30] [29] Boston Journal of Chemistry, November, 1871. [30] See Mr. Proctor's interesting discussion of this subject in the Monthly Notices of the R.A.S., vol. xxxii. 4. This eruptive force, whatever be its nature, is probably common to the sun and the so-called fixed stars. If so, the dispersed fragments of ejected matter ought to be found in the spaces intervening between sidereal systems. Accordingly, the phenomena of comets and meteors have demonstrated the existence of such matter, widely diffused, in the portions of space through which the solar system is moving. 5. According to Mr. Sorby the microscopic structure of the aerolites he has examined points evidently to the fact that they have been at one time in a state of fusion from intense heat,--a fact in striking harmony with this theory of their origin. 6. The velocity with which some meteoric bodies have entered the atmosphere has been greater than that which would have been acquired by simply falling toward the sun from any distance, however great. On the theory of their sidereal origin, this excess of velocity has been dependent on the primitive force of expulsion. The shower of aerolites which fell at Pultusk, Poland, on the 30th of January, 1868,[31] is not only a remarkable illustration of the fact here stated, but also of another which may be accounted for by the same theory, viz.: that meteoric bodies sometimes enter the solar system in groups or clusters. [31] See Chapter VII.