igS CARNEGIE INSTITUTION OF WASHINGTON. 



whole of known matter be theoretically dispersed through the space 

 now occupied by it. The light of a star in a flight of fifty years 

 does not encounter enough dark matter to seriously dim its bright- 

 ness. All the matter that lies between us and the uttermost visi- 

 ble stars does not cut off as much light as a thin cloud. If all the 

 matter now aggregated in the stellar system, on any reasonable esti- 

 mate of its mass (and the known distribution and movements of the 

 celestial bodies limit such an estimate), were distributed through the 

 space now occupied by the stars, it would not help the case much, 

 so far as the meteoritic assemblage is concerned. To illustrate, if 

 the matter of the solar system were scattered through that portion 

 of space which may be said to be its fair apportionment — that is, the 

 space about it, stretching out half-way to the nearest stars — its tenuity 

 would be such that if the orbit of Neptune were to be regarded as 

 the hoop of a drag-net 5,600,000,000 miles in diameter, and were to 

 be made to sweep through this space at the rate of 12 miles per sec- 

 ond—the estimated velocity of the sun— it would take some 900,000,- 

 000,000 5'ears for it to sweep up the scattered matter. This is prob- 

 ably not an unfair illustration of the average tenuity of the sup- 

 posed dispersion, since the sun is apparently near the center of the 

 known system where star-grouping might be expected to be at least 

 as dense as the average of the whole. 



With such extreme tenuity of dispersion, even when all known 

 matter is converted into meteorites, and with such potent obstacles 

 to assemblage as are imposed by the high moving force of the 

 meteorites, it seems an imperative conclusion that the growth of a 

 meteoritic assemblage of the mass of the solar system must require 

 a period quite beyond comprehension. 



This conclusion led on the further inquiry whether a swarm of 

 meteorites could perpetuate itself as a swarm, through such a pro- 

 digious period. Must not the part first assembled pass on through 

 its own evolution, whatever that might be, without awaiting the 

 excessively delayed assemblage of the later portions ? If the mem- 

 bers of the swarm were in collisional relations, must not the kinetic 

 energy of the earlier assemblage have been exhausted long before 

 the accession of the latter part ? In other words, must not the first 

 assemblage have become solid at a relatively early stage in the 

 process and the remainder of the accessions have been added individ- 

 ually, as meteorites are now added to the sun and planets? Is it a 

 tenable view that the assemblage of a swarm should go on alone 

 without attendant evolution until the mass necessary for a solar 



