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larger one. Many of these, perhaps most of them, are in all probability 

 matter which left the sun when the rest did and which are now for the first 

 time brought near enough the earth to be permanently annexed. In a 

 reeion where no large masses existed, the matter would continue to revolve in 

 a finely divided state, such as we actually find in the zone of the minor planets. 

 This zone lies between the orbits of Mars and Jupiter. In it have been found 

 some 800 planets large enough to make a record on a photographic plate, and 

 there is little reason to doubt that the whole number is many times greater 

 ai d the size of most of them so small that we can never see them except as 

 they collectively make a faint band of light across the sky. In this zone 

 we find what we should expect with small sizes — that is, very elliptical orbits 

 and very high inclinations. One of these planets has an orbit of such eccen- 

 tricity that while its mean distance is considerably greater than that of Mars, 

 yet in one point in its orbit it comes much closer to the earth than any body, 

 except the moon, and two others have perihelion distances less than that of 

 Mars. 



Thus it is entirely possible that our planetary system resulted from a 

 spiral nebula, and it is entirely possible that spirals may result from close 

 approaches of two stars and we lay even say that it is all probable, at least 

 more probable than any other plan yet proposed. 



There are still some difficulties. We must say that if our system resulted 

 from a spiral, this spiral was not at all on the scale observed among the 

 spirals in the sky. Such a nebula, having a radius equal to that of Nep- 

 tune's orbit, were it no farther away than the nearest star, would be a very 

 insignificant object and might fail of detection entirely. At the probable 

 distance of most of these objects it would certainly be invisible. We can see 

 how a star might be torn apart so as to scatter material over a space the size 

 of Neptune's orbit, but the case is different when we consider some of the 

 large spirals in the sky. The largest is known as the Great Nebula of 

 Andromeda. It covers an arc of over a degree in the sky. Assuming a 

 parallax of 0".l, which is probably larger than the real value, this nebula 

 from end to end must extend over a space more than 1,800 times the size of 

 Neptune's orbit, or 54,000 times the size of the Earth's orbit. 



We have never determined accurately the distance of a single nebula and 

 so do not know the real size of any one of them, compared to the solar system, 

 but there is no reason to suppose they are nearer than many of the faint stars. 

 If this is true, their volumns are vast beyond comprehension and their density 

 an inconceivably small fraction of the density of our best vacuum. It has 



