+-----------+-----------+-------------+--------------+--------------+ | Density | Mol. vel. | Mol. vel. of| Mol. vel. of | Mol. vel. of| | in terms | 500 yards | 1000 yards | 2000 yards | 3000 yards | | of water | a sec. | a sec. | a sec. | a sec. | +-----------+-----------+-------------+--------------+--------------+ |10⁻²⁹ | 25,000,000| 200,000,000| 1,500,000,000| 5,000,000,000| |10⁻³⁰ | 62,500,000| 500,000,000| 4,000,000,000|13,000,000,000| |1·5 × 10⁻³¹|160,000,000|1,300,000,000|10,000,000,000|30,000,000,000| +-----------+-----------+-------------+--------------+--------------+ All known stars have weights comparable with that of the sun. Thus if, as Newton conjectured, the stars first came into being as condensations of this kind, then the entries in this table ought to be comparable with unity. Newton’s conjecture, in the form in which we have just considered it, is clearly untenable, since all the calculated weights are many millions of times that of the sun. If there ever existed a primaeval chaos of the kind we are now considering, it would not condense into stars, but into enormously more massive condensations, each having the weight of millions of stars. THE BIRTH OF THE GREAT NEBULAE Now it is significant that bodies are known in space having weights equal to those just calculated, namely the great extra-galactic nebulae. There are two nebulae whose weights can be determined with fair accuracy, namely the Great Nebula in Andromeda (Plate IV, p. 30) and the nebula N.G.C. 4594 in Virgo (Plate XV). Hubble estimates these to be as follows: Nebula _M_ 31: weight = 3500 million times that of sun ” N.G.C. 4594: ” = 2000 ” ” These estimates are again probably both on the low side, but their general order of magnitude is such as to suggest that the condensations which would first be formed out of the primaeval nebula must have been the great extra-galactic nebulae, and not mere stars. It is of course at best only a conjecture that the great nebulae were formed in this manner—if for no other reason because we can never know whether the hypothetical primaeval nebula even existed—but it seems the most reasonable hypothesis we can frame to explain the fact that the present nebulae exist. These nebulae are so generally similar to one another that it seems likely that they must all have been produced by the action of the same agency, and that which we have just considered provides a reasonable explanation which, apart from the postulated existence of the continuous primaeval nebula, is based on _verae causae_. The great nebulae are of course not exactly similar, and our next inquiry must be as to the origin of their differences. [Illustration: PLATE XV The Nebula N.G.C. 4594 in Virgo] [Illustration: _Mt Wilson Observatory_ The Nebula N.G.C. 7217]