The comparatively thin layer of gas which now lies in the equatorial plane is similar in one respect at least to Newton’s matter “evenly disposed throughout an infinite space.” Disturbances can be set up in it in a variety of ways, and any disturbance, no matter how slight, must result in the creation of a series of condensations. As before, those below a certain limit of size disappear of themselves, while those above this limit continually increase in intensity until they have absorbed all the gas in the equatorial plane. Again, as with the hypothetical primaeval chaos, we can calculate the minimum size of condensation which can be expected to have a permanent existence, and once again the result proves to be highly significant. Hubble’s estimates of the total weights of two conspicuous nebulae have already been given. As the distances, and therefore also the sizes, of both these nebulae are known, it is an easy matter to calculate the average density of the gas throughout the whole nebula. The average density in _M_ 31 is found to be about 5 × 10⁻²² of that of water; the corresponding number for N.G.C. 4594 is 2 × 10⁻²¹. These figures give us some idea of the density of matter in the outer regions of the nebulae. Although these densities are about a thousand million times as great as the estimated density of the original primaeval nebula of space, they are still almost inconceivably low. There is still only about one molecule to the cubic inch, and a single breath from the lungs of a fly could fill a large cathedral with air of this density. On proceeding to calculate the weights of the smallest condensations which could form and persist in a gas of this low density, we obtain the results shewn in the following table. The molecular velocities are taken rather low, so as to allow for the cooling which must occur when the gas is spread out in the equatorial plane of the nebula. Again the weights of the condensations are given in terms of the weight of the sun. And the significant fact emerges that most of the entries in the table represent weights comparable with that of the sun. We are dealing with stellar weights at last; the condensations which must form in the outer regions of the great nebulae will have weights comparable with those of the stars. +--------------+----------------+----------------+----------------+ | Density in | Mol. vel. of | Mol. vel. of | Mol. vel. of | |terms of water|100 yards a sec.|300 yards a sec.|500 yards a sec.| +--------------+----------------+----------------+----------------+ | 10⁻²¹ | 1·7 | 36 | 220 | | 10⁻²² | 5 | 130 | 625 | | 10⁻²³ | 17 | 360 | 2200 | +--------------+----------------+----------------+----------------+ THE BIRTH OF STARS