ORIGIN OF THE SOLAR SYSTEM JEANS 145 



years,^ and the measures on the various other nebulae show sub- 

 stantially similar results; you will see that there is little room for 

 doubting that the arms consist of matter flowing out of the nucleus. 

 On measuring the actual velocities of flow it is found that in nebula 

 M. 51 (pi. 3, fig. 1) a particle of the jet makes a complete 

 revolution around the nucleus in about 45,000 years; in M. 81 (pi. 

 4, fig. 1) the corresponding figure is about 58,000 years, and in M. 

 101 (pi. 3, fig. 2) about 85,000 years. From these; figures it is 

 possible to estimate the density of the matter in the nucleus. It is 

 found that the densities must be of the order of 10^^® gm. per cubic 

 centimeter, a figure representing a vacuum more perfect than any 

 obtainable in the laboratory. The small amount of gas in an ordi- 

 nary electric light bulb, if spread out through St. Paul's Cathedral, 

 would still be something like 10,000 times as dense as the nucleus 

 of a spiral nebula. 



The nebula shown in Plate 2, Figure 2 exhibits a lumpy or granu- 

 lated appearance in its arms. In M. 51 (pi. 3, fig. 1) this takes the 

 form of pronounced condensations, and in the outer regions of M. 

 101 (pi. 3, fig. 2) and M. 81 (pi. 4, fig. 1) these condensations have 

 further developed into detached and almost star-like points of light. 



When gas is set free out of an ordinary nozzle into a vacuum it 

 immediately spreads into the whole of the space accessible to it. 

 Wliy then does not the jet of gas shot off from the equator of the 

 nebula do the same? The explanation is to be found in the gigantic 

 scale on which this latter process takes place. As we increase the 

 scale of the phenomenon the mutual gravitational attraction of the 

 particles of gas becomes of ever greater importance until finally, by 

 the time nebular dimensions are reached, gravitation overcomes the 

 expansive influence of gas pressure and is able to hold the jet to- 

 gether as a compact stream. But, as soon as this happens, dynamical 

 theory predicts that a further phenomenon ought to appear. As 

 regards the distribution of density along the filament, the influence 

 of gas-pressure is in the direction of keeping the density spread out 

 uniformly, while that of gravitation is toward making the stream 

 condense with compact globules. Wlien nebular dimensions are 

 reached the latter tendency prevails, and the issuing jet of gas breaks 

 up into drops much as a jet of water issuing from a nozzle does, 

 although for a very different physical reason. In the photographs 

 reproduced in Plate 2, Figure 2; Plate 3, Figures 1 and 2; and 

 Plate 4, Figure 1, we can trace this process going on. 



Dynamical theory not only predicts that these globules of gas 

 must form, but also enables us to calculate their size, mass, and 



- The points surrounded by small circles are stars which are believed to have no physi- 

 cal connection with the nebula. 



