November 21, 1913] 



SCIENCE 



721 



exception is the well-known Orion nebula, 

 which is classified under the irregular 

 nebulse. May not then these irregular neb- 

 ulse give birth to the stars? 



It turns out that this one object has ex- 

 actly the radial velocity of the first stream 

 helium stars; that is, we find exactly the 

 motion we must expect in this nebula, if it 

 were the birthplace of stars. We shall not, 

 of course, on this single fact base far-reach- 

 ing conclusions; but we have a right, in 

 my opinion, to say that here is a fact that 

 singularly strengthens what had already 

 been concluded from other facts. 



We see, moreover, that the observation of 

 the radial velocity of other irregular neb- 

 ulae must, ere long, furnish us with a 

 crucial test of the theory. 



There is another problem involved in our 

 observations which might seem to be of no 

 less importance than the one just men- 

 tioned. How have we to explain the fact 

 that the internal velocity of the stars grad- 

 ually increases with age? The astronomer 

 who, in the study of the motion of the 

 heavenly bodies, has found hardly a trace 

 of any other force than gravitation will 

 naturally turn to gravitation for such an 

 explanation. It really seems a necessity 

 that, under the influence of their mutual 

 gravitation, bodies, which at the outset 

 have little or no relative motion, must get 

 such a motion; they must come to fall 

 toward each other, and this velocity, up to 

 a certain limit at least, must increase with 

 time. 



Thus far, there is no great difficulty. But 

 now let us look farther back in time, back 

 to the time in which the stars had not yet 

 been formed, in which matter was still in 

 its primordial state. If it be true that mu- 

 tual attraction of the stars has generated 

 such an enormous amount of internal mo- 

 tion in the time needed by the stars to 

 develop from the helium type to the second 



and third type, how have we to explain the 

 fact that we find that same matter nearly 

 at rest at the first stage of evolution at 

 which we meet it ? How have we to explain 

 that in pre-helium ages gravitation has 

 produced no effect? 



He who believes in the creation of mat- 

 ter at a finitely remote epoch may find no 

 difiiculty in the question; but to him who 

 does not, it is simply astonishing to see 

 matter behaving as if there were no gravi- 

 tation at all. What may be the explana- 

 tion ? Is there no gravitation in primordial 

 matter, or is there another force exactly 

 counterbalancing its effects? 



1 shall offer no solution. I simply wish 

 to point out that here is a problem which 

 must be interesting to the physicist no less 

 than to the astronomer. 



Passing now to other inferences, I wish to 

 draw your attention to a question already 

 alluded to: does the observed fact of the 

 preference of the star motions for two defi- 

 nite directions lead us with necessity to the 

 assumption that our system has been 

 formed by the meeting of two independent 

 star clouds? Or is it still possible, and in 

 that case more plausible, to explain it with- 

 out sacrificing the unity of the system ? In 

 other words, is our universe a dual system, 

 or is it one unit? 



Suppose- a very elongated system of 

 stars which are originally at rest; now let 

 these be left to their mutual attraction. It 

 is evident that the stars, in opposite parts 

 of the cloud, will begin to fall towards each 

 other. Two streams will be set up, opposite 

 in direction, approximately parallel to the 

 axis of the cloud, though in no wise abso- 

 lutely and exclusively so. In other words, 

 we get two preferential directions of mo- 

 tion. There is no real difficulty in the fact 



2 The following supposition was first considered 

 in a lecture held at Harlem in 1906 ("Programme 

 de la Soc. Holl. des Se. pour 1906," p. liv). 



