60 NATIONAL ACADEMY OF SCIENCES 



per saltum, with that same right, I think, we conclude that the 

 order of evolution must be helium, first, second, third, or the 

 exact reverse. 



Now, then, there is much evidence that it is not the reverse 

 order; but I think it is better not to lose time in going into that 

 point just now; and, therefore, I will say that we have strong 

 confirmation here of what, on two or three different grounds, 

 is pretty generally considered as the order of the different ages 

 in a star's life. 



But to proceed: As the younger the stars, the smaller their 

 internal motion, it follows at once that if we go to a stage younger 

 still, if we go back to that matter from which the helium stars 

 have been evolved (and I call that matter primordial matter), 

 that internal motion of primordial matter must be smaller still. 



And, according to our second fact, the richness of the second 

 stream is smaller and smaller, the younger the class of stars we 

 consider. 



The number of the helium stars is already so great that, not 

 long ago, it was assumed that there was no trace of a second 

 stream in the helium stars at all. We conclude, that in this 

 still younger stage of evolution it is practically certain that 

 there can not be a second stream at all. 



Therefore, finally, we must expect that the particles of 

 primordial matter will all move in practically parallel lines, in 

 the direction and with the velocity of that stream in which all 

 but the totality of the helium stars move. 



It is a very general notion that that primordial matter, that 

 matter from which the stars are evolved, is the matter of the 

 nebulae. What precedes gives us the means of testing this theory 

 by observation. 



What, then, does observation show about the nebulae? The 

 number of available data is as yet extremely small. The deter- 

 mination of what we call astronomical proper motion of these 

 very ill-defined nebulae is extremely difficult, and has been up to 

 the present time invariably unsuccessful. We can say that we 

 know nothing about the astronomical motion of the nebulae. To 



