THE MILKY WAY. 259 



stituted, really the image of a gas properly so called ? 

 We know that Crookes introduced the notion of a 

 fourth state of matter, in which gases, becoming too 

 rarefied, are no longer true gases, but become what he 

 calls radiant matter. In view of the slightness of its 

 density, is the Milky Way the image of gaseous or of 

 radiant matter? It is the consideration of what is 

 called \\\& free path of the molecules that will supply 

 the answer. 



A gaseous molecule's trajectory may be regarded 

 as composed of rectilineal segments connected by 

 very small arcs corresponding with the successive 

 collisions. The length of each of these segments is 

 what is called the free path. This length is obviously 

 not the same for all the segments and for all the 

 molecules ; but we may take an average, and this is 

 called the ineatt free path, and its length is in inverse 

 proportion to the density of the gas. Matter will be 

 radiant when the mean path is greater than the 

 dimensions of the vessel in which it is enclosed, so 

 that a molecule is likely to traverse the whole vessel 

 in which the gas is enclosed, without experiencing a 

 collision, and it remains gaseous when the contrary 

 is true. It follows that the same fluid may be radiant 

 in a small vessel and gaseous in a large one, and this 

 is perhaps the reason why, in the case of Crookes' 

 tubes, a more perfect vacuum is required for a larger 

 tube. 



What, then, is the case of the Milky Way? It is 

 a mass of gas of very low density, but of very great 

 dimcnsion.s. Is it likely that a star will traverse it 

 withf)Ut meeting with any collision — that is to say, 

 without passing near enough to another star to be 



