THE MILKY WAY. 255 



that their trajectories are rectilineal ; nevertheless, from 

 time to time, two of them may come near enough 

 together to be deviated from their course, like a comet 

 that passed too close to Jupiter. In a word, in the eyes 

 of a giant, to whom our Suns were what our atoms 

 are to us, the Milky Way would only look like a 

 bubble of gas. 



Such was Lord Kelvin's leading idea. What can 

 we draw from this comparison, and to what extent is 

 it accurate? This is what we are going to enquire 

 into together ; but before arriving at a definite con- 

 clusion, and without wishing to prejudice the question, 

 we anticipate that the kinetic theory of gases will be, 

 for the astronomer, a model which must not be 

 followed blindly, but may afford him useful inspira- 

 tion. So far celestial mechanics has attacked only 

 the Solar System, or a few systems of double stars. 

 It retired before the aggregations presented by the 

 Milky Way, or clusters of stars, or resoluble nebulae, 

 because it saw in them only chaos. But the Milky 

 Way is no more complicated than a gas ; the statistical 

 methods based upon the calculation of probabilities 

 applicable to the one are also applicable to the other. 

 Above all, it is important to realize the resemblance 

 and also the difference between the two cases. 



Lord Kelvin attempted to determine by this means 

 the dimensions of the Milky Way. For this purpose 

 we are reduced to counting the stars visible in our 

 telescopes, but we cannot be sure that, behind the 

 stars we see, there are not others which we do not 

 see ; so that what we should measure in this manner 

 would not be the size of the Milky Way, but the scope 

 of our instruments. The new theory will offer us other 



