256 SCIENCE AND METHOD. 



resources. We know, indeed, the motions of the stars 

 nearest to us, and we can form an idea of the amount 

 and direction of their velocities. If the ideas ex- 

 pounded above are correct, these velocities must follow 

 Maxwell's law, and their mean value will teach us, so 

 to speak, what corresponds with the temperature of 

 our fictitious gas. But this temperature itself depends 

 upon the dimensions of our gaseous bubble. How, in 

 fact, will a gaseous mass, left undisturbed in space, 

 behave, if its elements are attracted in accordance 

 with Newton's law ? It will assume a spherical shape ; 

 further, in consequence of gravitation, the density will 

 be greater at the centre, and the pressure will also 

 increase from the surface to the centre on account of 

 the weight of the exterior parts attracted towards the 

 centre ; lastly, the temperature will increase towards 

 the centre, the temperature and the pressure being 

 connected by what is called the adiabatic law, as is 

 the case in the successive layers of our atmosphere. 

 At the surface itself the pressure will be nil, and the 

 same will be true of the absolute temperature, that is 

 to say, of the velocity of the molecules. 



Here a question presents itself I have spoken of 

 the adiabatic law, but this law is not the same for all 

 gases, since it depends upon the proportion of their 

 two specific heats. For air and similar gases this pro- 

 portion is 1.41 ; but is it to air that the Milky Way 

 should be compared ? Evidently not. It should be 

 regarded as a monatomic gas, such as mercury vapour, 

 argon, or helium — that is to say, the proportion of the 

 specific heats should be taken as equal to 1.66. And, 

 indeed, one of our molecules would be, for instance, the 

 Solar System ; but the planets are very unimportant 



