140 POPULAR SCIENCE MONTHLY. 



must, for its application, require data impossible to obtain. We shall, 

 therefore, confine ourselves to a brief outline of the main points of the 

 subject. A fundamental proposition of the whole theory is Lane's 

 law of gaseous attraction, which is as follows: 



When a spherical mass of incandescent gas contracts through the loss 

 of its heat by radiation into space, its temperature continually becomes 

 higher as long as the gaseous condition is retained. 



The demonstration of this law is simple enough to be understood by 

 any one well acquainted with elementary mechanics and physics, and it 

 will also furnish the basis for our consideration of the subject. 



We begin by some considerations on the condition of a mass of gas 

 held together by the mutual attraction of its parts. This attraction 

 results in a certain hydrostatic pressure, capable of being expressed as 

 so many pounds or tons per unit of surface, say a square inch. This 

 pressure at any point is equal to the weight of a column of the gas, 

 having a section of one square inch and extending from the point in 



Fig. 2. 



question to the surface. It is a law of attraction in a sphere of which 

 the density is the same at equal distances from its center, that if we 

 suppose an interior sphere concentric with the body, the attraction of 

 all the matter outside that interior sphere, on any point within it, is 

 equal in every direction, and, therefore, is completely neutralized. A 

 point is, therefore, drawn towards the center only by the attraction of 

 the sphere on the surface of which it lies. 



At every point in the interior the hydrostatic pressure must be bal- 

 anced by the elastic force of the gas. In the case of any one gas this 

 force is proportional to the product of the density into the absolute 

 temperature. This condition of equilibrium must be satisfied at every 

 point throughout the mass. 



Let the two circles in the figure represent gaseous globes, of the 

 kind supposed. The larger one represents the globe in a certain con- 

 dition of its evolution; the second its condition after its volume has 

 contracted to one half. The temperature in each case will necessarily 



