THE BIRTH OP THB SUN AND PLANETS 17 



leave open the question whether it passed through a 

 meteoritic phase. The broad principle is that, under the 

 influence of gravitation, the infinitely loose and scattered 

 particles have been brought together into a liquid and 

 then a solid condition. Watch the schoolboy taking up 

 the loosely-knit snow and forcing it into a hard ball. 

 So vast hands have closed round the flocculent matter 

 of the nebula, and squeezed it into balls. We do not 

 know precisely what gravitation is. Certainly it is not 

 an attraction of one particle by another, as even Sir 

 Isaac Newton pointed out. Most likely it is the pressure 

 of the environing ether on every side of the great nebula, 

 forcing the particles together. 



The more difficult question is how the condensing 

 mass came to leave behind the smaller masses, which 

 formed the planets, before it shrank into the sun. 

 Laplace's theory was that as the mass drew inwards and 

 at the same time revolved on its axis, there repeatedly 

 came a time when the fringe of matter at the outside 

 felt an equal pull towards the centre (from gravitation) 

 and from the centre (by the centrifugal force of the 

 revolution). So, time after time, a broad ring of matter 

 at the edge of the disc was detached. This matter 

 would settle round any thicker spot in the irregularity of 

 its texture. The ring would be slowly gathered into a 

 ball, and would be bound to continue the original revolu- 

 tion round the centre of the system. Thus, one by one, 

 beginning with the outermost, the planets were formed. 

 As they in turn were small nebulous masses, they would 

 cast off rings as they condensed, and so form moons or 

 satellites. 



There are astronomers who still think this system 

 tenable, with some modifications. The majority, how- 

 ever, find a grave difficulty in the retrograde motion of 

 some of the bodies in the system, and do not think the 



B 



