404 POPULAR LECTURES AND ADDRESSES. 



present mean density, or "175 of the density of 

 water ; but we cannot, with any probability of 

 argument or speculation, go on continuously 

 much beyond that. We cannot, however, help 

 asking the question, What was the condition of 

 the sun's matter before it came together and 

 became hot ? It may have been two cool solid 

 masses, which collided with the velocity due to 

 their mutual gravitation ; or, but with enormously 

 less of probability, it may have been two masses 

 colliding with velocities considerably greater than 

 the velocities due to mutual gravitation. This last 

 supposition implies that, calling the two bodies 

 A and B for brevity, the motion of the centre of 

 inertia of B relatively to A, must, when the 

 distances between them was great, have been 

 directed with great exactness to pass through 

 the centre of inertia of A ; such great exactness 

 that the rotational momentum, or " moment of 

 momentum," 1 after collision was no more than 



1 This is a technical expression in dynamics which means the 

 importance of motion relatively to revolution or rotation round an 

 axis. Momentum is an expression given about a hundred and fifty 

 years ago (when mathematicians and other learned men spoke and 



