And if the universe amounts to nothing more than this, shall we carry on the quotation: We are such stuff As dreams are made on; and our little life Is rounded with a sleep, —or shall we not? CHAPTER IV _Carving out the Universe_ We have commented on the surprising emptiness of space: six specks of dust in Waterloo Station about represent the extent to which it is occupied by stars in its most crowded parts. The comment might well have taken another form. Six specks of dust contain, let us say, a thousand million million molecules. Our model of space is empty because this great number of molecules happens all to be aggregated into as few as six lumps. In real space the unit of aggregation is the star, and an average star contains about 10⁵⁶ molecules—a number so large that it is quite useless to try to imagine it. The emptiness of space does not originate from any paucity of molecules; it originates from the circumstance that, apart from those which form the tenuous clouds of gas stretching from star to star, the molecules are aggregated together in the huge colonies we call stars, with about 10⁵⁶ members to each. Why should the molecules in space herd together in this way, when the molecules in the rooms in which I am writing and you are reading do not? Following a well-tried scientific method, we may attempt to discover why these aggregates have formed, by first examining what keeps them together now that they have formed. The earth’s atmosphere consists of about 10⁴¹ molecules. Why do they stay pressed down into an atmosphere instead of spreading out through space? The answer is of course provided by the earth’s gravitation. A bullet fired from the earth’s surface with a speed of 6·93 miles a second or more will fly off into space, because the earth’s gravitational pull is inadequate to hold it back when it moves with so high a speed. But a bullet fired with a speed of less than 6·93 miles a second does not leave the earth; its speed is inadequate to take it clear of the earth’s pull. Thus the molecule-bullets which form the earth’s atmosphere, flying with speeds less than a third of a mile a second, have no chance at all of getting away. The earth’s gravitation continually pulls them back to earth, so that the earth retains its covering of air. At rare intervals a molecule may experience a succession of exceptionally lucky collisions with other molecules, and so attain a speed of more than 6·93 miles a second. A molecule which arrives at the outside of the earth’s atmosphere with such a speed will leave the earth altogether, and join the interstellar crowd of stray molecules. The earth is continually shedding its atmosphere in this way, but calculation shews that the loss, even in millions of millions of years, is quite insignificant, so that we may regard the earth’s atmosphere as permanent.