f 



1908,1 THE PHYSICS OF THE EARTH. 177 



To obtain any adequate conception of their size we must betake ourselves 

 to a scheme of threefold magnification. Lord Kelvin has shown that if a 

 drop of water were magnified to the size of the earth the molecules of water 

 would be of a size intermediate between that of a cricket ball and of a marble. 

 Now each molecule contains three atoms, two being of hydrogen and one of 

 oxygen. The molecular system probably presents some sort of analogy with 

 that of a triple star; the three atoms replacing the stars, revolving about one 

 another in some sort of a dance which cannot be exactly described. I doubt 

 whether it is possible to say how large a part of the space occupied by the 

 whole molecule is occupied by the atoms ; but perhaps the atoms bear to the 

 molecule some such relationship as the molecule to the drop of water re- 

 ferred to. Finally, the corpuscles may stand to the atom in a similar scale 

 of magnitude. Accordingly, a threefold magnification would be needed to 

 bring these ultimate parts of the atom within range of our ordinary scales 

 of measurement. . . . 



" The community of atoms in water has been compared with a triple 

 star, but there are others known to the chemists in which the atoms are to 

 be counted by fifties and hundreds, so that they resemble constellations." 



Such general discussions by these iFlustrious physicists, Kelvin,^ 

 Poincare and Darwin, are not to be construed too Hterally, and yet 

 they clearly indicate the general belief among the foremost men 

 of science that the spaces between the particles of matter are im- 

 mense in comparison with the dimensions of the particles themselves. 



From Lx>rd Kelvin's discussion of the size of atoms treated 

 in the above section, we have seen that the diameters of these bodies 

 is of the order of 1/500,000,000 of a centimeter, or 1/1,270,000,000 

 of an inch. The average space between the molecules being 

 1/100,000 of a centimeter, or about 5,000 times the diameter, is of 

 the order of 1/254,000 of an inch. This is decidedly below the 



^ In a well-known paper on gravitating matter, Lord Kelvin compares 

 the stars of the Milky Way to the atoms of a bubble of gas. For a giant for 

 whom our suns would be what atoms are to us, the stars would be beyond 

 the reach of the keenest vision and the Milky Way appear to behave as a 

 gaseous medium. M. Poincare has discussed the problems of the universe 

 from this point of view in an address to the Astronomical Society of France 

 {Bulletin Astronotniquc dc la Socicte Astronomique de France, April, 1906; 

 an excellent translation in Popular Astronomy for October, 1906). It is 

 remarkable that Democritus, founder of the atomic theory among the 

 Greeks (460-360 B. C), should also have recognized that the Milky Way is 

 composed of a mass of stars too dense to be seen separately by the unaided 

 vision (cf. "Aristotle's Meteorology," Lib. I., Ch. VIII., Sec. 4). Thus Lord 

 Kelvin's conceptions do not differ greatly from those of Democritus of 

 Abdera, though the modern theories are much better established than the 

 atomic theories were among the Greeks. 



