

THE SIZE OF A TOMS. 2 1 3 



the same as phosphorescence ; and we may expect 

 that substances will be found continuously bridging 

 over the difference of quality between this uranium 

 glass, which glows only for a few thousandths of a 

 second, and the luminous sulphides which glow for 

 hours or days or weeks after the cessation of the 

 exciting light. 



The most decisive and discriminating method of 

 estimating the size of atoms I have left until my 

 allotted hour is gone that founded on the kinetic 

 theory of gases. Here is a diagram (Fig. 43) of a 

 crowd of atoms or molecules showing, on a scale 

 of 1,000,000 to i, all the molecules of air, of which 

 the centres may at any instant be in the space of 

 a square of i/io,oooth of a centimetre side and 

 i/ioo,ooo,oooth of a centimetre thick. The side 

 of the square you see in the diagram is a metre, 

 and represents i/io,oooth of a centimetre. The 

 diagram shows just 100 molecules, being i/io,oooth 

 of the whole number of particles (io 6 ) in the cube 

 of i/io,oooth centimetre, or all the molecules in a 

 slice of i/io,oooth of the thickness of that cube. 

 Think of a cube rilled with particles, like these 



