20 ORGANIC EVOLUTION CONSIDERED 



— 273.7° C, if this represents the temperature at 

 which all heat is absent, there is no molecular motion 

 in matter, and that the molecules, if they are cubes of 

 equal size, would fill the whole space so that there 

 would be no pores. The small amount of expansion 

 which solids and liquids undergo, due to a change of 

 temperature, shows that if they could be reduced to 

 the absolute zero, they would occupy nearly as much 

 space as they occupy at 0° C. For example,' a cubic 

 centimetre of steel on being cooled from 0° C to 

 — 273° C would lose about Yioo of its volume if 

 it followed the law of expansion and contraction 

 which has been established for higher temperatures. 

 Glass would lose Y125 and zinc 1 /m of its volume. 



From this it would seem that the molecules of 

 solids at ordinary temperatures are probably very 

 close together, and, for the most part, in contact with 

 each other. 



Lord Kelvin estimates from the kinetic theory of 

 gases that in glass or water "there are probably some- 

 thing like 600 molecules to the wave-length " of violet 

 light, " and almost certainly not fewer than 200, or 300, 

 or 400."* 



Taking 600 as the number of molecules of water in 

 a wave-length of 1 violet light, which is about Y2,5oo 

 of a millimetre in length, the size of the molecule of 

 water would be about Vi5,ooo,ooo of a centimetre, or 

 V37)6oo,ooo of an inch in diameter, — or, more accurately, 

 this would be the average distance between the 

 centers of adjacent molecules. 



The thickness of the film of the soap bubble at the 

 dark part, just before it breaks, is said to be 1 /m 

 the length of the sodium wave of light, which is equal 

 to Y847,ooo of a centimetre, or about Y2>ii8,ooo of an inch. 

 This represents the thinnest portion of matter that has 



♦Popular Lectures and Addresses, by Sir W. Thomson, p. 193. 



