MATTER AND FORCE. 83 



ally withdrawn, the antagonist to their union being re- 

 moved, the molecules prepare for new arrangements and 

 combinations. Like the particles of iron in our magnetic 

 experiment, the water molecules are endowed with attractive 

 and repulsive poles, and they arrange themselves together 

 in accordance with these attractions and repulsions. Solid 

 crystals of water are thus formed, to which we give the fa- 

 miliar name of ice. To the eye of science these ice-crystals 

 are as precious as the diamond as purely formed, as deli- 

 cately built. Where no disturbing causes intervene, there 

 is no disorder in this crystalline architecture. By their own 

 constructive power molecule builds itself on to molecule 

 with a precision far greater than that attainable by the 

 hands of man. We are apt to overlook the wonderful when 

 it becomes common. Imagine the bricks and stones of 

 this town of Dundee endowed with locomotive power. Im- 

 agine them attracting and repelling each other, and arrang- 

 ing themselves in consequence of these attractions and re- 

 pulsions to form streets and houses and Kinnaird Halls ; 

 would not that be wonderful ? Hardly less wonderful is 

 the play of force by which the molecules of water build 

 themselves into the sheets of crystal which every winter 

 roof your ponds and lakes. 



If I could show you the actual progress of this molecu- 

 lar architecture, its beauty would delight and astonish you. 

 A reversal of the process may be actually shown. The 

 molecules of a piece of ice may be taken asunder before 

 your eyes, and from the manner in which they separate, you 

 may to some extent infer the manner in which they aggre- 

 gate. When a beam is sent from our electric lamp through 

 a plate of glass, a portion of the beam is intercepted, and 

 the glass is warmed by the portion thus retained within it. 

 When the beam is sent through a plate of ice, a portion 

 of the beam is also absorbed ; but instead of warming the 

 ice, the intercepted heat melts it internally. It is to the 



