MATTER AND FORCE. 65 



each other; atom is added to atom, and molecule to 

 molecule, not boisterously or fortuitously, but silently 

 and symmetrically, and in accordance with laws more 

 rigid than those which guide a human builder when he 

 places his materials together. Imagine the bricks and 

 stones of this town of Dundee endowed with structural 

 power. Imagine them attracting and repelling, and 

 arranging themselves into streets and houses and Kin- 

 naird Halls would not that be wonderful? Hardly 

 less wonderful is the play of force by which the mole- 

 cules of water build themselves into the sheets of ice 

 which every winter roof your ponds and lakes. 



If I could show you the actual progress of this 

 molecular architecture, its beauty would delight and 

 astonish you. A reversal of the process of crystallisa- 

 tion 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 go together. 

 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 inter- 

 nally. It is to the delicate silent action of this beam 

 within the ice that I now wish to direct your attention. 

 Upon the screen is thrown a magnified image of the 

 slab of ice: the light of the beam passes freely through 

 the ice without melting it, and enables us to form the 

 image; but the heat is in great part intercepted, and 

 that heat now applies itself to the work of internal 

 liquefaction. Selecting certain points for attack, 

 round about those points the beam works silently, un- 

 doing the crystalline architecture, and reducing to the 



