MATTER AND FORCE. 85 



through this state of motion, to its final crystalline re- 

 pose. 



[ ft I can show you something similar. Over a piece of 

 perfectly clean glass I pour a little water in which a crystal 

 has been dissolved. A film of the solution clings to the 

 glass, and this film will now be caused to crystallize before 

 your eyes. By means of a microscope and a lamp, an image 

 of the plate of glass is thrown upon the screen. The beam 

 of the lamp, besides illuminating the glass, also heats it ; 

 evaporation sets in, and, at a certain moment, when the 

 solution has become supersaturated, splendid branches of 

 crystals shoot out over the screen. A dozen square feet of 

 surface are now covered by those beautiful forms. With 

 another solution we obtain crystalline spears, feathered 

 right and left by other spears. From distant nuclei in 

 the middle of the field of view the spears shoot with magical 

 rapidity in all directions. The film of water on a window- 

 pane on a frosty morning exhibits effects quite as wonderful 

 as these. Latent in this formless solution, latent in every 

 drop of water, lies this marvellous structural power, which 

 only requires the withdrawal of opposing forces to bring it 

 into action. 



Our next experiment on crystallization you will probably 

 consider more startling even than these. The clear liquid 

 now held up before you is a solution of nitrate of silver a 

 compound of silver and nitric acid. When an electric cur- 

 rent is sent through this liquid the silver is severed from 

 the acid, as the hydrogen was separated from the oxygen 

 in a former experiment ; and I would ask you to observe 

 how the metal behaves when its molecules are thus succes- 

 sively set free. The image of the cell, and of the two wires 

 which dip into the liquid of the cell, are now clearly shown 

 upon the screen. Let us close the circuit, and send the 

 current through the liquid. From one of the wires a beau- 

 tiful silver tree commences immediately to sprout. Branches 



