MATTER AND FORCE. 67 



of perfectly clean glass I pour a little water in which 

 certain crystals have been dissolved. A film of the 

 solution clings to the glass. By means of a micro- 

 scope and a lamp, an image of the plate of glass is 

 thrown upon the screen. The beam of the lamp, be- 

 sides illuminating the glass, also heats it; evaporation 

 sets in, and at a certain moment, when the solution has 

 become super-saturated, splendid branches of crystal 

 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 dis- 

 tant 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 these formless solutions, latent in every drop of 

 water, lies this marvellous structural power, which 

 only requires the withdrawal of opposing forces to 

 bring it into action. 



The clear liquid now held up before you is a solu- 

 tion of nitrate of silver a compound of silver and 

 nitric acid. When an electric current 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 successively 

 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 beautiful silver tree commences imme- 

 diately to sprout. Branches of the metal are thrown 

 out, and umbrageous foliage loads the branches. You 

 have here a growth, apparently as wonderful as that of 



