MATTER AND FORCE. 87 



is called carbonic acid gas. Two atoms of oxygen and one 

 of carbon unite to form the molecule of carbonic acid which, 

 as I have said, is the material from which wood and vege- 

 table tissues are mainly derived. The carbonic acid of the 

 air being subjected to an action somewhat analogous to 

 that of the electric current in the case of our lead and silver 

 solutions, has its carbon liberated and deposited as woody 

 fibre. The watery vapor of the air is subjected to similar 

 action ; its Irydrogen is liberated from its oxygen, and lies 

 down side by side with the carbon in the tissues of the 

 tree. The oxygen in both cases is permitted to wander 

 away into the atmosphere. But Avhat is it which thus tears 

 the carbon and the hydrogen from the strong embrace of 

 the oxygen ? What is it in Nature that plays the part of 

 the electric current in our experiments ? The rays of the 

 sun. The leaves of the plants absorb both the carbonic 

 acid and the aqueous vapor of the air ; these leaves an- 

 swer to the cells in which our decompositions by the electric 

 current took place. In the leaves the solar rays decompose 

 both the carbonic acid and the water, permitting the oxygen 

 in both cases to escape into the air, and allowing the carbon 

 and the Irydrogen to follow the bent of their own forces. 

 And just as the molecular attractions of the silver and the 

 lead found expression in the production of those beautiful 

 branching forms seen in our experiments, so do the molecular 

 attractions of the liberated carbon and Irydrogen find ex- 

 pression in the architecture of grasses, plants, and trees. 



In the fall of a cataract and the rush of the wind we 

 have examples of mechanical power. In the combinations 

 of chemistry and in the formation of crystals and vegetables 

 we have examples of molecular power. But before pro- 

 ceeding further I should like to make clear to you the 

 present condition of the surface of our globe with reference 

 to power generally. You have learned how the atoms of 

 oxygen and hydrogen rush together to form water. I have 



