338 THE LIFE OF THE PLANT 



are connected with those of the electric light. The visible, kinetic 

 energy of the sunlight, expended in the decomposition of carbonic 

 acid in plants, takes the form of latent potential energy, con- 

 cealed in carbon after it has been liberated from the carbonic 

 acid. This potential energy of carbon then passes over to the 

 zinc during the process of deoxidation of the zinc ore ; carbon 

 burns down and metallic zinc is obtained capable of combus- 

 tion. Zinc oxidises in the galvanic battery, burns down, and 

 its potential energy transforms itself into the actual energy of 

 the electric current, appearing as light in the white-hot carbons. 

 Such is the complicated chain of transformations of energy, which 

 connect phenomena that take place on our planet with the 

 activity of the sun. We can, however, form a more definite 

 idea as to the significance of the radiation of energy from the 

 sun, by making an approximate calculation of the amount of 

 energy afforded by the sun. We can determine the number of 

 units of heat cast by the sun upon a given area of our planet ; 

 and then, knowing the mechanical equivalent of heat, we can 

 express the energy of sunlight in units of mechanical work. 

 According to the calculations of Mouchot the sunlight that 

 falls during eight or ten hours on a bright day upon the 

 surface of a square metre in Paris can do work approxi- 

 mately equal to one horse-power. Ericsson calculated that if all 

 the heat of the sun that falls upon the roofs of Philadelphia 

 were to 'be used, it would amount to the force of 5000 steam 

 engines, each of 20 horse -power. Further, on calculating 

 the enormous amount of heat that falls upon the earth, he 

 exclaims : ' Archimedes undertook to lift the world by a lever, 

 whereas I maintain that by concentrating the heat of the sun 

 we could obtain a force capable of arresting the motion of the 

 earth.' Both Mouchot and Ericsson, however, did not confine 

 themselves to calculations : they made experiments, which 

 clearly demonstrated the stores of energy presented by the 

 rays of the sun. Mouchot made several very simple kinds of 

 apparatus, in which it was possible to boil water, soup and 

 vegetables as well as to bake bread exclusively by means of the 

 heat of the sun. In the end he also made some steam and hot- 

 air engines set in motion by the sun. Of all the applications 

 of sunlight suggested by Mouchot, his pumps for irrigating 

 fields are perhaps the most curious. They not only act by 



