Philosophy of Botany. 225 



Toward the end of the century the same method was also re- 

 sorted to for the investigation of animal life, and the discovery 

 of the circulation of the blood was the first important result. 

 In the eighteenth century, lastly, plants also undergo the trial 

 of experiment, and the Englishman, Hales, was the first one to 

 consider the vital action of plants as the result of the action of 

 physical forces, and to determine them with weights and meas- 

 ure. He compares the force which propels the sap of the 

 bleeding grapevine upward in the spring, to a column of mer- 

 cury of a definite height, or with the pressure of the crural 

 artery of the horse. He weighs the quantity of water which 

 a pear tree or a sunflower absorbs from the soil in twenty-tour 

 hours; he sets forth in the year 1727 a static of vegetation 

 which resolves the whole of vegetable life into a physical 

 problem. 



The Frenchman, Duhamel, published in 1758 a physical 

 treatise on trees, wherein he investigates the laws by which 

 the sap circulates in the wood and bark ; and in the same year 

 appeared a book on the functions of the leaves, by Bonnet, of 

 Geneva, wherein he attempts to define the cause of the move- 

 ment of leaves toward the light, and their transpiration. 



In this way enters the physiology of plants, based upon 

 physical science, into the rank of the exact sciences. 



As soon as, toward the end of the eighteenth century, 

 Chemistry awakes out of the obscure hallucinations of al- 

 chemistic dreams, we find her at once engaged in the servict; 

 of botany. The Belgian, Ingenhauss, and the Englishman, 

 Priestley, discovered the wonderful interaction between sun- 

 light and terrestric atmosphere, vegetable and animal life, 

 demonstrating how the carbonic oxide, exhaled by animals, 

 is inhaled by the plants, and inversely, that the oxygen which 

 the plants emit under the influence of light is indispensable 

 for the life of animals. Toward the end of the century Theo- 

 dore Saussure, of Geneva, shows how, by the nutrition of 

 plants, the moving force is supplied by the light and heat of 

 the sun, carbonic oxide by the air, and water and ammonia b\ 

 the soil ; further, that the ashes of the plant are not accidental 

 impurities, but indispensable elements, which the plants take 

 up from the soil with their roots, and thereb}- lies the founda- 



