212 GRAVITY, LIGHT, AND CALORIC. L.ECT. XI. 



LECTURE XL 



PHYSIOLOGICAL ACTION OF GRAVITY, LIGHT, AND CALORIC. 



ARGUMENT. 1 . Action of Gravity. Experiments of Hunter and Knight on 

 the influence of gravity on the germination of seeds. 



2. Action of Light. Influence of light on the development, and on the 

 colours, of animals. Influence of light on the respiration of plants ; 

 illustrative fact observed in the process of the Daguerreotype. Influence 

 of light on the germination of seeds, and on the direction of the roots of 

 plants. 



3. Action of Caloric. A certain degree of heat is essential to vitality. 

 Actions of contact take place only at certain temperatures; these pro- 

 bably have some agency in the phenomena of living beings, especially 

 in fecundation. Influence of temperature on the life of frogs. Relation 

 between respiration and temperature.. Capability of man and other 

 mammals to bear extremes of heat and cold. Refrigerating process of 

 the animal body. 



WE have hitherto spoken of the development of heat, 

 electricity, and light, in organized living beings; but we 

 must now study their action upon these bodies. 



jlction of Gravity. I think it unnecessary to say that, in 

 speaking of the action of gravity upon living beings, I do 

 not mean to notice that which is exercised upon bodies 

 generally ; which makes them, when left to themselves, fall 

 to the earth, and which causes them to press on the surface 

 which supports them, and to maintain themselves in equili- 

 bria when their centre of gravity is supported or sus- 

 pended.. 



