LIGHT AND PLANTS. 117 



is, and those organs, had not been adapted to 

 each other. But the subject is here introduced 

 that the reader may the more readily receive 

 the conviction of combining purpose which must 

 arise, on finding that an agent, possessing these 

 very peculiar chemical properties, is employed 

 to produce also those effects of illumination, 

 vision, &c., which form the most obvious portion 

 of the properties of light. 



CHAPTER XIV. 

 Sound. 



BESIDES the function which air discharges as the 

 great agent in the changes of meteorology and 

 vegetation, it has another office, also of great and 

 extensive importance, as the vehicle of sound. 



1. The communication of sound through the 

 air takes place by means of a process altogether 

 different from anything of which we have yet 

 spoken : namely, by the propagation of minute 

 vibrations of the particles from one part of the 

 fluid mass to another, without any local motion 

 of the fluid itself. 



Perhaps we may most distinctly conceive the 

 kind of effect here spoken of, by comparing it 

 to the motion produced by the wind in a field 

 of standing corn ; grassy waves travel visibly 



