MAGNETISM. 115 



collateral results of the existence of the same 

 cause by which electrical agency operates ; an 

 agency which, as we have already seen, has im- 

 portant offices in the processes of vegetable life. 

 And thus magnetism belongs to the same system 

 of beneficial contrivance to which electricity has 

 been already traced. 



We see, however, on this subject very dimly 

 and a very small way. It can hardly be doubted 

 that magnetism has other functions than those we 

 have noticed. 



CHAPTER XIII. 



The Properties of Light with regard to 

 Vegetation. 



THE illuminating power of light will come under 

 our consideration hereafter. Its agency, with 

 regard to organic life, is too important not to be 

 noticed, though this must be done briefly. Light 

 appears to be as necessary to the health of plants 

 as air or moisture. A plant may, indeed, grow 

 without it, but it does not appear that a species 

 could be so continued. Under such a privation, 

 the parts which are usually green, assume a 

 white colour, as is the case with vegetables grown 

 in a cellar, or protected by a covering for the 

 sake of producing this very effect ; thus, celery 

 is in this manner blanched, or etiolated. 



