37 



zinc and copper plates. Iron can be used instead 

 of the more expensive zinc, but the current fur- 

 nished by the iron and copper battery is almost too 

 weak to be of any service. 



The advantages of this method over that in which 

 the geomagnetifer is used are the absence of under- 

 ground wires which corrode and have to be replaced 

 in time, and the power to furnish a more constant 

 supply of current. The one disadvantage is the 

 interference the plates and wires offer to cultivation. 



The earth-battery would seem to be the more use- 

 ful in raising berries and vegetables in greenhouses 

 or small plots, and the geomagnetifer in growing 

 potatoes or cereals in fields. 



When electricity can be generated cheaply enough 

 by dynamos or other means, its application to soils 

 in which crops are growing can be regulated to suit 

 every condition ; each soil and crop will get its dose 

 as exactly and as effectively as the physician pre- 

 scribes tonics and nutrients possibly more so. 



In using the arc-light the main things to be con- 

 sidered are the proximity of the plants to it, the 

 nature and color of the shade interposed between 

 it and the plants, the length of time the plants are 

 exposed to its rays, and the hours of the day or 

 night most suitable for its use. 



If the arc-light is too near the plant, though not 

 near enough to affect it by its heat, it may, never- 

 theless, scorch its leaves and cause serious damage. 

 The ultra-violet rays are said to be responsible for 

 this, as they, according to Professor Rowlee, " pro- 

 duce great activity in the protoplasmic contents of 



