90 PLANT PRODUCTS 



I/emstrom. He succeeded by using electricity at a high 

 tension conveyed by wires over a field. He employed an 

 ordinary town current to drive a small electric motor, 

 driving in its turn a small influence static electric machine. 

 Subsequently the work was taken up by Professor Priestley 

 at Bristol and I^eeds. The method now adopted is to use 

 a transformer with a rectifier to give positive electricity 

 at a high tension. The details have by no means yet been 

 worked out. But the latest ideas suggest that wires are 

 best distributed overhead at about five feet in height, and 

 that the wires should be made as thin as possible. Under 

 these conditions a very marked increase in crops has been 

 obtained. The actual cost of the electric energy is quite 

 small, but the initial expense of the machinery is considerable, 

 and at present requires skilled attention. Until details 

 have been worked out on the large experimental scale, it 

 will be difficult to make a commercial success of this method. 

 Many points remain yet to be discovered, such as the relation- 

 ship of light and varying humidity of the air, the strength 

 of the discharge, and the relationship between electrification 

 and the manure used. All these points require to be investi- 

 gated thoroughly. The great advance, however, which has 

 been made since I/emstrom's days by Priestley, Jorgensen, 

 and Blackwood promises future progress. 



The Partial Sterilization of Soils. It is a very old, 

 well-known fact that the application of heat, and all kinds of 

 poisonous substances to soils, may increase the ultimate crop 

 obtained, even though some injury may occur at the moment. 

 From the elementary cottage idea of putting a flower-pot into 

 the oven for a short time, up to the laboratory researches of 

 Dr. Russell, the subject of application of heat to a soil has 

 been freely discussed. In nature this process occurs in hot 

 climates where solar radiation may raise the surface tempera- 

 ture of the soil up to 60 Cent. (140 Fahr.). Under these 

 conditions many pests in the soil are destroyed, so that the 

 ultimate growth of the crop is improved. In greenhouses 

 steam is not infrequently employed for the purpose of heating 

 the soil on a moderately large scale. In a similar way all 



