1904.] PUBLIC DOCUMENT — \o. 33. 15 



has been used for these experiments alone for some years. 

 Previous to using the soil it was sifted through a sieve of |- 

 inch mesh, and thoroughly mixed. After using the soil for 

 a few experiments, it was taken out, resifted and thoroughly 

 incorporated again ; and occasionally the boxes were shifted 

 about, that is, the normal or untreated boxes were substi- 

 tuted or chanoed for those which had been treated. With a 



l^b g, b 



12 3 4 



Fig. 2. — Longitudinal section of boxes employed in the electrical experiments, 

 showing different methods of treatment. Size of boxes, 53 by 32 by 7 inches. 

 1, copper and zinc electrodes connected; 2, direct cuiTent with rheostat; 3, 

 atmospheric; 4, wire electrodes. C, copper electrodes; Z, zinc electrodes; 

 r, rheostat; b, gravity cells; w, wires; g, ground wire; p, collecting pole, 47 

 feet high. 



few exceptions, copper or zinc plates were used for electrodes. 

 These were made the same size as the ends of the boxes, 

 and in a few exceptional cases two series of wires, strung on 

 a frame about three inches apart, were employed instead of 

 the plate electrodes. One of these frames of wires was 

 buried near the surface, the other being buried near the bot- 

 tom of the box. The current, therefore, had to pass from 

 one frame tf) the other in a vertical direction through the 

 soil. The strength of the currents was in most cases ob- 

 tained with the aid of a Weston milammeter, capable of read- 

 ing Jq of a milliampere, or about 2"o^7o" ^^ ^^^ ampere. The 

 interrupted induced currents were estimated, and represent 

 only approximate determinations. 



In the radish experiment the seed was sown directly in 

 the treated boxes, whereas in the case of the lettuce the 

 plants were transplanted into the treated boxes when of 

 suitable size to make good growth ; the latter plants, there- 

 fore, were not stimulated during the whole period of devel- 

 opment. Gravity cells were used in all cases except with 

 the interrupted induced current, in which case sal-ammoniac 

 cells were employed. 



