EXPERIMENT STATION BULLETINS. 587 



The last column in the first series shows that after 24 hours the resist- 

 ance of the water in which four seedlings had grown had decreased con- 

 siderably over that in which no seedlings had grown. Likewise the resist- 

 ance of the solution in which eight seedlings had grown is lower than 

 that of the solution in which four seedlings had grown. It may also be 

 observed that the decrease is not exactly proportional to the number of 

 individuals growing in the solution. In the last column of the second 

 series it may be noticed that the water in which no seedlings had grown 

 had a slightly higher conductivity after boiling than before. No expla- 

 nation is here offered. In the case of the other two culture solutions it 

 is seen that boiling reduced the conductance by about 13% of what it was 

 before boiling. 



6. CONCLUSIONS. 



In this paper the authors have endeavored to draw attention to the 

 many biological problems, for the investigation of which tlie Wheatstone 

 Bridge apparatus seems especially fitted. A plea is also made for a more 

 general use of the principles of physical chemistry and a closer attention 

 to the acquisition of quantitative rather than qualitative results. 



Why this apparatus has not been more widely used, is in all probability 

 due to the fact that it possesses, in its customary form a number of 

 sources of error, which preclude accurate measurement. In addition, the 

 taking of a measurement necessitates a considerable strain on the nerves 

 of the operator. It is obviously very difficult to listen tlirough a telephone 

 for a point at which a continuous but gradually decreasing sound stops. 



We have enumerated some of the errors inherent in the general method 

 and believe that modifications, such as we have suggested make the appa- 

 ratus much more precise as well as easier to operate. The induction coil 

 was abandoned for a 60 cycle rotary converter, which gives a pure sine 

 wave on a current of constant potential. The use of a frequency of 60 

 cycles per second was not attended with any appreciable error. It has 

 been shown that in assuming polarization at GO cycles we are assuming 

 something neither apparent nor real. We have used this frequency for 

 a period of two years, and have had no trouble from polarization but what 

 couid be eiiminatea. 



The alternating current galvanometer is a better detector than the 

 telephone for four reasons: — 1. An exact balance can be made quickly 

 and with ease. 2. Direction of deflection will indicate whether heating 

 or polarization is taking place and the correction can be quickly made. 



3. The circuit need not be closed more than an instant at a time and 

 therefore the possible error due to heating or polarization is negligible. 



4. The eye is more accurate than the ear. The customary resistance 

 coils are replaced by Curtis Coils since these are wound for annulment 

 of inductance and capacity. The dial decade type, a more recent form 

 than the one used in these experiments is better because of the ease and 

 convenience of manipulation. They also have as great a range and are 

 better because the resistance can be increased by unit increments. At- 



