18 ABSORPTION AND EXCRETION OF SALTS BY ROOTS. 
Deduction as to plans.—Experiments A, B, and C showed that fur- 
ther work should be so planned as to study absorption and excre-— 
tion (1) in solutions of magnesium. nitrate more dilute than a 
? 
(2) in comparable solutions of oe nitrate; (3) in equivalent solu- 
a 9 Cas (4) in equivalent solu- 
: 100 10 ae 
tions grading by hundredths from —— 100 Mg to 7 00 ° Me +700 Ca; and 
(5) in solutions containing magnesium and calcium in constant ratio, 
but varying in concentration so as to be comparable with the solu- 
tions of magnesium alone and calcium alone. 
tions grading by tenths fr om 70 5 Me to 75 
RECORD OF THE EXPERIMENTS. 
In the experiments recorded below the conductivity of the culture 
solutions was determined with the simplest form of the Wheatstone 
bridge, a scale 100 mm. in length with sliding contact. The ap- 
paratus specified by Ostwald and Luther+ gave ‘oma satisfactory 
results. Sharp tone minima were obtained with solutions varying in 
resistance from 5,000 to more than 100,000 ohms. The familiar Ar- 
rhenius resistance cell was used, with bright platinum electrodes. In 
making a series of readings the known resistance was kept constant 
at 10,000 ohms. Because of the great range in concentration of the 
solutions, the tone minimum was frequently too far from the center 
of the bridge to be consistent with the highest accuracy. Neverthe- 
less, the concentration changes in the culture solutions were com- 
paratively of such magnitude that the error thus introduced and 
the error introduced by disregarding the small change in temperature 
during the course of an experiment could be disregarded. It is 
believed that the correction of small errors can have no significance 
in culture experiments involving the use of very dilute solutions, 
because the single great error due to the presence in the solutions of 
unknown or uncontrolled substances is greater than all other errors 
combined. Every culture solution contains, in addition to the 
known quantity of known substances, a variable quantity of other 
substances, dissolved from the apparatus or absorbed from the air. 
When solutions are dealt with which are as dilute as those used in 
M M 
350,000 and = 10,000 in concentration) the 
amounts of known and of unknown electr ee are of the same order 
of magnitude. Consequently, when conductivity measurements show 
that minute amounts of material have been removed from a dilute solu- 
our experi 
1 Ostwald, W., and Luther, R. Physiko-Chemische Messungen, 2d ed., p. 395 et seq. 
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