138 
FIFTEENTH REPORT. 
IMPROVED METHODS FOR THE QUANTITATIVE DETERMINA- 
TION OF DILUTE SOLUTIONS OF ELECTROLYTES. 
(Preliminary contribution) . 
BY RUFUS PERCIVAL HIBBARD. 
The use of a Wheatstone slide wire bridge or Kohlrausch’s improve- 
ment of 1 lie same in botanical investigations is relativelv new. To those 
ot you who may not be familiar with its uses in plant physiological 
studies, let me state that among the many, it affords perhaps the most 
accurate method for determining the concentration of solutions. The 
concentration of a dilute solution is an index of the number of ions in 
that solution, and in the same way the electrical resistance is dependent 
on the number of ions. The greater the number of ions, the less the 
resistance to the passage of an electric current, and the less the num- 
ber of ions the greater the resistance to the passage of an electric cur- 
rent. So by calculating the specific resistance and inserting the values 
in certain formulae, the concentration of the solution can be determined. 
Recent work has shown that for purposes of greater accuracy and 
speed, it lias been necessary now to make some modifications in the 
present form of the apparatus. With the changes suggested in the fol- 
lowing pages, one can obtain a high degree of precision with greater 
rapidity and with much less strain on the nerves. The correct bridge 
setting is determined by the aid of the eye instead of the ear, which is a 
feature of some advantage. It seems probable that electrolytic con- 
ductivity methods will be of considerable value towards a better under- 
standing of many of the fundamental problems in the life of plants and 
animals. In this connection T might call your attention to the work 
of fibber on blood reactions. Rugarsky and Tangl on serum, and the 
work of many zoologists and pathologists. Of the American inves- 
tigators who have used the method recently, perhaps the work of W. 
J. V. Osterhout 1 2 has attracted the most attention. True & Bartlett- have 
also made use of the Wheatstone Bridge in their investigations on “Con- 
centration Relations of Dilute Solutions of Calcium and Magnesium 
Nitrates to Pea Roots.” Bayliss 3 has published an extensive series of 
observations on the hydrolysis of various proteids by the action of tryp- 
sin. The method used was that of measuring the increase in electrical 
conductivity due to the reaction of the enzyme. 
No further enumeration of the literature is necessary to show one 
the evident value of conductivity methods. A study of the articles 
mentioned above will convince one also of the necessity of eliminating 
errors, many of which are capable of correction. The work was begun in 
the summer of 1912, when I was making a study of “Adsorption” and 
its relation to biology. T first used the ordinary slide wire or meter- 
1 "The PERMEABILITY of PROTOPLASM to IONS and the THEORY of ANTAGONISM,” 
Science N. S. Vol. XXXV NO. 890, p. 112 (1912). 
2 U. S. Dept, of Agri. Bur. Plant Ind., Bui. No. 231 (1912). 
3 Arch, des Sciences Biologique: Vol. XI Suppl. p. 261, St. Petersburg, (1904). 
