:238 J. M. Lewis: 



Recently its application to the preparation of bacteriological 

 media having a prearranged exact hydrogen ion concentration 

 has proved to be of the greatest utility. Beside this, the investiga- 

 tion of the acidity of soil extracts, which is now known to bear 

 an important relation to plant growth, has been shown to be 

 another field in which the method should be of great use. (6) 



In the presence of these facts it will be evident that an instru- 

 ment of precision for the measurement of such slight variations 

 in hydrogen ion concentration is of the greatest value to the bio- 

 logical chemist, and it is safe to assume that the hydrogen elec- 

 trode will also occupy an important place in the clinical armamen- 

 tarium of the future. 



Theory of Electrode Potential. 



Before proceeding to a description of the hydrogen electrode 

 in detail, it will be necessary to discuss in general the prin- 

 ciples which underlie its use. The practical application of the 

 method received a great impulse from the work of Nernst, who 

 showed that when a metal made contact with a solution of one of 

 its salts, the electric charge, or potential, which it acquires with 

 respect to the solution, bears a constant relation to the concen- 

 tration of the dissolved salt. 



As this law holds good whatever the salt may be, it is evident 

 that the e.m.f. developed depends only upon the concentration 

 in the solution of ions identical with the metal employed. (7) 



When any substance goes into solution — and all substances ex- 

 hibit a certain tendency to dissolve — we must conceive the process 

 to be accompanied by electrical phenomena. 



In the case of a soluble salt, its solution will be followed in- 

 stantly by the dissociation of a certain fraction into ions bearing 

 complementary charges of electricity. As such a system is in a 

 state of equilibrium, the electrical phenomena are not manifest. 



Where metals are concerned, however, the case is different, 

 since a metal, when dissolving, can only give off to the solution 

 positively charged ions, and, depending on the degree to which 

 it does so, the metal will become negatively charged. 



We may take a concrete case. Let us suppose a rod of zinc 

 to make contact with a solution of the chloride of this metal. 

 Two opposing forces now come into operation. One is repre- 

 sented by a tendency of the zinc rod to go into solution, and 

 become to this extent negatively charged, i.e., to give off posi- 



