1 76 PRINCIPLES OF GENERAL PHYSIOLOGY 



of these dimensions for a particular vessel is determined by measuring in it the 

 resistance of a solution whose value in conductivity is known from previous 

 measurements in a vessel whose dimensions can be measured directly. Actual 

 details may be obtained from the book by Findlay (1906, pp. 144-181) or from 

 the article by Asher (1911, pp. 161-174). In the present pages general principles 

 only need be referred to. As is familiar to the reader, the measurement of the 

 resistance of metallic conductors by the Wheatstone bridge method is capable of 

 extreme accuracy. The same method, with modifications, is also employed t<>i 

 solutions of electrolytes. These modifications are due to the fact that, if a current 

 is sent for any appreciable length of time between metallic electrodes immersed in 

 such a solution, the current falls off greatly in strength owing to deposition of ions 

 on each electrode of opposite sign to themselves, polarisation, as it is called. For 

 this reason, accurate measurements by the ordinary galvanometer method are 

 impossible. The difficulty is got over in the method of Kohlrausch by the use of 

 a current which rapidly changes its direction, before any appreciable polarisation 

 has had time to develop. Fjach electrode is made anode and cathode in turn. A 

 small induction coil, with a very rapidly vibrating interrupter, is used for the pur- 

 pose and the alternating induced currents from the secondary coil are sent through 

 the electrolyte. But this again necessitates the use, as detector of the zero point, 

 of some instrument which responds to alternating currents, since the ordinary 

 galvanometer does not, except when the changes of direction do not occur at 

 frequent intervals. A telephone is generally used. 



When the solutions have a very high resistance, it is found to be difficult to get enough 

 current through to give sharp readings with the telephone. In such cases, the method of 

 Whetham (1900) is of great value. In this, the change of direction of the current is effected 

 by a rotating commutator, and, in order to enable a delicate galvanometer of the ordinary 

 type to be used, the alternating current is rectified again before going to the galvanometer. 

 This is done by a second commutator on the same axis as that which originally makes tin- 

 alternating current. It is obvious that this method allows of great variations in the electro- 

 motive force used to drive the current through the electrolyte, and in the sensibility of the 

 galvanometer. 



In practice, especially for physiological purposes, the conductivity vessels with fixed 

 vertical electrodes, and provided with stoppers, will be found of most value (see the catalogue 

 of Fritz Kohler, Cat, E. 1906, No. 1326). 



We may now return to the consideration of further evidence in favour of the 

 electrolytic dissociation theory. 



IONIC CONDUCTIVITY 



Let us take the molecular conductivity of the following series of salts in 

 O'OOOl molar concentration as given by Kohlrausch and Maltby (1899). 



Chloride. Nitrate. 



K - - 129-05 125-49 



Na - 108-06 104-53 



Li - - 98-06 94-38 



The precise units in which these are expressed does not matter for our present 

 purpose, since all are in the same units. 



The difference between KC1 and NaCl is 20-99 and between KNO 3 and 

 NaNO 3 is 20-96, practically identical. Again, the difference between KC1 and 

 LiCl is 30-99 and between KNO 3 and LiNO 3 is 31-11. What does this imply? 

 Obviously that it does not matter whether, in changing Na or Li for K, we 

 take a chloride or a nitrate; that is, the metallic part of the salt makes a certain 

 contribution to the conductivity which is independent of the acidic radical 

 associated with it. Similarly, the difference between KC1 and KNO 3 is 3-56, 

 between NaCl and NaNO 3 3-53, and between LiCl and LiNO 3 3-68, so that 

 the same consideration applies to the other radical. This fact may perhaps be 

 clearer if put in a symbolic form : 



(K + Cl) - (Na + Cl) = (K + N0 3 ) - (Na + NO 3 ) 

 and (K f Cl) - (K + NO 3 ) = (Na + Cl) - (Na + NO 3 ) 



can only hold, if K, Na, Cl and NO 3 each has a definite value independent of that 

 of any of the others. 



