ELECTROLYTES AND THEIR ACTION 175 



solution, and found the absorption band in the same situation in all. Various 

 salts of dyes show the same behaviour (see Fig. 53). 



Before we pass on to consider other evidence, the question of electrolytic 

 conductivity must be dealt with. 





The passage of an electric current through a solution being due to the ions 

 present between the electrodes, it is clear that the amount of current that 

 will pass through a given solution will depend, in the first place, on the size of 

 the electrodes the larger they are, the more ions there will be between them. 

 The current that passes, other things being equal, is in direct proportion to 

 the area of the electrodes when the column of solution between them is of the 

 same cross section as the electrodes, so that no spreading of the current takes 

 place. In the second place, owing to the fact that the velocity with which 

 the ions move is finite, the greater the distance between the electrodes the 

 longer it will take for an ion to carry its charge to the opposite electrode and 

 the less electricity will be carried in unit time, i.e., the current will be less. 

 In comparing the conductivity of one solution with that of another, it is there- 

 fore necessary to agree to some arbitrary dimensions. The unit of conductivity 

 is taken, accordingly, as that of a body of which a column one centimetre long 

 and one square centimetre in cross section has a resistance of one ohm (Nernst, 

 1911, p. 361). The resistance is the reciprocal of the conductivity; if one 

 solution has twice the resistance of another, only half the current will pass 

 through it, so that its conductivity is half that of the other. 



If, then, a body of the dimensions given above has a resistance w in ohms 



(usually written o>) its conductivity (K) is -- in reciprocal ohms, frequently 



called mhos (i.e., ohm spelt backwards). The actual conductivity of a particular 

 solution is called the " specific conductivity " of that solution ; but in order to 

 compare solutions of different salts with one another, it is convenient to have 

 an expression in which the molar concentration is taken into account. The 

 "molecular conductivity" is now understood as the actual conductivity divided 



by the concentration in gram-equivalents per cubic centimetre (77), i.e., -, and is 



denoted by A. It is clear that the conductivity of the solution of an electrolyte 

 depends on its concentration, since it is the ions into which the solute dissociates 

 that conduct the current, and the more there are in the space between the 

 electrodes, the more current will pass. The value of taking gram-equivalents 

 instead of gram-molecules is that salts with multivalent ions are more readily 

 compared with those with univalent ions. Thus, if equimolar solutions of KC1 

 and K 2 SO 4 are compared, we must remember that the second salt, at an equal 

 degree of dissociation, has twice the conducting power of the first, since it gives 

 ions with four charges, two negative and two positive, while the first only gives 

 one negative and one positive. 



It will be remembered that, in the statement of the theory of electrolytic 

 dissociation given by Arrhenius (page 173 above), the " inactive molecules " are said 

 to be converted into active molecules on dilution. This is the expression of the 

 experimental fact that the molecular conductivity, or the number of ions into 

 which a gram-equivalent is dissociated, increases as the solution is diluted. By 

 plotting successive values of the molecular conductivity at increasing dilutions in 

 the form of a curve, the value at infinite dilution, that is, what it would be if 

 completely dissociated, can be extrapolated. Equivalent conductivity may also 

 be expressed in terms of the volume of solution in cubic centimetres which contains 

 1 gram-equivalent; the symbol <f> is generally used, so that the equivalent 



conductivity may be expressed as K<J>. < is, of course, equal to -. 



The methods of measuring conductivity may be now considered. What is 

 actually measured is the resistance of a stratum of known dimensions. The value 



