ABSORPTION 401 



dissociated molecules becomes greater. The bodies which behave in 

 this way are electrolytes that is, their solutions conduct a current of 

 electricity ; bodies which do not exhibit this behaviour do not con- 

 duct in solution. And there are many reasons for believing that the 

 dissociation of the electrolytes is the essential thing in electrolytic 

 conduction. We may suppose that in a solution of an electrolyte 

 sodium chloride, for' instance a certain number of the molecules 

 fall asunder into a kation (Na),* carrying a charge of positive elec- 

 tricity, and an anion (Cl), carrying an equal negative charge. These 

 electrical charges, it must be remembered, are not created by the 

 passage of a current through the solution. We do not know how 

 they arise, but the ions must be supposed to be electrically charged 

 at the moment when the molecule is broken up. And the ions of 

 different substances must each be supposed to carry the same 

 quantity of electricity. But since they are all wandering freely in 

 the solution, no excess of negative or of positive electricity can 

 accumulate at any part of it in other words, no difference of poten- 

 tial can exist. When electrodes connected with a voltaic battery 

 are dipped into a solution of an electrolyte, a difference of potential, 

 an electrical slope, is established in the liquid, and the positively 

 charged kations are compelled to wander towards the negative pole, 

 the negatively charged anions towards the positive pole. In this 

 way that movement of electricity which is called a current is main- 

 tained in the solution. It is clear that the greater the number of 

 ions, and the faster they move in the solution, the greater will be 

 the quantity of electricity carried to the electrodes in a given time, 

 when the difference of potential between the electrodes, or the 

 steepness of the electric slope, remains constant. In other words, 

 the specific conductivity of a solution of an electrolyte varies as the 

 number of dissociated molecules in a given volume and the speed of 

 the ions. It increases up to a certain point with the concentration, 

 because the absolute number of dissociated molecules in a given 

 volume increases. The molecular conductivity that is, the conduc- 

 tivity per molecule, or, strictly, the ratio of the specific conductivity 

 to the molecular concentration, increases with the dilution, because 

 the relative number of dissociated molecules, as compared with un- 

 dissociated, increases. At a certain degree of dilution the molecular 

 conductivity reaches its maximum, for all the molecules are dissoci- 

 ated. The ratio of the molecular conductivity of any given solution 

 to this maximum or limiting value is therefore a measure of the pro- 

 portion between the number of dissociated, and the total number 

 of molecules. The molecular conductivity of the salts dissolved in 

 the liquids of the animal body, for the degree of dilution in which they 

 exist there, is such that we must assume them to be for the most part 

 dissociated. 



Absorption of the Food. In the preceding chapter we have 

 traced the food in its progress along the alimentary canal, and 

 sketched the changes wrought in it by digestion. We have 

 next to consider the manner in which it is absorbed. Then, for 

 a reason which has already been explained, instead of following 

 its fate within the tissues, until it is once more cast out of the 



* J. J. Thomson has shown that the chemical atoms must be 

 assumed to consist of smaller corpuscles or particles of electricity 

 called electrons. 



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