ABSORPTION 363 



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

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

 can exist. When electrodes connected with a voltaic battery are 

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

 (p. 533), 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 maintained 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 

 conductivity per molecule, or, strictly, the ratio of the specific con- 

 ductivity to the molecular concentration, increases with the dilution, 

 because the relative number of dissociated molecules, as compared 

 with undissociated, increases. At a certain degree of dilution the 

 molecular conductivity reaches its maximum, for all the molecules 

 -are dissociated. The ratio of the molecular conductivity of any given 

 solution to this maximum or limiting value is therefore a measure of 

 the proportion 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 

 liave traced the food in its progress along the alimentary 

 canal, and sketched the changes wrought in it by diges- 

 tion. 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 body in the form of 

 Avaste products, it will be best to drop the logical order and 

 pick up the other end of the clue in other words, to 

 pass from absorption to excretion, from the first step in 

 metabolism to the closing act, and afterwards to return and 

 iill in the interval as best we can. 



And here, first of all, it should be remembered that the epithelial 

 surfaces, through which the substances needed by the organism 

 enter it, and waste products leave it, are, physiologically considered, 

 outside the body. The mucous membranes of the alimentary, 



