PRELIMINARY PHYSICO-CHEMICAL DATA 423 



behave in this way are electrolytes that is, their solutions conduct a 

 current of electricity; bodies which do not exhibit this behaviour do 

 not conduct in solution. And there are many reasons for believing 

 that the dissociation of the electrolytes is the essential thing in elec- 

 trolytic conduction. We may suppose that in a solution of an electro- 

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

 fall asunder into a kation (Na-f-),* 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 sub- 

 stances 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 potential can exist. When electrodes 

 connected with a voltaic battery are dipped into a solution of an elec- 

 trolyte, 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 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 conductivity to the molecular concen- 

 tration 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 molec- 

 ular 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. 



Surface Tension. This is a property of surfaces which is typically 

 illustrated in such instances as a globule of mercury, a drop of water 

 on a greasy slide, or a drop of oil suspended in a liquid with which it 

 does not mix. The tendency of such drops to assume the spherical 

 form when not large enough to be distorted by gravity is due to the 

 fact that the surface layer is under a certain tension in virtue of which 

 it strives to contract and to render the surface of the drop as small as 



* It has been shown that the chemical atoms themselves are not homo- 

 geneous, but are all built up of simpler particles and possess a certain struc- 

 ture. All atoms, e.g., contain electrons, minute particles charged with negative 

 electricity. The number of electrons in an atom appears to be not far from 

 half its atomic weight. Thus in the carbon atom there are 6 electrons, in the 

 oxygen atom 8, and in the hydrogen atom probably only i. There is 



^ evidence that the electrons in the atom are divided into groups or rings 



k one within another (Thomson) . 



