FROM THE SMALL INTESTINE 623 



solution of NaCl, 0*682 grm. in 100 grm. water, was found to 

 give an actual A which was 1*95 times greater than its theoretical 

 value. In other words, the actual A corresponded to a solu- 

 tion having T95 times more molecules in solution than were 

 apparently there. Arrhenius accounted for this discrepancy 

 by believing that these substances underwent electrolytic 

 dissociation or ionisation, and that the ions conducted them- 

 selves as independent molecules in the solution. If, then, equi- 

 molecular solutions of two substances be made, one of which, 

 undergoes ionisation and the other not, the former will contain 

 a larger number of molecules in a given volume, and will there- 

 fore have an osmotic pressure greater by the degree of ionisa- 

 tion which it has undergone. By determining the A of a solu- 

 tion we ascertain the number of inactive molecules plus the 

 number of active molecules or ions, if any, present in a given 

 volume of the solvent. The dissociative power of solvents, 

 i.e. their ability to split up a substance into its ions, is very 

 different. Hence the osmotic pressure of equimolecular solutions 

 of the same substance in various solvents may be very different. 

 Water has the greatest dissociative power of any solvent. 



The question arises, how is it known when a substance in 

 solution has undergone ionisation and the degree to which it 

 has taken place ? Now, it is the possession of free ions which 

 makes a solution an electric conductor or electrolyte. Ions 

 are the electrically charged particles which together constitute 

 a molecule of an electrolyte, and under the influence of an 

 electric current the anions migrate towards and discharge at 

 the anode, and the kations at the kathode. It must be under- 

 stood that free ions are actually present in the aqueous solution 

 of an electrolyte because of the dissociative power of the 

 solvent, and that they are not first produced by the action 

 of the electric current. Ions move freely in all directions 

 through a solution until under the influence of an electric 

 current they are driven in two definite directions. Solutions 

 of various substances, therefore, may be divided into conductors 

 and non-conductors, according as the substance undergoes 

 ionisation or not. Thus, 100 per cent, sulphuric acid is a non- 

 conductor, but if water be added it undergoes ionisation and 

 conducts. Up to a certain limit the greater the dilution the 

 greater the ionisation. Since the possession of ions makes a 



