352 TRANSACTIONS OP SECTION Bt 



We are indebted to America not only for these researches, and for the 

 voluminous material of H. C. Jones and his collaborators dealing with hydrates 

 in solution, but also to A. A. Noyes and his school for accurate experimental 

 work and for systematic treatment of solutions on the theoretical side. They, 

 and also Van Laar, have shown how solutions not coming within the ordinary 

 range of dilute solutions to which van't Hoff's simple law is applicable, may 

 in some cases at least be made amenable to mathematical treatment. Van't Hotf 

 chose one simplification of the general theory by considering only very dilute 

 solutions, for which very simple laws hold good, just as they do for dilute 

 gases. Even a single gas in the concentrated or compressed form diverges 

 widely from the simple gas laws; much more then may concentrated solutions 

 diverge from the simple osmotic pressure law. The other simplification is to 

 consider solutions of which the components are miscible in all proportions and 

 are without action on each other; and this method has been developed with 

 marked success from the point of view of osmotic pressure and other colligative 

 properties. 



The outstanding practical problem in the domain of electrolytic solutions is 

 to show why the strong electrolytes are not subservient to the same laws as 

 govern weak electrolytes. If we apply the general mass-action law of chemistry 

 to the electrically active and inactive parts of a dissolved substance (the ions 

 and un-ioni6ed molecules) as deduced from the conductivities by the rule of 

 Arrhenius, we find that for a binary substance a certain formula connecting con- 

 centration and ionisation should be followed, a formula which we know by the 

 name of Ostwald's dilution law. This law seems to be strictly applicable to 

 solutions of feeble electrolytes, but to solutions of strong electrolytes it is 

 altogether without application. Wherein lies the fundamental difference between 

 these two classes of solutions ? Two kinds of explanation may be put forward. 

 First, the ionised proportion may not be given accurately for strong electrolytes 

 by the rule of Arrhenius; or second, the strong electrolytes do not obey the 

 otherwise general law of active mass, which states that the activity of a sub- 

 stance is proportional to its concentration. The first mode of explanation has 

 been practically abandoned, for other methods of determining ionisation give 

 values for strong electrolytes in sufficient agreement with the values obtained 

 by the method of Arrhenius. The other explanation is that for some reason 

 the law of active mass is, apparently or in reality, not obeyed by some or all 

 of the substances in a solution of a strong electrolyte. An apparent disobedience 

 to the law of mass-action would, for example, be caused by the formation 

 of complexes such as Na 2 Cl,, or Na,Cl + or NaCl 2 ~ in a solution of sodium 

 chloride. Mere hydration, e.g., the formation of a complex NaCl, 2H 2 0, would 

 not affect the mass-action law in dilute solution, and the electrolyte would obey 

 the dilution law in solutions of the concentration usually considered. A some- 

 what similar explanation, which takes into account the properties of the solvent, 

 is that the ionising power of the solvent water undergoes a noticeable change 

 when the concentration of the ions in it increases beyond a certain limit. 



I should wish now to draw attention to a point of view which has not, so 

 far as I am aware, been fully considered. To begin with we may put to our- 

 selves the question : Is it the ions in the solution which are abnormal or is it 

 the non-ionised substance ? A simple consideration would point at once to it 

 being the non-ionised portion. We have, for example, in acetic acid a substance 

 which behaves normally, so that the ions H + and Ac", as well as the undissociated 

 molecule HAc are normal. Similarly in ammonium hydroxide the ions NH 4 * 

 and OH- as well as the non-ionised Nil, and NH,,OH all behave normally. 

 When we mix the two solutions there is produced a substance, ammonium acetate, 

 which behaves abnormally. Now, on the assumption that the equilibrium we 

 are now dealing with is 



NH 4 * + Ac-^I^NH 4 Ac, 



which of these molecular species is abnormal in the relation between its con- 

 centration and its activity? Probably not the ions Nr!,,* and Ac - , because 

 these were found to act normally in the solutions of acetic acid and ammonia. 

 The presumption is rather that the abnormal substance is the undissociated 

 ammonium acetate, for this occurs only in the abnormal acetate solution, and 

 not in the normal acetic acid and ammonia. This view, that it is the non- 



