142 Dr. M. Wildermann on the Law of 



III. The Law of Dilution as deduced from the 

 Freezing-Point. 



We proceed to illustrate one of the most important, if not 

 the most important, of the foundations of the theory of disso- 

 ciation, namely, Ostwald's law of dilution based upon the 

 freezing-point. (Planck's name also should be mentioned 

 here.) It was notably Ostwald who first showed the relation 

 between the numbers of dissociated and undissociated molecules 

 to depend upon the action of mass, and who took the pains to 

 verify the same experimentally in the case of two hundred 

 acids by means of their electric conductivity. From the 

 above-mentioned generalization of Arrhenius it follows that 

 the law of dilution ought also to be deducible from the 

 freezing-point, since the freezing-point as well as the electric 

 conductivity enables one to know the degree of dissociation, 

 and it may be at the same time interesting, inasmuch as we 

 have not here to do with velocity of ions or with the theory 

 of the electric conductivity. On the other hand, from the inti- 

 mate connexion which exists between Yan't Hoff's laws and 

 the dissociation theory, it follows that the law of dilution must 

 find its experimental confirmation in dilute solutions. This 

 has not been possible before now, because the constant de- 

 duced from the law of dilution is greatly affected by the 

 experimental error, and the freezing methods were till now 

 far from being sufficiently exact for this purpose. But we 

 have now made a sufficient advance in the freezing methods 

 to be able to undertake this proof under favourable circum- 

 stances. Here we have to take into account two conditions : 

 first, the degree of dissociation must not be small, espe- 

 cially because, in consequence of the nature of the calculation, 

 the experimental error affects not the whole value observed, 

 but only that part of it which gives the degree of dissociation; 

 secondly, the degree of dissociation must very considerably 

 change with the degree of dilution, or else the influence of 

 dilution is difficult to ascertain. The law of dilution can 

 therefore only be proved in the case of a few substances 

 which fulfil these conditions. Dichloracetic acid and ortho- 

 nitrobenzoic acid are suitable for this purpose. 



Below are given the constants calculated according to the 

 law of dilution in the cases of dichloracetic acid and ortho- 

 nitrobenzoic acid. In the last Table are given under " a cor- 

 rected " the concentrations of the solutions, and under A the 

 degree of dissociation, when t=l'S7. The constant K given 

 by the law of dilution appears indubitably, but is smaller 



