V.c. iii. ; ON SOLUBILITY, 831 
An extensive communication on the solubility of mixed crystals 
published by Fock,** cast doubt upon the validity of Nernst’s rule 
relating to the solubility of mixtures of salts with a common ‘ ion.’ 
In place of Nernst’s rule that the molecular concentration of the un- 
dissociated substance is a constant quantity, he suggested that the 
sum of the molecular concentration of the undissociated portion and 
of the square of the concentration of the ‘ ions ’ is a constant.* 
1898 An investigation of the influence of potassium iodide and of 
* lead nitrate on the solubility of lead iodide in water, carried out 
by Noyes and Woodworth,** in which they applied the principle of mass 
action, led them to conclude that the product of the concentration of 
lead ‘ions’ into the square of the concentration of the iodine ‘ ions ’ 
remains constant. The theoretical aspect of this relationship was 
enlarged upon*® in terms of the dissociation theory, and shortly after- 
wards *! results of solubility measurements of silver benzoate in water 
and in aqueous solutions of nitric acid and chloracetic acid were shown 
to be in very fair agreement with the above hypothesis. Those con- 
clusions were verified further by Noyes and Chapin,®? who studied 
the effect produced by sodium acetate and sodium formate on the 
solubility of benzoic acid in water. 
1899 Meyerhoffer and Saunders °’ studied the solubility of the 
* reciprocal salt pair Na,SO,/KCl and determined what were 
the various solid phases which can be formed and the conditions of 
their existence. 
Grave doubt was thrown upon the general utility of Noyes’ results 
by Arrhenius,°? who proved that the undissociated portion of a spar- 
ingly soluble salt by no means remains constant; as had been assumed 
hitherto, during the addition of a salt containing a common ‘ ion’: 
a result which, if true, entirely invalidates Noyes’ method of cal- 
culating diminutions of solubility. 
1900 Touren”* studied the variation in solubility of potassium 
* nitrate when in the presence of potassium chloride and bromide 
respectively. He found that equivalent—molecular—quantities of the 
halogen salts have equal effect upon the solubility of the nitrate, but 
that the nitrate and the bromide do not reduce the solubility of the 
chloride by the same amount. Results of a somewhat similar nature 
to the above were published by Winteler** who carefully measured 
the solubility of potassium chloride and of sodium chloride respectively 
in solutions of potassium hydroxide and sodium hydroxide, and of 
sodium chlorate and potassium chlorate in solutions of sodium chloride 
and potassium chloride respectively. Von Hnde ** showed that the 
addition of increasing quantities of hydrogen chloride or of potassium 
chloride to aqueous solutions of lead chloride reduced the solubility of 
the lead salt to a minimum, after which it increased. 
1901 - Groschuff,8° when endeavouring to determine the solubility 
* of acid barium oxalate, found that this salt is decomposed by 
water ; his determinations, therefore, have reference to measurements of 
the solubility of normal barium oxalate in presence of oxalic acid. 
* Tn a later publication‘ this author again called the Nernst rule into question, 
