98 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
judge by the curves given by Etard and Engel, which all show sharp breaks, the maximum solubilities of the other salts are to be attributed to the same cause.
A true maximum in an equilibrium has been proved by Troost and Hautefeuille {15) for the dissociation of silicon chloride, and by Ditte {16) for the dissociation of hydrogen selenide. In these cases it must be assumed that the quantity of heat which is absorbed by the splitting up of the compound is at first positive, then at the maximum dissociation it changes its sign, so that at higher tempera- tures the decomposition is accompanied by an evolution of heat.
More recently an interesting example of a true recurring point has been found by Knupflfer {17) in the reaction —
TlCl + KSCN aq Z^ TISCN + KCl aq
solid. Mild.
at 32°.
The reaction expressed by the equation —
Pbia + KaS04 ^ PhSO* + 2KI
solid. dissolved. solid. dissolved.
has been investigated by A. Klein {18) \ the electrical measure- ments show that below 8° the reaction is endothermic, Le. is accompanied by an absorption of heat, and above 8° it is exothermic, i,e. takes place with evolution of heat.
The phenomenon that a chemical reaction at a sufficiently high temperature sometimes take place in the opposite direc- tion to that in which it proceeds at the ordinary temperature is of the greatest importance in chemistry. Substauces which are not stable at the ordinary temperature may be formed at high temperature, and in the same way substances may be formed at low temperature which at the normal temperature decompose with absorption of heat.
Influence of Pressure.— Besides the temperature we must also take into consideration the pressure when dis- cussing the condition of a substance. As regards the influence of pressure the following statement is quite general —
�� �
