( 460 ) 



The heat of solution is here tliei-moclieinically positive : and zero 

 in F and C. The explanation is now as follows : 



The dissolution of the solid substance a considered by itself will 

 be accompanied bv absorption of heat; the formation of the compound, 

 this being" exothermic, with development of heat. In that region of 

 temperature in which the formation of the compound chiefly takes 

 place, it may, therefore, happen that the heat of formation of the 

 compound exceeds that which is absorbed during the dissolution. This 

 is the case from F to C, between which temperatures, as seeji in 

 the triangle, the dissociation isotherms are the most divergent and 

 the quantity of the compound formed, therefore, increases most 

 rapidly with a decreasing temperature. 



The solubility -isotiierms for a.^ will, at a sufticiently low tempe- 

 rature, approach to the side h^ ah. If at a lower temperature the 

 combination is complete, the intersection of the dissociation- and melting- 

 isotherm wiW finally come very close to the angular point of the 

 compound and the projected melting point line will, therefore, become 

 an asymptote to the straight line PQ which indicates the composition 

 of the compound. It is also possible that the melting point curve 

 does not intersect the straight line PQ for the second time but 

 remains to the right of PQ. 



When a larger quantity of the compound is already present at 

 a higher temperature, the bend is shifted further to the left, so 

 that it may happen that the melting point line is not intersected 

 by PQ. 



If the formation of the compound takes place over a larger temi)e- 

 rature interval, the bend BCD F may disappear from the line and 

 there may only remain a more strongly inclined part. 



In a similar manner the solubility line for the compound ah or 

 for the component h^ may be determined. The different forms \vhich 

 these lines assume either wholly or in those parts which, OAving to 

 their mutual interference, are alone capable of existence, are again 

 entirely dependent on the manner in which the solution-isotherms 

 shift in regard to the dissociation-isotherms. 



In this way all the known cases of the meeting of the lines of 

 the components with those of the compound may be deduced. It 

 also is shown how it is possible that the compound which exists 

 partly in the liquid cannot separate in the solid condition and must, 

 therefore, again be decomposed when the components solidify. 



Another case, as yet unknown, may also be possible namely that 

 after the solidification of the two components by cooUng a liquid is 

 again formed from which on further cooling the compound is depo- 



