v GRADATION BETWEEN LIQUIDS AND SOLIDS. 81 



not be applicable to the same extent to crystallized silicate rocks, but it 

 seems to me highly probable that they apply in large measure to many. 

 In so far as Barus's final conclusion is applicable, there may result all grada- 

 tions, from solutions in which the water is the dominant constituent to 

 those in which it is the subordinate constituent. This principle of the 

 increased quantity of material which may be held in solution as a result of 

 combined high pressure and temperature is believed to possess very great 

 significance in alterations in the zone of anamorphism, and to be of impor- 

 tance in alterations in the belt of cementation. (See pp. 602-603, 659-661.) 



Relations of solution to absorption and liberation of heat. As already explained, wlieil 



material passes into solution the molecules are separated and acquire 

 kinetic energy, and are believed by many to change from the solid to the 

 gaseous form. This process absorbs heat. On the other hand, where the 

 volume of the solution is less than the volume of the solvent and salt sepa- 

 rately, the molecules of the solvent and salt combined are brought closer 

 together and heat is therefore liberated. In the reverse case, where the 

 volume of the solution is greater than that of the solvent and salt separately, 

 the molecules are pushed farther apart, and heat is absorbed. If the com- 

 pounds in solution separate into ions this process is believed to be usually 

 attended by liberation of heat." Whether there is a rise or fall of tempera- 

 ture of the solution will depend upon the relative values of these factors. 

 In the common case where there is decrease in the volume as a result of 

 solution, the heat thus liberated by change in volume plus the supposed 

 heat of ionization are together preponderant, and there is, therefore, libera- 

 tion of heat and a rise in temperature. However, in the case where there 

 is increase in the volume as a result of solution, the heat thus absorbed and 

 the heat absorbed in changing the salt from the state of a solid to that of a 

 gas is greater than that supposed to be liberated by dissociation. The first 

 two factors are dominant, and there is usually a marked absorption of heat 

 and, consequently, a fall in the temperature of the solution. This is illus- 

 trated by the solution of ammonium chloride in water. The volume is 

 considerably decreased and the fall in temperature is very decided. 



oNernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895, 

 p. 562. 



MON XLVII — 04 6 



