SUMMARY OF PHYSICO-CHEMICAL PRINCIPLES. BA 
such an increase in temperature; and it is this fact which, primarily, 
eives the preceding proposition its universal validity.”* “If we heat 
the system therefore, the reaction which takes place will be accompanied 
by absorption of heat; if we cool the system, the corresponding reaction 
will develop heat.” + ‘‘On the whole, the preponderating chemical re- 
actions at lower temperatures are the combinings (associations) which 
‘take place with the development of heat: while the reactions preponder- 
ating at higher temperatures are the cleayings (dissociations) which take 
place with the absorption of heat.” { This last is van’t Hoff’s law. 
The meaning of this law may be illustrated by the following reactions : 
At ordinary temperatures CO combines with O, producing CO,, with great 
liberation of heat; at very high temperatures CO, dissociates into CO and 
O, with very great absorption of heat. This illustration makes it clear 
that van’t Hoff’s law, as stated by Nernst, must replace that of Satz and 
Berthelot, that ‘“‘every chemical change gives rise to the production of 
those substances which occasion the greatest development of heat.” § 
However, this rule, according to Nernst, usually agrees with experiment, 
and he concludes that, “other things being equal, there is the more chance 
that a substance can be formed, the greater its heat of condensation.” || 
Tn general, in comparing substances which are chemically analogous, 
and soluble with difficulty, the heat of precipitation (= the negative 
value of the heat of solution) is greater the more insoluble the sub- 
stance is.” 4] 
Finally, the relations between heat, pressure, and chemical action in 
a solution may be generally expressed as follows: “ Every change of one 
of the factors of an equilibrium occasions a rearrangement of the system 
in such a direction that the factor in question experiences a change in a 
sense which is contrasted with the original change.” ** 
APPLICATION OF PHYSICO-CHEMICAL PRINCIPLES TO THE 
EARTH'S CRUST 
It is evident from the foregoing principles that within the superficial 
zone of rocks in which reactions take place directly under our observa- 
tion, and within the deeper-seated zone in which reactions have taken 
place and later have been brought within our observation, there may be 
* Nernst, loc. cit., p. 566. 
+ Outlines of general chemistry, by W. Ostwald: London and New York, 2d ed., 1895, p. 312. 
t Nernst, loe. cit., p. 583. 
2 Loe. ecit., p. 581. 
|| Loe. eit., pp. 585, 586. 
{ Loe. cit., p. 504. 
** Loc, cit., p. 567. 
XLII—Butt. Gron. Soc. Am., Vor. 9, 1897 
