504 JOHN JOHNSTON AND PAUL NIGGLI 
investigations show that the change of melting-point for the first 
1,000 atm. is less than 4o°;' further, that for each succeeding 
1,000 atm. the effect is progressively less, a fact which indicates 
that no maximum melting-point is realizable (if indeed it is pos- 
sible). 
There are only two substances for which a lowering of melting- 
point by uniform pressure is established, namely, water and bis- 
muth; and with water only at pressures up to about 2,100 atm. 
(where the m.p. is —22°), above which the melting-point rises 
steadily with pressure, reaching + 76° at 20,000 atmospheres. 
There may be a few other substances belonging to this category 
but their number is certainly small. In general, therefore, uniform 
pressure raises the melting-point; so that we may say that uniform 
pressure increases the rigidity of material—an increase which 
becomes very noticeable with substances (e.g., the oils commonly 
used to transmit pressure) whose freezing-point is not far removed 
from the ordinary temperature. 
EFFECT OF UNIFORM PRESSURE ON SOLUBILITY 
The influence of temperature upon the solubility of solids varies 
greatly from one substance to another; the more common effect is 
an increase of solubility with temperature, but there is a large 
number of substances (e.g., Ca(OH),, CaSO,2H.O, to name two 
common ones) whose solubility decreases with rise of temperature. 
The direction and magnitude of this change of solubility is deter- 
mined by the sign and magnitude of the heat of solution; an absorp- 
tion of heat corresponds to an increase of solubility with tempera- 
ture, an evolution of heat to a decrease. In applying this criterion, 
one must take care to choose the heat effect? appropriate to the 
dissolution of the particular solid phase which is in equilibrium with 
the solution under the particular conditions; for instance, to use 
the heat of solution of Ca(OH)., not of CaO, and at temperatures 
«With one exception, camphor, where 300 atm. pressure raised the melting- 
point by 38.7° (Hulett, Joc. cit.). 
2 The heat effect to be used is always that observed when 1 mol. of the solid is 
dissolved in a large volume of a solution nearly saturated with respect to the solid in 
question. 
