174 SAO DITE SHO Re SIROLDLEAN TES) 
Unequal expansion and contraction of the rock.—TYhe heat con- 
ductivity of stone is very low. A stone a few inches in thickness 
may be heated on one side to a temperature sufficiently high that 
it will not bear handling, while on the other side the stone may 
be comparatively cold. The actual expansion of different kinds 
of stone has been experimentally determined by W. H. Bartlett," 
in which he obtained the following results: 
Granite, .000004825 inch per foot for each degree F. 
Marble, .000005668 inch per foot for each degree F. 
Sandstone, .000009532 inch per foot for each degree F. 
The diurnal changes in temperature in this latitude are often 
as much as 50° F., while the annual variation in temperature 
exceeds 150, F. A difierence of 150° F. would make a ditter 
ence of one inch in a sheet of granite 100 feet in diameter. 
Each mineral of which a stone is composed has a different 
rate of expansion. Whenever a stone is heated each particle 
presses against its neighbors with almost irresistible force. 
When cooling begins, contraction sets in which initiates stresses 
pulling the individuals apart. The inequalities in the rate of 
expansion of the different mineral particles initiate stresses in 
rocks having a heterogeneous composition, which tend to sepa- 
rate the individual minerals from their neighbors. The result of 
these alternating temperatures is to weaken the rock and produce 
small cracks into which water may percolate or roots descend. 
Besides the unequal expansion and contraction of the mineral 
particles, there is an unequal expansion and contraction between 
the different laminae or hypothetical layers of the rock which 
are near enough to the surface to be affected by the atmospheric 
temperatures. The layer at the surface suffers the greatest 
change in temperature, and is therefore most affected. Each 
succeding layer is less affected until a point is reached where 
there is little or no change in the temperature the year around. 
Owing to the rapid diurnal changes in temperature in some regions 
forces are constantly at work tending to separate the superficial 
stratum from those immediately below. 
t American Journal of Science, Vol. XXII, 1832, p. 136. 
