20 THE PRINCIPLES OF SOIL MANAGEMENT 



changes in temperature. All substances change volume 

 with changes in temperature. Different minerals expand 

 at different rates, and the same mineral may have 

 different rates of expansion along different axes. So 

 that, when a rock made up of several minerals has its 

 temperature changed, it expands unequally, and a 

 strain is set up all through the mass, which, if severe 

 enough, and repeated often enough, will break it into 

 small fragments. Further, even if a rock did expand 

 uniformly in all its parts with changes of temperature, 

 these changes of temperature are far from uniform. 

 Heat is conducted slowly into a rock. Since the rock 

 may have very different temperatures at points a short 

 distance apart, as a result of this slight conductivity 

 a great strain may result from expansion due to tem- 

 perature differences. Merrill quotes Bartlett to the effect 

 that granite expands .000004852 inch per foot for each 

 degree of Fahr., marble .000005668 inch, and sandstone 

 .000009532 inch. While these movements appear 

 exceedingly small, they are multiplied through many 

 feet of rock and through many degrees of temperature. 

 The differences in temperature between day and night 

 on rock surfaces exposed to the sun is extreme, although 

 it varies with the color of the rock. (2) When water is 

 carried below its freezing-point, it may be exceedingly 

 destructive. In freezing, water expands about one- 

 eleventh of its volume. It has been determined that 

 water at a temperature of 1C. exerts an expansive 

 force of 150 tons per square foot, and that to keep it 

 from becoming ice would require the weight of a column 

 of granite 1,800 feet high. All rocks are somewhat 



