Homogeneity. This is a term of much general importance in physics. A substance is homogeneous when at edl points the properties are exactly the same. Two equal cubes cut from different portions of the substance, with their corresponding edges parallel, would be identical in properties, and could conceivably be interchanged without altering the properties of the mass. Among homogeneous substances may be mentioned glass, water, rock-crystal, and gold. Such are further subdivided into .isotropic and ceolotropic substances. The former have the properties exactly the same' in all directions; the above cubes could be interchanged in any liew positions. Glass and 'water are isotropic. AEolptropio substances have the properties at any two points exactly the same in corresponding directions; the cubes must be interchanged in such a way as will not alter the aspects of their faces. Rock crystal, selenite, and most other crystalline substances are examples of eeolotropic bodies.