72 PHYSICAL SCIENCE 



sink below the cryohydric point, these Hquid films 

 themselves solidify ; but, even then, the mass is 

 not a homogeneous solid, for the cryohydric con- 

 glomerate forms a cement-like connection between 

 the primary crystals of pure ice. We see now the 

 explanation of the fact that a block of natural 

 ice, taken from a glacier or lake, has a definite 

 structure, and may be resolved into a heap of 

 separate crystals by exposure to the sun. The 

 cryohydric cement dissolves first at the lower 

 temperature, and thus the primary crystals of 

 pure ice fall away from each other before the 

 temperature rises to their melting-point. 



Phenomena precisely similar to those we have 

 described appear when a fused metal is allowed 

 to solidify. Crystalline structures of pure metal 

 form in the liquid, and grow till the whole mass 

 becomes solid. These primary crystals usually 

 start as fernlike forms, of which a beautiful 

 example is shown in Fig. 2. This represents 

 the microscopic structure of a bronze ingot, 

 suddenly chilled from a temperature of 644° C. 

 If the crystals be allowed to grow by very slow 

 cooling, they may come to fill nearly the whole 

 mass, as in the case of the section of iron shown 

 in Fig. 3. Even in this case, with a substance 

 nearly as pure as can be obtained, the lines of 

 separation between the primary crystals are 

 clearly visible ; the primary crystals are differ- 

 ently orientated, and their faces reflect the incident 

 light at different angles. The crystals of zinc are 

 often remarkably large and well defined, and fine 

 specimens can be seen on surfaces of so-called 

 galvanised iron, such as is used for water-cisterns. 



