6 NEW YORK STATE MUSEUM 



pane. The formation of crystals may be reproduced in a very 

 striking manner; take for example a strong solution of salt 

 and set it aside in a shallow dish over night; after the water 

 has evaporated the bottom of the dish will be found covered with 

 small cubic crystals of salt. The forces of nature working much 

 more slowly but in a similar way have produced the vast deposits 

 of native salt or halite which sometimes yield very large crystals. 



Crystal masses 



When a number of crystals are formed in a limited space the 

 individual crystals intersect and lap over one another producing 

 what is known as crystal masses. If this intersecting is carried 

 to such an extent as to entirely fill the bounded space, leaving 

 no interstices between the crystals, the mineral is said to be 

 massive. The term massive in its broader sense includes mineral 

 masses which do not show definite crystal faces but which in 

 most cases can be shown to be distinctly crystalline by means 

 of cleavage and optical properties. Experiment and study of 

 mineral deposits show that a liquid substance which is cooled 

 slowly or a solution which is concentrated gradually tends to 

 form large and perfect crystals while substances which are solid- 

 ified rapidly produce small and ill defined crystals, often giving 

 rise to massive forms. A substance which displays no evidences 

 of crystallization is said to be amorphous as distinct from crys- 

 talline. Glass is a good example of an amorphous substance. 



Laws of crystals 



A complete study of all known forms of crystallized sub- 

 stances has shown that the formation of crystals is subject to 

 the following laws: 



1 Law of constancy of interfacial angles 



2 Law of symmetry 



3 Law of simple mathematical ratio 



Law of constancy of interfacial angles 

 In all crystals of the same substance the angle between any 

 two like faces is constant. Fig. 4 and 7 show two crystals of 



