Crystallogenesis. — Hensoldt. 375 
Acrothele, Kutorgina, Iphidea and Protospongia. The Agnos- 
i are very common in the upper beds of the formation, and the 
Orthis characterizes also the upper beds. 
[To be continued.] 
CRYSTALLOGENESIS. 
By Dr. H. Hensoldt. 
School of Mines, Columbia College, New York. 
II. 
The crystals of the isometric system, which are equi-axed, 
equi-expansive and singly refractive, are doubtless composed 
of particles which have experienced the least amount of com- 
pression, compatible with their symmetrical arrangement. 
They are either spherical or very nearly so, and it is significant 
that of the twenty-two elementary bodies of which the crystal- 
line forms have been hitherto ascertained, no less than fifteen 
are referable to the isometric system. In the crystals of the 
tetragonal and hexagonal system the molecules are so com- 
pressed that structural differences are developed in two direc- 
tions, and in the forms of the remaining systems three direc- 
tions of structural variation have been similarly originated. 
That the double refraction of crystals is not inherent in the 
molecules, but is an acquired property, is obvious from the 
fact that in crystallized silica (Quartz, Amethyst, etc.,) we 
have double refraction, while amorphous Silica (Flint, Opal, 
Tabasheer, etc.,) is singly refractive. If the molecules of water 
were identical in form with those of ice, they should also be 
endowed with the same properties, j^et in ice we have double 
refraction, while water is singly refractive. 
If we dissolve fifteen grains of Chloride of Sodium in an 
ounce of distilled water and allow a drop of this solution to 
evaporate slowly on a glass slide' under the microscope, we 
may learn, if we watch the process in its final stages with a 
"roth" objective of good definition — many of the secrets of crys- 
tal life. The commencement of the operation of crystalline 
'The 8lide should be previously cleaned with alcohol, and it in not 
advisable to afcelerate the evaporating process by artificial means, 
such as heating the slide over a spirit lamp. On the contrary, the 
slower the evaporation, the ]>etter the phenomena here referreil to are 
observed. Weak solutions give the best results, especially if the ex- 
periments are made in a cold room. 
