Agate— Physical Properties and Origin 11 
forms, such as animalculae and worms. Examination 
shows these views to be incorrect, and they are not 
held by investigators at present, especially as close 
imitations of all of these forms can be obtained by 
wholly inorganic processes. If one introduces crystals 
of green vitriol (ferrous sulphate) into water-glass 
(a solution of sodium silicate), green threads gradu- 
ally rise into the colloid and scatter through its sub- 
stance in such a manner as to closely imitate the moss 
agates. A comparison of a genuine moss agate with the 
formation given by green vitriol is shown in Plate V. 
Similar fibers, except that they are red instead of 
green, are produced by crystals of iron chloride 
when immersed in water-glass. The crystal (of iron 
vitriol or iron chloride) is at first covered with a thin 
coating of iron silicate. Through this porous coating, 
water is drawn by osmose 1 toward the crystal from the 
water-glass until the difference of specific gravity 
causes the lighter vitriol solution to break through 
the coating and rise into the water-glass. It then 
becomes coated with the silicate and the process is 
repeated until a tube rising to the surface is formed. 
The interior of these tubes is filled with undecomposed 
vitriol solution and being thinner than the water-glass, 
osmose is carried on in a direction transverse to the 
walls of the tube as well as toward the surface. This 
causes the deposition of concentric layers about the 
tubes corresponding to the banding of common agates. 
'Osmose, from the Greek word meaning to push, is a term 
applied to the mixing, or tendency to mix, of two liquids by 
passage through a membrane or porous wall separating them. 
Each liquid passes through the partition into the other, but 
generally, one, usually the thinner or less concentrated, does 
so with greater rapidity than the other, so that a difference 
of pressure or volume is produced on the two sides. The more 
rapid passage is properly called endosmose and the slower 
exosmose, though the names are sometimes given respectively 
to inward and outward osmose without reference to the ra- 
pidity of the process. (Adapted from the Standard Diction- 
ary.) 
[116] 
