166 MR GOODCHILD. 
is quite uncrystalline; the latter crystalline entirely. Light, 
heat, electricity, ete., affect the non-crystalline form quite 
alike in every direction, while in the case of the crystalline 
form, these forms of energy act differently in accordance 
with certain laws relating to the external form of the 
erystal. There are other properties besides these which 
need no more than a passing allusion here. 
Now, Opal, in its original unmodified condition, consists of 
non-crystalline (or colloid) Silica combined with a variable per- 
centage of Water. Some non-essential constituents may also 
be present without materially affecting its general properties. 
Chalcedony, on the other hand, consists of a mixture of 
opaline or colloid silica with one of the crystalline forms of 
SiO,. The precise nature of this crystalline silica has not 
yet been satisfactorily determined ; but, pending the results 
of further investigations, this fibrous constituent of Chalce- 
dony is termed Quartzine. Nobody knows yet what cause 
determines whether a solution of silica jelly in water shall 
consolidate as the hydrous colloid Opal, or shall eventually 
become Chalcedony. Nor does it appear to be known with 
certainty whether the colloid Opal may change in time into 
the hemi-crystalline Chalcedony, though the fact that all 
colloids tend to pass into their crystalline equivalents (just 
as Barley Sugar cannot be prevented from becoming Sugar 
Candy) would seem to point to some such conclusion. 
What, however, seems to have happened has been this: as 
the composition of the solutions varied, or, as the conditions 
under which they were deposited changed, different degrees 
of Surface Energy were set up between the solutions and the 
surface upon which they were being deposited. Solutions 
tending to consolidate as Chalcedony certainly were affected 
more powerfully by Surface Energy than by Gravitation, as 
is shown by the fact that in many cases they cling on in 
larger quantities to the roof of the cavity than they do to 
its sides, or even to the floor. Other solutions, which 
appear to have been originally deposited as Opal, have con- 
formed absolutely to Gravitational Energy, and have arranged 
themselves in perfectly parallel layers and in an absolutely 
horizontal position, without climbing the sides in the least. 
The difference in comportment of the two layers is very 
