AGATES, CARNELIANS, AND JASPERS. 161 
rain-water would first gather into little spheroidal forms. 
This shape they would naturally assume under the influence 
of Surface Tension, under which impulse the water molecules 
are led to arrange themselves in that form which offers the 
least surface in proportion to the volume of the whole—that 
is to say, they tend to group themselves as spheres. Now, if 
the quantity of water leaking in is exactly equal to the 
waste by evaporation, the spheroid remains unaltered. If 
evaporation exceeds the supply, the spheroids spread more 
and more over the adjoining surface, because that surface 
attracts them more than their own molecules attract each other. 
If the supply exceeds the waste, the size of the water globule 
gradually increases, and then gravitation begins to outbalance 
the force that held the molecules together in the spheroidal 
form. In other words, the. drops of water begin to lengthen 
out, and may continue to do so until gravitation overcomes 
the force of cohesion and some of the water begins to drip. 
Conceive, instead of pure water, some containing just 
enough silica jelly to make it slightly cohere, just as it 
would do if a very little soap were dissolved in it. Now, 
if the ingress of silica jelly takes place, as it is here 
assumed that it has done, at a rate sufficiently slow to 
permit of a certain amount of coagulation of the deposits as 
they are formed one after the other, it is obvious that the 
drops, as they lengthened, would not necessarily break, but 
would simply increase a little in diameter, and much more 
in length. I may illustrate this rather important point 
a little further by asking the reader to imagine a ceiling 
being painted several times in succession with a few days’ 
interval between each coat, and that each coat is put on 
rather thick, with paint of a somewhat more fluid kind than 
usual. In this case some of it would begin to collect into 
little globules, which, under the joint operation of gravitation 
and the oxidation of the paint, would lengthen out into tear- 
like drops. Repeat the process many times, and the whole 
would result in a little group of stalactitic masses pendant 
from the ceiling. Each fresh coat of paint over the hardened 
surface of the one previously laid on would accumulate in 
largest quantity at the base of each drop, because there 
the Surface Energy exerts its fullest effect. 
