AGATES, CARNELIANS, AND JASPERS. 157 
is only continued long enough, quite a thick deposit may 
accumulate, all of which may cling firmly to the ceiling. 
Suppose, now, that the quantity of dirty rain water 
leaking in on any one occasion may be enough to wet the 
ceiling outward to the plaster of the walls. In that case 
the stain will affect the plaster of the walls too. Note, 
further, that unless the quantity exceeds a certain amount, 
not a drop from the leak will descend to the floor—the 
ceiling and the walls will hold it all up. The principle 
upon which this depends is one of great importance in the 
making of agates, as well as in numerous other departments 
of Nature’s operations. It is customary now to refer to the 
attractive force which makes the water cling to the plaster 
instead of dropping on to the floor as Surface Energy or 
Surface Tension. It is the same cause which makes it so 
difficult to pour tea out of certain teapots without letting 
that fluid fall outside the tea cups. Ladies, when pouring 
out tea, have come, by force of habit, to give the teapot a 
slight jerk at the instant of beginning to pour out the tea. 
This gives Gravitation a little advantage at the outset, and 
turns the balance against Surface Tension, which is doing its 
best to make the fluid adhere to the surface of the teapot, 
and therefore to flow in connection with it as far as the 
shape of the surface permits. To put this into other words, 
the surface energy between the tea on the one hand, and 
the air and the surface of the teapot on the other, are 
sufficient to overcome Gravitation. If a teapot of a 
different kind were to be used it would probably be found 
that the result varied. So, too, if a different fluid were 
employed in the same teapot the result again would be 
different. In other words, the degree of surface energy 
between fluid and solid varies with the nature of each, 
and is probably not quite alike in any two cases. The 
principle thus illustrated is of the highest importance in 
the process of making an agate. 
In the actual process of agate growth it very commonly 
happens that the first coat or priming received by an agate 
consists of a film of one of the group of decomposition- 
products arising from the alteration of Augite by the aetion 
VOL. I. 13 
