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156 MR GOODCHILD. 
grained rock. The explanation of this is, of course, that 
every solid is more or less permeable. Nay, more; many 
solids are permeable in different degrees, not only to gases, 
or to fluids, but even to other solids. To illustrate this 
general principle it is well to reflect upon what would happen 
if one were to pack a box tightly full of golf balls, and 
were then to pour, say, some peas upon it. With a little 
jolting most of the peas would find their way down into the 
spaces between the golf balls; and if one were afterwards to 
shake a quantity of dry sand over the whole, the sand, in 
its turn, would all go in out of sight too. After the box 
was filled to the brim with all of these, one could still pour 
in a large quantity of water. And even after the box was 
apparently quite filled with the four it would still be possible to 
introduce other fluids without causing the water to overflow. 
Indeed, it would be difficult to state exactly when the ab- 
solute limit had been reached. The illustration will serve 
to show that the term “solid” is a relative term; and that 
there are few, if any, substances, however compact to the eye, 
that are not pervious to some extent by something else. 
So these weak aqueous solutions of silica jelly gradually, 
and with extreme slowness, make their way downward 
through the hard and close-grained rock, working their way 
to lower and lower levels as time goes on. 
What happens when a cavity is met with can well be 
illustrated by reference to a common and well-known fact :— 
When rain water leaks through the slating of a roof it 
usually carries with it more or less soot and dust in suspen- 
sion, or, in other words, it is dirty. The dirty water soaks 
into the back of the plaster of the ceiling, and, after a time, 
oozes through. If it come in in small quantities at a time 
the dirty water soaks into the ceiling right and left of the 
place of entry; and when the water dries up it leaves a 
dirty stain on the face of the plaster. Supposing now that 
this process is repeated many times, and that the water 
containing matter in suspension time after time makes its 
way through the plaster and deposits its dirty matter, coat 
upon coat, on the ceiling, it will be obvious that the last 
coat deposited would be left nearest the floor of the room, 
and the first one nearest the ceiling; and that if the process 
