l88o.] 
On Water and Air. 
5i 
of the kettle, and this is due to the hardness (as we call it) 
of the water. By boiling, a great amount of the mineral 
matter is precipitated and is rendered solid, and it settles 
upon the interior surface of the kettle, and produces that in- 
crustation to which I have referred. Here is a copper tube 
which belonged long ago to a boiler at the Athenaeum rib , 
and if after the ledture, you come forward and examine this 
tube vou will find it coated so that it is almost choked up 
with’ a series of beautiful concentric incrustations of the 
solid matter which was contained in the tap-water supplied 
to the Athenaeum Club and which was deposited upon the 
interior surface of the tube. 
Not only will boiling liberate and re-solidify dissolved 
matter, but evaporation does the same. If you go to St. 
Govor’s Well, in Kensington Gardens, and look at where 
the water drips or splashes down from the little outlet, you 
will see a red mark where it falls. That is oxide of iron, 
and if you taste the water you will find that it has an inky 
taste As the water splashes down, it is in part evaporated, 
and the iron is liberated there in the form of this oxide of 
iron. The reason why caverns are usually found in lime- 
stone strata is that limestone is more soluble in water than 
most other rocks ; and if you wander, as I have done, in 
limestone caves, you will usually find in each cave a stream 
of water which has washed out the limestone and produced 
the cavern ; and sometimes you see from the roofs of those 
caverns— I was going to say most beautiful icicles but 
stalactites hanging down. These are due to the water 
which has entered into the fissures in the roof above, and 
percolated through the roof and dissolved some of the 
limestone, and made its way into the cavern, where the 
water is in part evaporated ; and, in consequence of the 
evaporation, the solid matter has been deposited, and you 
find that, as it evaporates, these beautiful stalactites grow 
longer and longer from the roof towards the floor, then 
drops of water fall from the end of the stalactites upon the 
floor and there the water is still further evaporated, and a 
heap is produced called a stalagmite ; and. as the water con- 
tinues to drip, more and more of the solid matter is deposited 
below, so that the stalagmite grows from below upwards, 
while the staladtite grows from above downwards, and by- 
and-bye they meet in the centre, midway, with the point of 
the staladtite aftually in the centre of the stalagmite, and 
most wonderful and fantastic pillars are thus produced, it 
you visit any of the great limestone caves you will see 
examples of this kind. Here are some stalactites from 
E 2 
