ON EARTHS AND STONES, 803

mixed with the soil. And even then it is reckoned
a hot and forcing manure, chiefly fit for cold and wet
lands. ‘The principal use of lime, however, is as an
ingredient in mortar. This, you know, is the cement
by which bricks and stones are held together in
building. It is made of fresh-slacked lime and a pro-
portion of sand well mixed together; and generally
some chopped hair is put into it. The lime binds with
the other ingredients; and, in length of time, the
mortar, if well made, becomes as hard, or harder, than
stone itself.

G. I have heard of the mortar in very old build-
ings being harder and stronger than any made at
present.

Z. That is only on account of its age. Burning of
lime, and making of mortar, are as well understood
now as ever; but, in order to have it excellent, the
lime should be of good quality, and used very fresh.
Some sorts of lime have the property of making
mortar which will harden under water, whence it
is much valued for bridges, locks, wharfs, and the
hike. |

G. Pray, is not plaster of Paris a kind of lime?
I know it will become hard by only mixing water with
it; for I have used it to make casts of.

7. The powder you call plaster of Paris is made of
an earth named gypsum, of which there are several
kinds. Alabaster is a stone of this sort, and hard
enough to be used like marble. The gypseous earths
are of the calcareous kind, but they have naturally
a portion of acid united with them, whence they
will not effervesce on having acid poured on them.
But they are distinguished by the property, that,
after being calcined or burned in the fire, and reduced
to powder, they will set into a solid body by the addi-
tion of water alone. This makes them very useful
for ornamental plasters, that are to receive a form or
impression, such as the stucco for the ceiling. of
rooms.