3U5 TWENLTY-SECOND EVENING.

The different kinds of slate, too, are stones of the
argillaceous class; and very useful ones for covering
houses, and other purposes.

H. Ave writing-slates lke the slates used for cover-
ing houses ?

T. Yes; but their superior blackness and smooth-
ness make them show better the marks of the pencil.

G. You have mentioned something of sand and
flints, but you have not told us what sort of earths
they are.

Y. I veserved that till I spoke of the third great
class of earths. This is the szliceous class, so named
trom silex, which is Latin for a flint-stone. They
have also been called vitrifiable earths ; because they
are the principal ingredient in glass, named in Latin
vilrum.

G. 1 have heard of flint glass.

Z. Yes; but neither flint, nor any other of the kind,
will make glass, even by the strongest heat, without
some addition; but this we will speak of by-and-by.
1 shall now tell you the principal properties of these
earths. ‘They are all very hard, and will strike fire
with steel, when in a mass large enough for the stroke.
They mostly run into particular shapes, with sharp
angles and points, and have a certain degree of trans-
parency ; which has made them, also, be called erys-
fallme earths. They do not in the least soften with
water, like clays; nor are they affected by acids; nor
do they burn to lime, like the calcareous earths. As
to the different kinds of them, flint has already been
mentioned. It is a very common production in some
parts, and is generally met with in pebbles, or round
jumps. What is called the shingle on the sea-shore,
chiefly consists of it; and the ploughed fields, in some
places, are almost entirely covered with flint stones.

41, But do they not hinder the corn from growing ?

i. ‘The corn, to be sure, cannot take root wnon
them; but, I believe, it has been found that the pro-
tection they afford to the young plants which grow