THE YOUNG SCIENTIST. 
147 
whether smooth like the beech, or rough 
like the oak, just as you do in pencil 
drawing. The foliage is then represented 
by either the curved or the angular, zig- 
zag line. In all cases the line must fol- 
low the masses of the leaves. 
In treating rocks you will find that the 
rough texture will be obtained best by 
short, sharp cross-lines. 
Now it only remains for me to speak of 
the sky, which with beginners is always a 
puzzle. It is best to leave it alone until 
you have gained considerable delicacy as 
well as freedom of touch. The sky is 
sometimes the most important part of the 
whole picture, and I would advise you to 
make a special study of the manner in 
which our painter etchers are treating the 
subject. As a rule, you must either leave 
it white, or go over the whole space in- 
telligently; not necessarily covering it 
with lines, but indicating all that goes to 
make up the effect in outline. If you 
wish to represent a perfectly clear sky, by 
lining as engravers do, you must keep the 
lines close together at the top, and draw 
them finer and wider apart as you de- 
scend to the horizon; but they must 
be verij delicate, or you must treat them 
with stopping out varnish to make them 
print lightly. In cloudy skies the top is 
frequently the lighter part, and near the 
horizon the darkest ; but remember that 
no portion of sky or cloud must be as 
dark as the shadows on the ground. 
[It is, of course, impossible to give an actual 
illustration of these etchings in the journal, but 
in a future number we shall give a fac simile en- 
graving of a very fine etching, which has been 
sent to us by Mr. Hartley. This will enable our 
readers to judge of the capabilities of this 
method.-ED. Y. S.] 
Sponge Spicules. 
TTTHEN our young readers take a sponge 
** in their hands, they probably little 
think of wfiat it is, and of its curious 
structure. The common sponge used in 
the bath-room, is the type of a very large 
family, the members of which differ 
widely amongst themselves. Some kinds 
inhabit our fresh water streams and 
lakes, but these are generally small, and 
comparatively few people, even amongst 
those who have given some attention to 
the inhabitants of fresh water, are famil- 1 
iar with them. The sponges vary so much 
in appearance and character that some 
of them would hardly be recognised as 
belonging to the same group. Thus, for 
example, few would suppose that the 
coarse sponge used by stable boys, and 
for other rough work, is any relation to 
the beautiful Venus Flower Basket, which 
is so prized as an ornament for the man- 
tel piece ? and yet they are both sponges. 
The beautiful frame work which we 
know as Venus' Flower Basket, is really 
the siliceous or flinty skeleton of a kind 
of sponge, and other kinds of sponges are 
also provided with these flinty frames, 
though none are so perfect and beautiful 
as the one just named. In general the 
SPONGE SPICULES. 
flinty part of the sponge takes the form of 
a collection of spicules, and these spicules 
are found in the most diverse forms, some 
of which we have figured in the accom- 
panying engraving. Some, as the upper- 
most one, is simply a rough body armed 
with spines; others take the form of 
stars ; others of curious club-shaped rods, 
while others take the form of anchor- 
