lue, but if it requires a mortar and
pestle to break, its quality is pronounced good. From an analysis of
this singular bark, that of old trees has been found to give 30.8 per
cent of ash, and that of young 23.30 per cent. Of the different layers
of old bark, the outer gave 17.15 per cent, the middle 37.7, and the
inner 31. The wood of the tree, in comparison with the bark, is
relatively poor in silex, the duramen of an old tree giving only 2.5
per cent of silex.

* * * * *

GLASS SPONGES.

The natural history of sponges had, up to the middle of this century,
been comparatively neglected. Until 1856, when Lieberkuhn published
his treatise on sponges, very little or nothing had been written on
the subject. Later, Haeckel did much to determine their exact nature,
and it is now universally admitted that sponges form one of the
connecting links between the animal and the vegetable kingdom.

Sponges, generally considered, consist of fine porous tissue, covered,
during life, with viscid, semi-liquid protoplasm, and are held in
shape and strengthened by a more or less rigid skeleton, consisting
chiefly of lime or silica. The tissue consists of a very fine network
of threads, formed probably by gradual solidification of the threads
of protoplasm. The inorganic skeleton is formed by larger and smaller
crystals and crystalline threads. In the various families of sponges
the quantity of inorganic matter varies greatly; some sponges are
nearly devoid of an inorganic skeleton, while other families consist
chiefly of lime or silica, the organic tissue being only rudimentarily
developed.

As observed in their natural state, sponges are apparently lifeless.
When, however, a live sponge is placed in water containing some finely
powdered pigment in suspension, it will be noticed that in regular,
short intervals water is absorbed through the pores of the tissue and
ejected again through larger openings, which are called "osculae."
Following up these into the interior, we find them divided into
numerous branches, the walls of which are, under the microscope, found
to be covered with minute cells, fastened at one end only and
oscillating continually. By means of these cells the sponge receives
its nourishment.

Sponges with very rigid inorganic skeletons may be divided into two
classes--calcareous and silicious--according to whether the skeleton
is chiefly composed of lime or silica.

Our eng