process of incubation, or sitting for a certain length of time, are, in
the various species, different both in figure and colour, as well as in
point of number. They contain the elements of the future young, for the
perfecting of which in the incubation a bubble of air is always placed
at the large end, between the shell and the inside skin. It is supposed
that from the heat communicated by the sitting bird to this confined
air, its spring is increased beyond its natural tenor, and, at the same
time, its parts are put into motion by the gentle rarefaction. By this
means, pressure and motion are communicated to the parts of the egg,
which, in some inscrutable way, gradually promote the formation and
growth of the young, till the time comes for its escaping from the
shell. To preserve an egg perfectly fresh, and even fit for incubation,
for 5 or 6 months after it has been laid, Réaumur, the French
naturalist, has shown that it is only necessary to stop up its pores
with a slight coating of varnish or mutton-suet.