The Book of Household Management by Mrs. Beeton

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.