252 Dr John Davy on the Hygs of Birds. 
or the time of its incubation,—another proof of the great 
porosity of the shell and its membranes. 
Without any exception yet known, the eggs of all birds 
consist of albumen and yolk. These in the egg of the 
common fowl exhibit different and opposite reactions, and 
different electrical states,—the albumen being alkaline, the 
yolk acid; and in the instances of the eggs of the smaller 
birds, I have, as regards the reaction, observed the same ; I 
have had no opportunity to test their electrical condition ; 
but judging from analogy, that also may be inferred to be 
similar. 
I shall now briefly advert to some points of difference. 
I need hardly notice that of form, and that each kind has 
commonly a specific shape as well as size, and that though 
one pole is commonly bigger than the other, the one, as 
already mentioned, which, in the act of laying, is first pro- 
truded, and which contains the air-vesicle, is never, that I 
am aware of, smaller than its opposite, seemingly a happy 
provision, securing the successful propulsion of the egg 
without delay under the influence of an adequate impelling 
power: did the smaller end first present, it might act as a 
wedge, and the processes of distension and of laying might 
be protracted, and the projecting pole in danger. 
As is well known, the colour of eggs varies with the 
species. White may be mentioned as the prevailing 
colour; next, blue, or bluish-green of different shades ; 
next, brown of different shades, passing into red. Even of 
the same species, the colour, as of the eggs of the common 
fowl, is not precisely the same—some being distinguished 
for their whiteness, as those of the dorking and game-fowl ; 
some for their brown hue, as those of Cochin-china variety. 
In lustre and polish, too, the several kinds exhibit differ- 
ences—some having a lustre approaching almost that of — 
the pearl, as the egg of the common pigeon; others being 
destitute of lustre, giving them a dull appearance. The 
markings, spots, and partial colouring are so various as to 
almost baffle description, as any one may satisfy himself by 
referring to any work in which they are depicted. 
These diverse markings—these different colours of eggs— 
from such experiments as I have made, I am induced to 
