Everybody has seen a common fowl. It is an active creature which runs
about and sometimes flies. It has a body covered with feathers, provided
with two wings and two legs, and ending at one end in a neck terminated
by a head with a beak, between the two parts of which the mouth is
placed. The hen lays =eggs=, each of which is enclosed in a hard shell.
If you break an egg the contents flow out and are seen to consist of the
colourless glairy “white” and the yellow “yolk.” If the white is
collected by itself in water and then heated it becomes turbid, forming
a white solid, very similar to the vegetable albumin, which is called
=animal albumin=.
If the yolk is beaten up with water, no starch nor cellulose is obtained
from it, but there will be plenty of fatty and some saccharine matter,
besides substances more or less similar to albumin and gluten.
The feathers of the fowl are chiefly composed of horn; if they are
stripped off and the body is boiled for a long time, the water will be
found to contain a quantity of =gelatin=, which sets into a jelly as it
cools; and the body will fall to pieces, the bones and the flesh
separating from one another. The bones consist almost entirely of a
substance which yields gelatin when it is boiled in water, impregnated
with a large quantity of salts of lime, just as the wood of the wheat
stem is impregnated with silica. The flesh, on the other hand, will
contain albumin, and some other substances which are very similar to
albumin, termed =fibrin= and =syntonin=.
In the living bird, all these bodies are united with a great quantity of
water, or dissolved, or suspended in water; and it must be remembered
that there are sundry other constituents of the fowl’s body and of the
egg, which are left unmentioned, as of no present importance.
58. =Certain Constituents of the Body are very similar in the Wheat
Plant and in the Fowl.=
The wheat plant contains neither horn, nor gelatin, and the fowl
contains neither starch, nor cellulose; but the albumin of the plant is
very similar to that of the animal, and the fibrin and syntonin of the
animal are bodies closely allied to both albumin and gluten.
That there is a close likeness between all these bodies is obvious from
the fact that when any of them is strongly heated, or allowed to
putrefy, it gives off the same sort of disagreeable smell; and careful
chemical analysis has shown that they are, in fact, all composed of the
elements =Carbon=, =Hydrogen=, =Oxygen=, and =Nitrogen=, combined in
very nearly the same proportions. Indeed, =charcoal=, which is impure
carbon, might be obtained by strongly heating either a handful of corn,
or a piece of fowl’s flesh, in a vessel from which the air is excluded
so as to keep the corn or the flesh from burning. And if the vessel were
a still, so that the products of this =destructive distillation=, as it
is called, could be condensed and collected, we should find water and
ammonia, in some shape or other, in the receiver. Now ammonia is a
compound of the elementary bodies, nitrogen and hydrogen; therefore (§
50) both nitrogen and hydrogen must have been contained in the bodies
from which it is derived.
It is certain, then, that very similar nitrogenous compounds form a
large part of the bodies of both the wheat plant and the fowl, and these
bodies are called =proteids=.
59. =Proteid Substances are met with in Nature only in Animals and
Plants; and Animals and Plants always contain Proteids.=
It is a very remarkable fact that not only are such substances as
albumin, gluten, fibrin and syntonin, known exclusively as products of
animal and vegetable bodies; but that every animal and every plant, at
all periods of its existence, contains one or other of them, though, in
other respects, the composition of living bodies may vary indefinitely.
Thus, some plants contain neither starch nor cellulose, while these
substances are found in some animals; while many animals contain no
horny matter and no gelatin-yielding substance. So that the matter which
appears to be the =essential= foundation of both the animal and the
plant is the =proteid= united with =water=; though it is probable that,
in all animals and plants, these are associated with more or less
=fatty= and =amyloid= (or starchy and saccharine) substances, and with
very small quantities of certain mineral bodies, of which the most
important appear to be =phosphorus=, =iron=, =lime=, =and potash=.
Thus there is a substance composed of water + proteids + fat + amyloids
+ mineral matters which is found in all animals and plants; and, when
these are alive, this substance is termed =protoplasm=.