er then to ascertain what
parts of our food are nutritious, we must compare the composition of
the blood with the composition of the various articles taken as
food.

Two substances require especial consideration as the chief
ingredients of the blood; one of these separates immediately from
the blood when it is withdrawn from the circulation.

It is well known that in this case blood coagulates, and separates
into a yellowish liquid, the serum of the blood, and a gelatinous
mass, which adheres to a rod or stick in soft, elastic fibres, when
coagulating blood is briskly stirred. This is the fibrine of the
blood, which is identical in all its properties with muscular fibre,
when the latter is purified from all foreign matters.

The second principal ingredient of the blood is contained in the
serum, and gives to this liquid all the properties of the white of
eggs, with which it is indeed identical. When heated, it coagulates
into a white elastic mass, and the coagulating substance is called
albumen.

Fibrine and albumen, the chief ingredients of blood, contain, in
all, seven chemical elements, among which nitrogen, phosphorus, and
sulphur are found. They contain also the earth of bones. The serum
retains in solution sea salt and other salts of potash and soda, in
which the acids are carbonic, phosphoric, and sulphuric acids. The
globules of the blood contain fibrine and albumen, along with a red
colouring matter, in which iron is a constant element. Besides
these, the blood contains certain fatty bodies in small quantity,
which differ from ordinary fats in several of their properties.

Chemical analysis has led to the remarkable result, that fibrine and
albumen contain the same organic elements united in the same
proportion,--i.e., that they are isomeric, their chemical
composition--the proportion of their ultimate elements--being
identical. But the difference of their external properties shows
that the particles of which they are composed are arranged in a
different order. (See Letter V).

This conclusion has lately been beautifully confirmed by a
distinguished physiologist (Denis), who has succeeded in converting
fibrine into albumen, that is, in giving it the solubility, and
coagulability by heat, which characterise the white of egg.

Fibrine and albumen, besides having the same composition, agree also
in this, that both dissolve in concentrated muriatic acid, yielding
a solution of an intense purple colour. Th