ions of the Royal Society of Edinburgh,
and contained internal evidence, without a repetition of his
experiments, that he was totally unacquainted with the principles of
chemical analysis. But his experiments have been carefully repeated
by qualified persons, and they have completely proved his ignorance:
his rhodium is iron, and his silicon an impure incombustible coal.

LETTER VI

My dear Sir,

One of the most remarkable effects of the recent progress of science
is the alliance of chemistry with physiology, by which a new and
unexpected light has been thrown upon the vital processes of plants
and animals. We have now no longer any difficulty in understanding
the different actions of aliments, poisons, and remedial agents--we
have a clear conception of the causes of hunger, of the exact nature
of death; and we are not, as formerly, obliged to content ourselves
with a mere description of their symptoms. It is now ascertained
with positive certainty, that all the substances which constitute
the food of man must be divided into two great classes, one of which
serves for the nutrition and reproduction of the animal body, whilst
the other ministers to quite different purposes. Thus starch, gum,
sugar, beer, wine, spirits, &c., furnish no element capable of
entering into the composition of blood, muscular fibre, or any part
which is the seat of the vital principle. It must surely be
universally interesting to trace the great change our views have
undergone upon these subjects, as well as to become acquainted with
the researches from which our present knowledge is derived.

The primary conditions of the maintenance of animal life, are a
constant supply of certain matters, animal food, and of oxygen, in
the shape of atmospheric air. During every moment of life, oxygen is
absorbed from the atmosphere in the organs of respiration, and the
act of breathing cannot cease while life continues.

The observations of physiologists have demonstrated that the body of
an adult man supplied abundantly with food, neither increases nor
diminishes in weight during twenty-four hours, and yet the quantity
of oxygen absorbed into his system, in that period, is very
considerable. According to the experiments of Lavoisier, an adult
man takes into his system from the atmosphere, in one year, no less
than 746 pounds weight of oxygen; the calculations of Menzies make
the quantity amount even to 837 pounds; but we find his weight at
the