Environmental Engineering Reference
In-Depth Information
equally well to autotrophs and heterotrophs. Edward
Smith (1857) collected about 1,200 samples of expired
air and proved that respiration is a function of one vari-
able, the ingested food, and one constant, a distinct indi-
vidual metabolic rate. During the 1840s the advances
in human energetics were closely associated with the
genesis and progress of general energetic principles in
the work of a German physician, Julius Robert Mayer
(1814-1878).
Mayer's ideas had their origin in a voyage he made
in 1840 as a ship's doctor. When bleeding his patients in
Java (the ancient practice of bleeding as a cure for many
ailments persisted well into the nineteenth century), he
noticed that their venous blood was much brighter
than the blood of patients in Germany (Caneva 1993).
Mayer's correct explanation was that blood in the tropics
does not have to be oxidized as much as in the temperate
regions because less energy is needed for body metabo-
lism in warm climates. But if less heat is lost in the tropics
because of radiation, how about the heat lost as a result
of physical work (that is, expenditure of mechanical en-
ergy), which obviously warms its surroundings regardless
if done in Europe or tropical Asia? Unless we posit some
mysterious origins for it, that heat must also come from
the oxidation of blood, and hence heat and work must
be equivalent and convertible at a fixed rate.
Mayer published the first quantitative estimate of
the equivalence in 1842, and eventually he extended the
idea of energy conservation to all natural phenomena,
including electricity, light, and magnetism. He gave
details of his calculation based on an experiment with
gas flow between two insulated cylinders (Mayer 1842;
1851). His result was considerably below the real value,
mainly because of incorrect numbers for the specific heat
of air at constant pressure and constant volume that he
1.2
Ideal Carnot cycle.
and food intake provided the foundation for studies of
heterotrophic metabolism. On the other hand, Lav-
oisier's insistence on the caloric, an imponderable fluid
carrier of heat, was an intellectual cul-de-sac (R. Fox
1971).
In contrast to Carnot's ideas, the work of Jean-
Baptiste Dumas and Jean-Baptiste Boussingault (1842)
was immediately influential, and their explanations of
grand biospheric cycles underpinning the planetary bio-
energetics were essentially correct. At the same time,
Justus Liebig (1803-1873) was the first scientist to as-
cribe the generation of CO
2
and water to food oxidation
(Liebig 1843), offering a fundamentally correct view of
heterotrophic metabolism. He also introduced the pow-
erful concept of
limiting nutrients that
is applicable
