ubstance, is supposed to
be projected from one particle to another, in a manner which is quite
independent of a medium, and which, as Neumann has himself pointed out,
is extremely different from that of the propagation of light. In other
theories it would appear that the action is supposed to be propagated in
a manner somewhat more similar to that of light.

But in all these theories the question naturally occurs: "If something
is transmitted from one particle to another at a distance, what is its
condition after it had left the one particle, and before it reached the
other?" If this something is the potential energy of the two particles,
as in Neumann's theory, how are we to conceive this energy as existing
in a point of space coinciding neither with the one particle nor with
the other? In fact, whenever energy is transmitted from one body to
another in time, there must be a medium or substance in which the energy
exists after it leaves one body, and before it reaches the other, for
energy, as Torricelli remarked, "is a quintessence of so subtile a
nature that it cannot be contained in any vessel except the inmost
substance of material things."

Hence all these theories lead to the conception of a medium in which the
propagation takes place, and if we admit this medium as an hypothesis, I
think we ought to endeavour to construct a mental representation of all
the details of its action, and this has been my constant aim in this
treatise.

ELIE METCHNIKOFF

The Nature of Man

Elie Metchnikoff, Sub-Director of the Pasteur Institute in Paris,
was born May 15, 1845, in the province of Kharkov, Russia, and has
worked at the Pasteur Institute since 1888. The greater part of
Metchnikoff's work is concerned with the most intimate processes of
the body, and notably the means by which it defends itself from the
living agents of disease. He is, indeed, the author of a standard
treatise entitled "Immunity in Infective Diseases." His early work
in zoology led him to study the water-flea, and thence to discover
that the white cells of the human blood oppose, consume, and
destroy invading microbes. Latterly, Metchnikoff has devoted
himself in some measure to more general and especially
philosophical studies, the outcome of which is best represented by
the notable volume on "The Nature of Man," which was published at
Paris in 1903.

_I.--Disharmonies