n the tropics. Led by this
reflection to consider the body as a machine dependent on outside forces
for its capacity to act, he passed on into a novel realm of thought,
which brought him at last to independent discovery of the mechanical
theory of heat, and to the first full and comprehensive appreciation
of the great law of conservation. Blood-letting, the modern physician
holds, was a practice of very doubtful benefit, as a rule, to the
subject; but once, at least, it led to marvellous results. No straw is
go small that it may not point the receptive mind of genius to new and
wonderful truths.

MAYER'S PAPER OF 1842

The paper in which Mayer first gave expression to his revolutionary
ideas bore the title of "The Forces of Inorganic Nature," and was
published in 1842. It is one of the gems of scientific literature, and
fortunately it is not too long to be quoted in its entirety. Seldom if
ever was a great revolutionary doctrine expounded in briefer compass:

"What are we to understand by 'forces'? and how are different forces
related to each other? The term force conveys for the most part the idea
of something unknown, unsearchable, and hypothetical; while the term
matter, on the other hand, implies the possession, by the object in
question, of such definite properties as weight and extension. An
attempt, therefore, to render the idea of force equally exact with that
of matter is one which should be welcomed by all those who desire to
have their views of nature clear and unencumbered by hypothesis.

"Forces are causes; and accordingly we may make full application in
relation to them of the principle causa aequat effectum. If the cause
c has the effect e, then c = e; if, in its turn, e is the cause of a
second effect of f, we have e = f, and so on: c = e = f... = c. In a
series of causes and effects, a term or a part of a term can never, as
is apparent from the nature of an equation, become equal to nothing.
This first property of all causes we call their indestructibility.

"If the given cause c has produced an effect e equal to itself, it has
in that very act ceased to be--c has become e. If, after the production
of e, c still remained in the whole or in part, there must be still
further effects corresponding to this remaining cause: the total effect
of c would thus be > e, which would be contrary to the supposition c =
e. Accordingly, since c becomes e, and e becomes f, etc., we must regard
these various magni