Marcu 1, 1901.] 
as Abel, Cauchy, Gauss, Jacobi, Riemann, 
Weierstrass. The general laws of these 
sciences were established at the beginning 
of the century, and remain essentially 
unaltered. The great discovery for me- 
chanics in the nineteenth century is the 
law of the conservation of work. Since 
this law lies at the very foundation of 
mechanics, it is of fundamental importance 
for the whole science. ‘The discovery and 
development of this law belongs to the 
field of physics, yet it was not developed 
by physicists alone. Mayer was a physi- 
-cian, Joule a brewer, Colding an engineer, 
and Helmholtz was at that time a physiol- 
ogist. 
The influence of this law on mechanics 
was pointed out by Lagrange in two equa- 
tions—the one for motion, the other for rest 
orequilibrium. In the light of the iaw that 
the amount of work cannot change, these 
two equations became relatively very sim- 
ple. This lawisas follows: The total amount 
of work is unalterable. It should be observed 
that work can have two forms, the form 
of movement (or kinetic energy), and the 
form where a weight moves a clock—where 
the capability of doing work is connected 
with the weight, therefore, with a force, 
(potential energy). The law should be ex- 
pressed thus: The swm of the two kinds of 
work is unalterable. 
We can then say, in general, of our 
three fundamental sciences, that at the close 
of the nineteenth century they rest on a 
foundation that is practically perfect. 
If we now turn to the experimental sci- 
ences, physics and chemistry, we find that 
there is no sharp division between them. 
Recently a celebrated chemist said that 
Lavoisier and Bunsen were not chemists, 
but physicists, and to show what inherent 
connection exists between the two, Bunsen 
himself said ‘a chemist who is not a physician 
is nothing.’ 
The physicist has to do chiefly with the 
SCIENCE. 
339 
transformations of force; the chemist with 
the transformations of matter. 
Turning now to physics, therefore, to the 
problem of the transformations of natural 
forces or corresponding work forms, the 
developments in the nineteenth century 
are closely connected with the fundamental 
conceptions that natural processes are to 
bereferred to purely mechanical movements 
and forces. Since light, sound, heat, elec- 
tricity and magnetism are only different 
forms of movement, the possibility exists of 
transforming these into one another. This 
is the first great advance in physics. It 
was Faraday especially who believed in 
the correlation of energy. This reciprocal 
transformation of work forms, of course, 
takes place in daily life in the steam engine, 
dynamo, ete., and from this energy heat 
and light can be reproduced. 
The second great advance is, of course, 
the law of the conservation of energy, or of 
work, in terms of which the total amount 
of work remains unchanged. 
A third important step was taken. If 
one form of work can be transformed into 
another and this transformation takes place 
quantitatively, then the question still re- 
maining is, Which way will the transforma- 
tions take place? This was answered by 
Carnot and Clausius in what has become 
the- second law of thermodynamics. This 
is often formulated thus : Heat always flows 
from a warmer to a colder body. Helm- 
holtz formulates it in terms of ‘ free work.’ 
We now consider the last fundamental step 
which has been taken—how quickly do 
transformations take place in nature ? 
This brings us to the views in reference to 
the inner nature of natural processes—views 
which have been developed in the nine- 
teenth century. As an example, let a local 
increase in pressure be produced in the air 
by, say, an explosion. This increase in 
pressure tends to equalize itself, and the 
excess of pressure moves through the air as 
