78 Original Articles. [ Jan. 
vapour, which enables it to overcome the vast mechanical resistance 
that is set in opposition to it. And, in like manner, it is the heat of 
the solar rays which pumps up terrestrial waters in the shape of vapour, 
and thus supplies to Man a perennial source of new power in their 
descent by the force of gravity to the level from which they have been 
raised .* 
The power of the steam-engine, indeed, is itself derived more 
remotely from those same rays; for the Heat applied to its boilers 
is but the expression of the chemical change involved in com- 
bustion; that combustion is sustained either by the wood which is 
the product of the vegetative activity of the present day, or by the 
coal which represents the vegetative life of a remote geological epoch ; 
and that vegetative activity, whether present or past, represents an 
equivalent amount of Solar Light and Heat, used up in the decomposition 
of the carbonic acid of the atmosphere by the instrumentality of the 
growing plant.| Thus in either case we come, directly or indirectly, 
to Solar Radiation as the mainspring of our mechanical power ; the vis 
viva of our whole microcosm. Modern physical inquiry ventures even 
one step further, and seeks the source of the Light and Heat of the Sun 
itself. Are these, as formerly supposed, the result of combustion ; or 
are they, as surmised by Mayer and Thomson, the expression of the 
motive power continually generated in the fall of aérolites towards 
the Sun, and as continually annihilated by their impact on its surface ? 
Leaving the discussion of this question to Physical Philosophers, I 
proceed now to my own proper subject. 
It is now about twenty years since Dr. Mayer first broadly 
announced, in all its generality, the great principle now known as that 
of Conservation of Force; as a necessary deduction from two axioms 
or essential truths—ewx nihilo nil fit, and mil fit ad nhilum—the validity 
of which no true philosopher would ever have theoretically questioned, 
but of which he was (in my judgment) the first to appreciate the full 
practical bearing. Thanks to the labours of Faraday, Grove, Joule, 
Thomson, and Tyndall, to say nothing of those of Helmholtz and 
other distinguished Continental savans, the great doctrine ex- 
pressed by the term ‘“ Conservation of Force” is now amongst the 
best established generalizations of Physical Science; and every 
thoughtful Physiologist must desire to see the same course of inquiry 
thoroughly pursued in regard to the phenomena of living bodies. 
This ground was first broken by Dr. Mayer in his remarkable treatise, 
‘Die Organische Bewegung in ihrem Zusammenhange mit dem 
Stoffwechsel’ (‘On Organic Movement in its relation to Material 
Changes,’ Heilbronn, 1845); in which he distinctly set forth the 
principle that the source of all changes in the living Organism, 
animal as well as vegetable, les in the forces acting upon it from 
without ; whilst the changes in its own composition brought about by 
* See on this subject the recent admirable address of Sir William Armstrong, 
at the Mecting of the British Association at Newcastle. 
+ This was discerned by the genius of George Stephenson, before the general 
doctrine of the Correlation of Forces had been given to the world by Mayer or 
Grove. 
