11G 
ON HEAT AS A MODE OF MOTION. 
Whether this theory be correct or not, there is nothing more wonderful than 
the influence of the sun’s heat. By the act of vaporization the sun lifts me¬ 
chanically all the moisture of our air, it condenses and falls as rain, or freezes 
and is deposited as snow. Once more the sun liberates the solidified liquid, and 
permits it to roll by gravity to the sea. No streamlet glides to a lower level that 
has not been previously lifted by the sun. More than this, and if possible more 
wonderful (if any natural phenomenon can be considered as more wonderful than 
another), is the sun’s influence upon vegetable life. Trees and vegetables grow 
upon the earth, and when burned they give rise to heat, and hence to mechanical 
energy. Whence is this power derived ? In the building of plants, carbonic acid 
is the material from which the carbon of the plant is derived ; the solar beam the 
agent of separation, setting the oxygen free, and allowing the carbon to aggre¬ 
gate in woody fibre. Without the sun the reduction of the carbonic acid can¬ 
not be effected, and an amount of sunlight is consumed exactly equivalent to the 
molecular work done. But we must not stop at vegetable life, which is the 
source of all animal life. The sun separates the carbon from its oxygen, and the 
animal consumes the vegetable thus formed, and in its arteries a reunion of the 
severed elements takes place and produces animal heat. The warmth of our 
bodies, and every mechanical energy we exert, trace their lineage directly to the 
sun. Not therefore in a poetical, but in a purely mechanical sense, are we chil¬ 
dren of the sun. 
So far Professor Tyndall; and it is hoped enough has been strung together to 
give an idea of the theory propounded, and to awaken an interest in the lectures 
themselves. It would lead us too far for our present purpose to enter upon the 
details of the subject; and yet I may say, braving the charge of affectation, 
that no romance contains more exciting passages than these strictures upon heat. 
The laws of expansion, the liquid and gaseous states of matter, the physical pro¬ 
perties of ice, the phenomena of latent heat, of potential and dynamic energy; 
leading onwards to the consideration of larger physical phenomena, such as wind 
and rain, the Gulf-stream, snow and glaciers, find their fitting place in these dis¬ 
cussions. With radiant heat we have already been made acquainted, while for 
those who like practical applications there is u a discourse upon the Mer de 
Glace,” containing the well-known notice on Professor Forbes. 
Two episodes have resulted from these lectures, the first of which ran hard 
upon a quarrel. Mr. Joule took exception to the manner in which Mayer’s claims 
to notice had been stated; accordingly" he wrote a letter to the editors of the 
* Philosophical Magazine ’ upon the matter ; part of it is as follows :— 
u Geutlemen,—Will you permit me to trouble your readers with a few re¬ 
marks on the subject of my friend Professor Tyndall’s lecture at the Royal In¬ 
stitution, reported in your last number ? In this lecture he enforces the claims 
M. Mayer, a philosopher whose merit has perhaps been overlooked by some of 
our English physicists, and unaccountably so by his fellow-countrymen. I my¬ 
self was only imperfectly acquainted with his papers, when, in good conscience 
and with the materials at command, I gave a sketch of the Dynamical theory of 
Heat, in my paper published in the Philosophical Transactions for 1850. hi. 
Mayer’s merit consists in having announced, apparently without knowledge of 
what had been done before, the true theory of heat. This is no small merit, and I 
am the last person who would wish to detract from it. But to give to Mayer, 
or indeed to any single individual, the undivided praise of propounding the 
dynamical theory of heat, is manifestly unjust to the numerous contributors 
to that great step in physical science.” (Here follow quotations from Locke, 
Rumford, Davy, and Seguin.) “ From the above extracts, it will be seen that a 
great advance had been made before Mayer wrote his paper in 1842. Mayer dis¬ 
courses to the same effect as Seguin, but at greater length, ,with greater perspi- 
