ON HEAT AS A MODE OF MOTION. 
113 
any show of reason be affirmed that I merely render sensible the heat bidden in 
the ice, for that quantity is only a small fraction of the heat contained in the 
water.” He made the experiment, and liquefied the ice by pure friction ; and 
the result has been regarded as the first which proved the immateriality of heat. 
When a hammer strikes a bell, the motion of the hammer is arrested, but its 
force is not destroyed ; it has thrown the bell into vibrations, which affect the 
auditory nerve as sound. A hammer descending on a leaden bullet has its de¬ 
scending motion arrested at a given point; the motion is not destroyed, but is 
transferred to the atoms of the lead, and announces itself to the proper nerves 
as heat. This then is the theory attempted to be worked out, that heat is a 
kind of molecular motion ; and that by friction, percussion, and compression, 
this motion may be generated, as well as by combustion. 
But what is the relation of the heat developed by mechanical action to the 
force which produces it ? The man who first raised the idea of the equiva¬ 
lence between heat and mechanical energy to philosophic clearness ir. his own 
mind was a physician. Hr. Mayer, of Ileilbronn, in Germany, enunciated the 
exact relation which subsists between heat and work, giving the number which 
is now known as the “mechanical equivalent of heat,” and following up the state¬ 
ment of its principle by its fearless application. All honour is due to one who, 
without external stimulus, while pursuing his profession as town physician in 
Heilbronn, raised the conception of the interaction of natural forces to clearness 
in his own mind. In 1842 Mayer had calculated the mechanical equivalent of 
heat from data which only an original thinker could have turned to account; 
it w r as from the velocity of sound in air that Mayer determined the mechanical 
equivalent of heat. In 1845 he published his memoir on “ Organic Motion.” 
It was the accident of bleeding a feverish patient at Java, in 1840, that led him 
to speculate on these subjects. He noticed that the venous blood in the tropics 
was of a much brighter red than in colder latitudes, and his reasoning on this 
fact was the origin of his investigations. In 1848 appeared his essay on “ Ce¬ 
lestial Dynamics.” Nevertheless, however honourable it may be to Mayer to 
have elaborated in his own mind this grand idea, it is to Mr. Joule, of Manches¬ 
ter, that we are mainly indebted for the experimental treatment of the subject. 
Entirely independent of Mayer, he persisted for years in his attempts to prove 
the invariability of the relation which subsists between heat and ordinary me¬ 
chanical force ; he made numerous experiments ; he caused disks of cast iron to 
rub against each other, and measured the lieat produced by their friction, and 
the force expended in overcoming it. 
He urged water through capillary tubes, and determined the amount of heat 
generated by the friction of the liquid against the sides of the tubes ; the result 
being that under all circumstances, the quantity of heat generated by the same 
amount of force is fixed and invariable ; that is to say, that however the tempe¬ 
rature may differ in consequence of the different capacity for heat of the substance 
employed, the absolute amount of heat generated by the same expenditure of 
power, is in all cases the same. In this way it was found that the quantity of 
heat which w r ould raise one pound of w T ater one degree of Fahrenheit in tem¬ 
perature, is exactly equal to wdiat would be generated if a pound weight, after 
having fallen through a height of 772 feet, had its moving force destroyed by 
collision with the earth. Conversely, the amount of heat necessary to raise a 
pound of water one degree in temperature would, if all applied mechanically, be 
competent to raise a pound weight 772 feet high, or it would raise 772 lbs. 
one foot high. The term “foot-pound” has been introduced to express the 
lifting of one pound to the height of a foot. Thus the quantity of heat neces¬ 
sary to raise the temperature of a pound of water one degree being taken as a 
standard, 772 foot-pounds constitute what is called the Mechanical Equivalent 
of Heat. 
