114 
ON HEAT AS A MODE OF MOTION. 
Suppose a ball of lead falls from a height of twenty-six feet, the heat generated 
is proportional to the height through which the body falls. Lead falling through 
772 feet would generate heat sufficient to raise its own temperature 30°, its u ca¬ 
pacity ” being one-thirtieth of that of water : hence, in falling through twenty- 
six feet ( i . e. about one-thirtieth of 772), the heat generated would, if all con¬ 
centrated in the lead, raise its temperature one degree. 
But if motion be imparted to a body by other means than gravity, the de¬ 
struction of this motion also produces heat. A bullet striking a target is in¬ 
tensely heated ; the mechanical equivalent of heat enables us to calculate the 
amount of heat generated by the bullet, its velocity being known. The greater 
the height from which a body falls, the greater the striking force, owing to the 
greater acquired velocity ; but the velocity imparted to the body is not propor¬ 
tional to the height from which it falls ; the height augments in the same pro¬ 
portion as the square of the velocity. On the other hand, the heat generated by 
the collision of the falling body increases simply as the height; consequently, the 
heat generated increases as the square of the velocity . 
It is manifest from these considerations that if we know the velocity and 
weight of any projectile, we can calculate the amount of heat developed by the 
destruction of its moving force. Knowing, for example, the weight of the earth, 
and the velocity with which it moves through space, a simple calculation would 
enable us to determine the exact amount of heat which would be developed, sup¬ 
posing the earth to be stopped in its orbit. Mayer and Helmholtz have made 
this calculation, and found that the quantity of heat generated by the shock 
would be quite sufficient, not only to fuse the entire earth, but to reduce it in 
great part to vapour. Thus, by the simple stoppage of the earth in its orbit, 
“ the elements” might be caused “ to melt with fervent heat.” Our earth moves 
in its orbit with a velocity of 68,040 miles an hour; stop this motion but an in¬ 
stant, and heat would then be generated sufficient to raise the temperature of a 
globe of lead of equal size 384,000 degrees Centigrade. 
Adopting this theory, it has been supposed by Mayer (1848), Waterston, and 
Professor William Thomson (1853), that the sun itself derived its heat from 
the showering down of meteoric matter upon its surface; and whatever be the 
value of this speculation, this pouring down of meteoric matter would be com¬ 
petent to produce its heat and light. 
Leaving the sun, let us see whether in this way may not be explained some of 
the most familiar phenomena of combustion. Place a diamond (held fast in a 
loop of platinum wire) in an ordinary flame ; heat it to redness and then plunge 
it in a jar of oxygen gas ; see how it brightens and glows, and burns like a ter¬ 
restrial star. Why does it brighten, and glow, and burn? The diamond is pure 
carbon, and on the surface of this carbon are showered the atoms of oxygen on 
every side ; and could we measure the velocity of the atoms when they clash, and 
could we find their number and weight, multiplying the weight of each atom by 
the square of its velocity, and adding all together, we should get a number re¬ 
presenting the exact amount of heat developed by the union of the oxygen and 
carbon : by whatever force urged, whether by chemical affinity, or pure attrac¬ 
tion, otherwise called gravity, it matters not, the action is the same. Every 
oxygen atom as it strikes the surface, and has its motion of translation destroyed 
by its collision with the carbon, assumes the motion which we call heat; and this 
heat is so intense, the attractions exerted at these molecular distances are so 
mighty, that the crystal is kept white-hot, and the compound, formed by the 
union of its atoms with those of oxygen, flies away as carbonic acid gas. 
But let us leave the diamond as we have before left the sun, and endeavour in 
the same way to explain the action of an ordinary flame. What is it ? Take a 
jet of gas. Within the flame we have a core of unburnt gas, outside we have 
the oxygen of the air. The external surface of the core is in contact with the 
