ON HEAT AS A MODE OF MOTION. 115 
air, and here the atoms clash together, and by their collision produce both light 
and heat. 
But the exact constitution of the flame, as revealed by Davy, is worthy of our 
attention. Coal-gas is a hydrocarbon, that is, a chemical union of hydrogen and 
carbon. When the combustion is incomplete the soot and lamp-black escape; 
here, as in the burning diamond, we have this compound gas in the presence of 
the oxygen of the air ; we apply heat, and the gas bursts into flame. The oxygen 
first closes with the hydrogen and sets the carbon free ; the solid particles of 
carbon thus scattered in the midst of burning matter are raised to a state of in¬ 
tense incandescence, and becoming white-hot, it is to them mainly the light of 
our lamps is due. The carbon eventually closes with the oxygen and becomes 
carbonic acid. To the existence then of solid carbon particles is due the light; 
and therefore by passing a current of air through such a flame, sufficient oxygen 
would be introduced to combine with the excess of carbon, and a considerable 
diminution of light would be the result. 
To return to the theory ; it is to the clashing together of the oxygen of the 
air and the constituents of our gas and candles that the light and heat of our 
flame is due : it is the impact of the atoms of oxygen against the atoms of sul¬ 
phur which produces the flame observed when sulphur is burnt in oxygen; to 
the collision of the same atoms against the phosphorus are due the heat and daz¬ 
zling light which result from the combustion of phosphorus in oxygen gas : in 
short, all cases of combustion are to be ascribed to the collision of atoms which 
have been urged together by their mutual attractions. That the arrest of me¬ 
chanical motion results in heat may be illustrated in innumerable ways ; a weight 
falls to the earth, and in so doing is found to be heated ; a cannon ball is fired 
against a target, the result being the same. The mode of motion changes, but it 
still continues motion ; the motion of the mass is converted into a motion of the 
atoms of the mass ; and these small motions, communicated to the nerves, produce 
the sensation which we call heat. 
Heat then is developed by the clashing of sensible masses and atoms; work is 
expended in giving motion to these atoms, and heat produced. But we can re¬ 
verse the process, and by the expenditure of heat execute work. One pound of 
coal produces by its combination with oxygen an amount of heat which, if me¬ 
chanically applied, would suffice to raise a weight of 100 lbs. to a height of 
twenty miles above the earth’s surface. Conversely, 100 lbs. falling from a 
height of twenty miles and striking against the earth, would generate an amount 
of heat equal to that developed by the combustion of a pound of coal. 
Let us turn our thoughts for a moment from the earth to the sun. The whole 
heat emitted by the sun in a minute would be competent to boil 12,000 millions 
of cubic miles of ice-cold water. Whence is the sun’s heat derived, and by what 
means is it maintained? Now, upon any theory of the assumption that the sun is 
merely a burning body, its light and heat must eventually and speedily come to 
an end ; in fact it would burn out. We have seen already heat generated by 
collision, that is, by an arrest and change of motion. The maximum velocity 
with which a body can strike the earth is about seven miles in a second; the 
maximum velocity with which it can strike the sun is 390 miles in a second. 
And as heat developed by the collision is proportional to the square of the velo¬ 
city destroyed, an asteroid falling into the sun with the above velocity, would 
generate about 10,000 times the quantity of heat generated by the combustion 
of an asteroid of coal of the same weight. 
Indeed the existence of the sun’s heat has been thus explained, and by this, 
u the Meteoric theory,” it is assumed that meteorites raining down upon the sur¬ 
face of the sun form a perpetual store of heat and light; moreover though this 
necessarily implies an increase in the actual magnitude of the sun, such aug¬ 
mentation would scarcely be appreciable during the lapse of ages. 
