Heat travels with light from the sun, but it can be produced by friction - by percussion - by chemical action - by electrical action - by vital power. "Pouillet has carefully ascertained," says Professor Pepper, in his admirable "Cyclopaedic Science Simplified," "the total heating effect of the sun's rays upon the earth; and estimating the whole heating power of the sun as 2300 millions of parts, he calculates that less than one of those parts only reaches our earth, and yet it would melt a layer of ice thirty-five yards thick over the whole surface of our globe. This proportion of heat is not all available: some of it is at once converted into power by setting the air in motion, to create the winds; another portion raises the water of the ocean into vapour, which, descending in the form of rain on high levels, such as the mighty water-shed which supplies the great lakes (discovered by Speke and Grant and Sir Samuel Baker), which are the sources of the Nile, flows down to the lowlands, giving rise to water power, which is again the equivalent for heat.

Another part stimulates and increases the growth of plants; and thus in ages long since passed away, the heat of the sun's rays was not all lost, as the elder Stephenson insisted, but was stored up ready for man to use in another form - viz., coal. It was therefore called potential heat. The plants, being the food of animals, again contribute to the production of animal heat and muscular force. The sources of heat are all connected with motion of some kind.

"Friction is a notable illustration, and it was by causing two pieces of ice to rub one against the other that Sir Humphry Davy gene-rated heat, and liquefied the ice. Like Dr. Young, who proved that light could turn a corner, and established by his experiments with inflection a sort of basis upon which the undulatory theory of light was again reconstructed, so this famous experiment of Davy supplied a great fact, and gave the first blow to the old theory which said that the ice melted because latent heat was made sensible heat, when it was well known that water at a temperature of 320 Fahrenheit contains much more heat than ice; how, then, could the ice, already deficient in heat, supply enough to satisfy the condition of water? There are plenty of illustrations of the generation of heat by friction. The flint and steel; the attrition of dried wood, as used by savage tribes; the famous experiments of Count Rumford whilst boring cannon, when enough heat was generated in two hours and a half to cause two and a half gallons of water to boil; the friction of railway-wheel axles, which have been known to become red hot and to set fire to the woodwork of the carriage.

In North America, a case is quoted where heat was intentionally generated by waste water power and used for heating purposes, the generator being two flat plates of iron which rubbed against each other.

Percussion

It was said formerly that metals when struck with a hammer, or with a die in the coining-press, became hot because their density was increased, and therefore their capacity or containing power for heat was altered; but it is clearly shown that this is not the true explanation. Lead, for instance, which becomes hot by percussion, does not increase in density and yet becomes hot - so hot that when projected from the steam gun, in the form of bullets, against a wrought-iron target, a flash of light is apparent in a darkened room. The heavy shot used for battering iron plates always becomes very hot after it has struck the plate.

Chemical Action

The bringing together of a number of atoms, however small, the clashing together (as Tyndall calls it) of particles to produce new compounds, as in the heating and combustion of finely powdered antimony when it is brought in contact with chlorine gas; or the heat generated by combustion or from other chemical changes, are all to be regarded as the result of motion which the eye cannot detect, but which must occur before the elements come in contact, combine, and form new compounds. There are many chemical changes accelerated by motion, and hence the stirring-rod is an important mechanical means to secure the more rapid union of particles.

"Electrical action is, of course, circulation, or motion".

Vital power is heat generated by chemical action - it is very slow combustion; we call it animal heat, without the presence of which in our bodies life would cease. This heat is caused (in scientific language) by "the combustion or burning of hydrogen and carbon in the capillary vessels".

The capillary vessels are vessels as small as hairs, from which they take their name: Capillaris is a Latin word meaning, " like a hair." These vessels run all over the body. We all know that wherever we prick or cut ourselves, blood will flow; no part of the body is exempt from this flow of blood, even if it be but the speck covered by a pin-prick. The blood flows from these tiny vessels. The blood in them contains hydrogen gas and carbon. By breathing we inhale (or ought to inhale) oxygen gas; this gas combines with the carbon in our blood, and causes combustion or heat by forming carbonic acid gas. Thus a constant fire is kept up within the human body; if it failed from want of food (which makes carbon), or from want of oxygen (pure air), we should perish. Animal heat is not very high, not hotter than 90 or 104 degrees; nevertheless, it slowly consumes the body; every muscle, nerve, organ is wasted away by it, and has to be perpetually renewed. The lamp of life requires to be constantly fed, just as an ordinary lamp requires to be supplied with oil, but the combustion in the former case (that of the animal) is much slower than in the case of the latter.

Food is the fuel of the body; when it is not supplied the capillary fires consume the human frame itself, while substance enough remains to keep them burning, and when that fails they expire. Life goes also, and the man is said to be starved to death.