To show this co-relation and capacity for transformation I will give a few examples. Rub a piece of iron briskly, or repeatedly strike it with a hammer, and it will become warm. We know from this, therefore, that motion will produce heat. Or strike a lucifer match against the box, and the rapid motion of the match against the hard surface will produce sufficient heat to cause the sensitive chemical substance at the end of the match to inflame, and so to give out heat and light. Or watch a horse trotting on a hard road at night, and sparks will every now and then fly from his feet.
This is because by the rapid forcible motion of his legs the iron of his shoes every now and then comes in contact with a stone, and a minute particle of steel being struck off with great force and rapidity it becomes red-hot, and thus presents another example of how motion can be converted into heat.
These are instances of heat and light being produced by the motion of friction. But now let us look at the converse case, of heat producing motion.
Light a piece of coal, and set a kettle of water on it. The flames resulting from chemical change soon leap out, and flicker, and flare, and presently the water too becomes agitated, and boils with energetic motion, and steam rushes up into the air.
Place water in a proper machine, and the force you get from the chemical change of the coal causing the water to form steam, can make a railway train weighing hundreds of tons rush along the rails at a mile a minute.
These are familiar examples of how man may set in action the correlative forces, but instances abound universally in all creation where the correlative forces are constantly producing each other by mutual conversion, and affecting thereby all sorts of natural changes.
By the stimulus of heat and light, etc., received from the sun, motion and chemical change are compelled throughout nature, both animate and inanimate. As an example of the latter take the case of water.
The water of the seas and lakes, etc., being affected by heat moves by evaporation into the air, and afterward descends as rain or dew to perform the well-known and indispensable uses pertaining thereto.
Then as to the way in which light and heat produce movement and chemical change in plants and animals, I will also cite one example, selecting plants as being the most ready of illustration.
It is under the stimulation of light and heat that the plant grows and performs its functions; and astonishing to say, the rays, both of light and heat, which fall on the plant are absorbed into it and fixed there.