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. That is to say, the chemical changes necessitated in the plant by the light and heat, result in these factors of change being themselves incorporated with the new wood, etc., in such a way as to be retained there in union with the atoms of carbon and other constituents of the tissue and products of the plant. Thus fixed they may remain for ages, until the wood, etc., is itself subjected to change, and then either as wood or as coal (if turned into such) it will—if burnt—again give out that light and heat which it received and appropriated during growth. Hitherto I have spoken of chemical decomposition only as produced by motion, and heat, and light, but now I must give a familiar instance in which you can produce great chemical change and movement amongst atoms by simply mixing two chemicals together. Add tartaric acid to a solution of carbonate of soda and a great commotion ensues, owing to the superior affinity of the tartaric acid for the soda; and which acid displaces the carbonic acid in previous union with the soda, and the latter acid is turned out and escapes by violent effervescence.