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.
This instance is but a typical example of chemical decomposition in general, and may occur in thousands of different ways in different chemicals, and producing not only chemical transformation, but the manifestation in many cases of heat, and light, and electricity, etc.
I will next speak of electricity and chemical affinity conjointly. The electrical spark will produce heat, light, chemical change, and movement.
Faraday showed also that electricity produces magnetism, and magnetism electricity—that indeed you can not produce the one without the other.
Again, electricity will set in motion chemical affinity or decomposition, and conversely chemical change will produce heat, light, electricity, motion, etc. To show this, place pieces of zinc and copper in an acid—that is, make a voltaic battery. The acid attracts the metal and sets up chemical action, and the result is that this chemical action by producing a change of state, sets free electricity, which being conducted by “the poles” or wires of the battery can there be made manifest in the following different ways:
Bring the poles together and heat, light, motion, etc., will be produced as witnessed in the dazzling electric light.
Or, to produce chemical change only, plunge the wires constituting “the poles” of the battery into water, and wonderful to say the molecules of water will have such motion imparted to them that they will be broken up into their constituent gases; and what is most marvelous, the oxygen will always be given off at one pole and the hydrogen at the other.
This is electrolysis or electro-chemical decomposition. Numbers of compounds in solution may have the molecular states of their atoms broken up thus, by the voltaic current, and curious to say of the atoms so dissevered, as above noted, those composing a given element will always be evolved by the same pole—the positive pole or the negative pole as the case may be. Some elements, that is to say, always appearing at the positive pole and others always at the negative.
Lastly I will say a few words specially as to motion.