In the growth of a tree, its wood and all its products are the result of certain external forces effecting chemical changes. Carbonic acid is decomposed, the carbon is retained, and oxygen given off, and assimilations of a complex character are in constant progress to produce the various compounds of oxygen, hydrogen, nitrogen, and carbon. Every condition of organised forms is due to the external excitation of light and heat, and in the chemical changes which take place, an equivalent of these principles, or powers--it signifies but little according to which view we may regard them--is absorbed, and retained as essential to the condition of the matter formed. Let us confine our attention to wood--although the position applies equally to every organic product. A cubic foot of wood is formed by the decomposition of a certain quantity of carbonic acid, by the vital function of the plant, excited by the solar rays, which are involved in the mass which nature by “her wondrous alchemy” has made. Eventually this cubic foot of wood is subjected to a process of chemical change--combustion; by the application of a single spark,--and in the disintegration of the wood, its carbon combining with oxygen to form carbonic acid, its hydrogen to form water, which is returned to the air, a large amount of light and heat is produced. This is exactly equivalent to the amount which was engaged in its formation. Indeed, the sunshine which fell upon the leaves of the forest tree, of which the log formed a part, has been hoarded up, and we again develope it in its original state of heat and light. The vast coal beds of England were formed by the rapid growth and quick decay of a peculiar class of plants under the influence of a tropical sun. They have been buried myriads of ages, under hundreds of feet of sandy rock. By the industry of the miner the coal is brought again to the surface, and we develope from it those powers by which it was formed. In the fire which gives comfort to our homes--in the furnace which generates force for the purposes of manufacture, or to propel the railway engine and its ponderous train--in the gas with which we illumine our streets and gladden during the long winter nights our apartments, we are developing that heat and light which fell upon the earth with all its quickening influences millions of ages before yet the Creator had called into existence the monarch Man, for whose necessities these wondrous formations were designed. FOOTNOTES: [43] The following table of the rays penetrating coloured glass has been given by Melloni, in his memoir _On the Free Transmission of Radiant Heat through Different Bodies_:--