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Show I I ' I 160 PROPERTIES OF AIR. substance) is in a state of air, it is literally, and _in the truest sense of the word a material-a material which the plastic hand of ~ature can mould and fashion into any new production for which it is adapted, with far more ease and certainty thhn the potter can out of the same clay mould "one vessel for honour, and another for dishonour," or the builder can apply the same bricks as part either of a palace or a pigsty. This, when we think seriously of it, is really the most wonderful part of the whole wide field of nature ; and it is the one in wiNch the foundations of all our knowledge of nature's working are laid. The solvent power of heat, which loosens the firm( cohesion of the diamond with as much ease and1 certainty as it melts ice into water, or the sunbeams into all those tints of colour that enliven the face of nature, overcomes all, but destroys or injures nothing. It holds all matter captive ; but the captivity is only that the purposes of matter may thereby be fulfilled ; for the moment that the proper ingredients of any compound come together in due proportion, and under the requisite circumstances, the heat which held their properties suspended lets them slip, and they instantly act, and the compound is formed, with the same ease and the same certainty as if it had existed from the beginning. Not only that, but the heat is as powerful in escaping away, and allowing the qualities of materials) which it had held in the state of air, to act, in the formation of new substances, as it is in the suspension of those properties, in bringing about the destruction of that which is old,-that which has already served the purpose of its being, and is occupying materials to no use. Oxygen and hydrogen, the component parts of water, can both be obtainerl in the state of pure and colourless air, the first a little heavier, at the same temperature. than the common air of the atmosphere, .. . ..... '. DECOMrP081TION. 161 and the second a great deal lighter. Each, in its separate state, may have a great variety of temperatures, and have its volume augmented by heat or the removal of pressure, or diminished by pressur_ e or by. cold ; and though that has not yet been I satisfactonly done by human experiment, there is n~t the least doubt, .that. by. sufficient cooling, both mig-ht be condensed mto hqmds, and crystallized into solids. We do not know that these elements of ~ater are absolutely simple; but we call them so, JUSt because we are not ab~e to ~esolve any of them mto two s~bstances bearmg different properties; and the ancients thought water simple and called it an " element," for the same reason. ' But we can work any of those (to us) simple substances through a very gr~at. range of temperature, and still get them back a~am m the very state with which we set out. But brmg them together in the proportions in which they fort? w~ter, a1_1d apply a lighted match, and the c?mbustwn IS ter~Ible, probably the most brilliant display of the actwn of heat with which we are acquamted, and perfectly irresistible in its effects. rWhen th~se e.lemen~s are in sufficient quantity, and free to m1x w1th their natural rapidity, as much heat , would come out ?f the materials of a pitcher of water, when passmg from the state of separate airs I or gases to ~hat of the compound liquid, as would suffic~ to kmdle .the globe, or loosen from their cohesiOn the particles of any substance in nature whether compound or simple. ' The progress .of ~ecomposition is always the same as that. w.hich IS produced by the action of heat .i t~e .sohd IS first changed to a liquid, and then the hq~ud m~o an air or vapour; but there are many cases m which the process is altogether invisible . and there are others in which the two parts of it fol~ow each other so ~losely that we cannot distingmsh the~. ~here IS no doubt, however, of the perfect umform1ty of the process; and that whenever 02 |