[210] _Comptes Rendus de l’Académie des Sciences_, 1840, No. 5. A good translation of Dumas’s Memoir appeared in the Philosophical Magazine, from which I extract the following familiar exposition of the laws of substitution:--“Let me make a comparison drawn from a familiar order of ideas. Let us put ourselves in the place of a man overlooking a game at chess without the slightest knowledge of the game. He would soon remark that the pieces must be used according to positive rules. In chemistry, the equivalents are our pieces, and the law of substitutions one of the rules which preside over their moves. And as in the oblique move of the pawns one pawn must be substituted for another, so in the phenomena of substitution one element must take the place of another. But this does not hinder the pawn from advancing without taking anything, as the law of substitution does not hinder an element from acting on a body without displacing or taking the place of any other element that it may contain.”--_Memoir on the Law of Substitutions, and Theory of Chemical Types._ [211] Liebig’s _Chemistry in its application to Agriculture and Physiology_: translated by Lyon Playfair, Ph. D. _Animal Chemistry, or Chemistry in its application to Physiology and Pathology_: by Justus Liebig; translated by Wm. Gregory. CHAPTER XII. CHEMICAL PHENOMENA. Water--Its Constituents--Oxygen--Hydrogen--Peroxide of Hydrogen--Physical Property of Water--Ice--Sea Water--Chlorine--Muriatic Acid--Iodine--Bromine--Compounds of Hydrogen with Carbon--Combustion--Flame--Safety Lamp--Respiration--Animal Heat--The Atmosphere--Carbonic Acid--Influence of Plants on the Air--Chemical Phenomena of Vegetation--Compounds of Nitrogen--Mineral Kingdom, &c. &c. Without attempting anything which shall approach even to the character of a sketch of chemical science, we may be allowed, in our search after exalting truths, to select such examples of the results of combination as may serve to elucidate any of the facts connected with natural phenomena. In doing this, by associating our examination with well-known natural objects or conditions, the interpretation afforded by analysis will be more evident, and the operation of the creative forces rendered more striking and familiar, particularly if at the same time we examine such physical conditions as are allied in action, and are sufficiently explanatory of important features.