We have now made a rapid survey of the history of chemistry, and traced the evolution of thought which has rendered possible the newer conceptions of the constitution of matter. We must now say a few words as to the nature of the various elements themselves, and give a brief account of some modern researches. A few practical hints as to experiments may also be of interest to the reader. We have seen that when two chemical elements combine, some third substance is formed, quite different in properties from the original two. Thus, water seems to us entirely different from the two invisible gases which compose it--oxygen and hydrogen. Yet a simple experiment will prove that such is the case. We can decompose water by means of an electric current, when the original gases are given off, in the proportion of two to one--hydrogen collecting at the negative pole, and oxygen at the positive. This process can be kept up until all the water has been decomposed, and only hydrogen and oxygen gases remain. This process of electrical decomposition is known as _electrolysis_. THE ELEMENTS _Hydrogen_ is the lightest of all gases, and, as we have seen, the simplest of the elements, in its constitution. Like all gases, it can be liquefied, and even frozen solid into a hard lump, like ice. On the other hand, even the densest of substances can be liquefied, and even turned into gas or vapor at a sufficiently high temperature. (Gases are rendered liquid or solid at a very low temperature.) Liquid air, for example, is so cold that when a can of it is set upon a block of ice the liquid air boils and gives off “steam”! _Oxygen_ gas constitutes about one-fifth of our atmospheric air (the other four parts being nitrogen) and is the most essential element in supporting life. Without it, life would at once become extinct. All forms of combustion take place very rapidly in oxygen, and the combustion going on within the human body is no exception to this rule. The atmospheric nitrogen acts as a sort of dilutant, being an inert gas. If a mouse be placed under a jar of pure oxygen gas, it will often run round and round until it drops dead with exhaustion. In an atmosphere of pure oxygen, we should soon burn up, and live our lives too rapidly. Oxygen has a great tendency to combine with various other elements, particularly metals. Thus, iron rust is due to the combination of oxygen with iron; the blackening and tarnishing of cooking pots is due to the slow oxidation of copper, etc. In breathing, we take in oxygen from the air, which combines with the gases in the lungs, forming carbon dioxide. Curiously enough, plants thrive upon this gas, which is so poisonous to human beings, and in turn give off oxygen. Hence the value of plants and flowers in the room, or in any densely inhabited area.