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METALLIC OXIDES.

213. The only other metal of much importance in,connection with vegetable chemistry is manganese ; itsoxide is occasionally present in soils, though only insmall quantity. In all its general characters it is verysimilar to iron, and, like that metal, it has a strongaffinity for oxygen. It is, however, far less abundantor common than iron.

214. It is unnecessary further to describe the metallicoxides and salts, as the oxides and salts of iron are byfar of the most interest in relation to plants; the otheroxides, though very important in the arts, are scarcelyever found in the soil, or in any vegetable substances.

215. What has been said of iron is, however, generallyspeaking, applicable to the other metals; they all havea more or less powerful affinity for oxygen, and underproper circumstances combine with it: some have sostrong an affinity for it that they are able to decomposewater, and hence rust when exposed to the weather ;others again are unable to combine with it at commontemperatures, but oxidise rapidly when heated ; andsome have so weak an affinity for oxygen, that they areunable to combine with it unless under the most favour-able circumstances, such as, for example, when boiled instrong nitric acid, which, from the large quantity ofoxygen it contains, is a very powerful oxidising agent(103), and is therefore able rapidly to corrode most ofthe metals.

216. Some of the metals only combine with oxygenin one proportion, whilst others combine in two or more