In section C, the definitions of groups of chemical elements are as follows: Alkali metals: Li, Na, K, Rb, Cs, Fr Alkaline earth metals: Ca, Sr, Ba, Ra Lanthanides: elements with atomic numbers 57 to 71 inclusive Rare earths: Sc, Y, Lanthanides Actinides: elements with atomic numbers 89 to 103 inclusive Refractory metals: Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W Halogens: F, Cl, Br, I, At Noble gases: He, Ne, Ar, Kr, Xe, Rn Platinum group: Os, Ir, Pt, Ru, Rh, Pd Noble metals: Ag, Au, Platinum group Light metals: alkali metals, alkaline earth metals, Be, Al, Mg Heavy metals: metals other than light metals Iron group: Fe, Co, Ni Non-metals: H, B, C, Si, N, P, O, S, Se, Te, noble gases, halogens Metals: elements other than non-metals Transition elements: elements with atomic numbers 21 to 30 inclusive, 39 to 48 inclusive, 57 to 80 inclusive, 89 upwards Section C covers : pure chemistry, which covers inorganic compounds, organic compounds, macromolecular compounds, and their methods ofpreparation; applied chemistry, which covers compositions containing the above compounds, such as: glass, ceramics, fertilisers, plastics compositions, paints,products of the petroleum industry. It alsocovers certain compositions on account of their having particular properties rendering them suitable for certain purposes, as in the case of explosives, dyestuffs, adhesives, lubricants, and detergents; certain marginal industries, such as the manufacture of coke and of solid or gaseous fuels, the production and refining of oils, fats and waxes, the fermentation industry (e.g., brewing and wine-making), the sugar industry; certain operations ortreatments, which are either purely mechanical, e.g., the mechanicaltreatment of leather and skins, or partly mechanical, e.g., thetreatment of water or the prevention of corrosion in general; metallurgy, ferrous or non-ferrous alloys. In all sections of the IPC, in the absence of an indication to the contrary, the Periodic System of chemical elements referred to is the one with 8 groups as represented in the table below. For example, group "Compounds containing elements of the 2nd Group of the Periodic System" refers to the elements of columns IIa and IIb. In the case of operations,treatments,products or articles having both a chemical and a non-chemical part oraspect, the general rule is that the chemical part oraspect is covered by section C. In some of these cases, the chemical part oraspect brings with it a non-chemical one, even though purely mechanical, because this latteraspect either isessential to the operation ortreatment or constitutes an important element thereof. It has seemed, in fact, more logical not to dissociate the different parts oraspects of a coherent whole. This is the case for applied chemistry and for the industries, operations andtreatments mentioned in Notes (1)(c), (d) and (e). For example, furnaces peculiar to the manufacture of glass are covered by class and not by class . There are, however, some exceptions in which the mechanical (or non-chemical)aspect carries with it the chemicalaspect, for example: Certain extractive processes, in subclass ; The chemical purification of air, in subclass ; Chemical methods of fire-fighting, in subclass ; Chemical processes andapparatus, in class; Impregnation of wood, in subclass ; Chemical methods of analysis or testing, in subclass ; Photographicmaterials and processes, in class, and, generally, the chemicaltreatment of textiles and the production of cellulose or paper, in section D. In still other cases, the pure chemicalaspect is covered by section C and the applied chemicalaspect by another section, such as A, B or F, e.g., the use of a substance or composition for: treatment ofplants or animals, covered by subclass; foodstuffs, covered by class ; ammunition or explosives, covered by class . When the chemical and mechanicalaspects are so closely interlocked that a neat and simple division is not possible, or when certain mechanical processes follow as a natural or logical continuation of a chemicaltreatment, section C may cover, in addition to the chemicalaspect, a part only of the mechanicalaspect, e.g., after-treatment of artificial stone, covered by class. In this latter case, a note or a reference is usually given to make the position clear, even if sometimes the division is rather arbitrary.