The following notes are meant to assist in the use of this part of the classification scheme; they must not be read as modifying in any way the elaborations. In this sub-section, the separation of different materials, e.g. of different matter, size, or state, is predominantly found in the following subclasses: , , , , . The classifying characteristics of these subclasses are: the physical state of the matter to be separated; the principle of the process used; particular kinds of apparatus. The first of these characteristics involves six different aspects, assembled in three groups: liquid/liquid or liquid/gas and gas/gas; solid/liquid or solid/gas; solid/solid. These subclasses are to be used according to the following general rules: is the most general class as far as separation other than solids from solids is concerned. Apparatus for separating solids from solids are covered by when the process concerned is regarded as the equivalent of "washing" in the sense of the mining art, even if such apparatus is a pneumatic one, especially pneumatic tables or jigs. Screens per se are not covered by this subclass but are classified in , even if they are being used in a wet process. All other apparatus for the separation of solids from solids according to dry methods are classified in . If the separation takes place as a result of the detection or measurement of some feature of the material or articles to be sorted it is classified in . It should also be noted that the separation of isotopes of the same chemical element is covered by , whatever process or apparatus is employed. LIQUID/LIQUID, LIQUID/GAS OR GAS/GAS SEPARATION Method General operations by centrifugal force, using centrifuges or free-vortex apparatus using magnetic or electrostatic effect Apparatus General operations by centrifugal force, using centrifuges or free-vortex apparatus , using magnetic or electrostatic effect SOLID/LIQUID OR SOLID/GAS SEPARATION Method General operations by centrifugal force using centrifuges or free-vortex apparatus using magnetic or electrostatic effect Apparatus General operations by centrifugal force using centrifuges or free-vortex apparatus , using magnetic or electrostatic effect SOLID/SOLID SEPARATION Method Dry methods material in bulk Individual sorting Screening, sifting, pneumatic sorting using pneumatic tables or jigs by magnetic or electrostatic effect by centrifugal force using centrifuges or free-vortex apparatus Wet methods General operations flotation, differential sedimentation screening Combinations Dry methods - wet methods Apparatus Dry methods material in bulk Individual sorting Screening, sifting, pneumatic sorting using pneumatic tables or jigs by magnetic or electrostatic effect by centrifugal force using centrifuges or free-vortex apparatus , Wet methods General operations flotation, differential sedimentation screening Combinations Dry methods - wet methods Attention is drawn to the Notes following the subsection title "SEPARATING; MIXING". In this class, the following terms or expressions are used with the meanings indicated: "other fluent materials" includes semiliquids, pastes, melts, solutions, dispersions, suspensions, particulate materials, gases or vapours; "particulate materials" includes powders, granules, short fibres or chips; "coating" means the materials applied. The coating may be a liquid having become solid after drying, e.g. paint. This class does not cover: combinations of operations covered by different subclasses of class , which are covered by subclass ; combinations of operations covered by any particular subclass of class with operations covered by other classes, e.g. with operations involving removal of material, which are also covered by subclass , except that if the operations covered by the other classes are subsidiary to the operations properly covered by a single subclass of the combination is classified in that subclass. Processes of a kind covered by this class but applied to non-metallic materials are classified in this class if they are applicable to metal and cannot be classified fully in another class. This class covers: operations not provided for in any other class; combinations of operations covered by different subclasses of classes , which combinations are covered by subclass , with the exception of subsidiary operations performed in conjunction with main operations covered by a single subclass; features, specific to machine tools, which relate to a requirement or problem of a nature which is not peculiar to a particular kind of machine tool, e.g. feeding work, which are covered by subclass , although the realisation of these features may differ according to the kind of machine tool concerned. The said subclass covers such features, in general, even if the feature or a specific function, in any particular case, is to some extent peculiar to, or is claimed only for, machine tools designed for one particular operation; only in exceptional cases are such features classified in the subclass for the machine tool concerned. Certain features of this general nature are, however, referred to subclasses relating to particular metal-working operations, especially , in which case the subclasses in question are not restricted, in respect of those features, to the kind of machine tool with which they are primarily concerned. In this class, the following terms or expressions are used with the meanings indicated: "metal-working" covers the working of other materials unless the context requires otherwise; "kind of operations" and similar expressions relate to such metal-working operations as boring, drilling, milling and grinding; "kind of machine" means a machine designed for a particular kind of metal-working operation (e.g. a lathe); "form of machine" means a machine of a particular kind adapted or arranged for a particular way of working or for particular work, e.g. face-plate lathe, tailstock lathe, turret lathe; "different machines" covers different forms of machines for performing the same type of metal-working operation, e.g. vertical and horizontal boring machines. If details, components, or accessories have no essential feature specific to machine tools, the more general class, e.g. , takes precedence. In this class, the following term is used with the meaning indicated: "grinding" is used in its most general sense to mean machining and covers, in particular, "corrective" operations. In this class, the following term is used with the meaning indicated: "portable" includes suspension for easy manual handling, e.g. in connection with spring-suspended portable apparatus for use along assembly lines. This class does not cover the working of plastics sheet material in a manner analogous to the working of paper, which is covered by class . In this class, the following term is used with the meaning indicated: "plastics" means macromolecular compounds or compositions based on such compounds. In this class, the following rules apply: The working of plastics is, as far as possible, classified primarily according to the particular shaping technique used, e.g. in subclass . A product is not classified in this class unless it results from a technique which is covered by this class. Classification according to production of particular articles in subclass is restricted to: aspects which are characteristic for the production of a particular article, and not classifiable in subclass or ; combined operations for making the particular article which are not fully classifiable in subclass . The codes of subclass are only for use as indexing codes associated with subclasses , , or so as to provide information concerning moulding materials or materials for reinforcements, fillers or preformed parts, e.g. inserts. The codes of subclass are only for use as indexing codes associated with subclass , so as to provide information concerning the articles produced by the techniques classified in subclass . This class covers subject matter restricted to adaptations or associations of handling sheets, webs, or blanks peculiar to paper-working, e.g. bag or box making, machinery. This class does not cover: making articles directly from paper pulp, which is covered by ; handling sheets, webs, or blanks of wider applicability, irrespective of whether described or claimed only for paper-working machinery, which is regarded as being of a more comprehensive nature and is covered by . In this class, the following term is used with the meaning indicated: "paper" covers material worked in a manner analogous to paper, e.g. plastics sheet materials, laminated materials or metal foils. Any machine, apparatus, tool or process is classified in this class in so far as it produces an effect or mark meant to be judged by the eye and in so far as such machine, apparatus, tool or process is not provided for elsewhere. In this class, the following term is used with the meaning indicated: "vehicle" means all vehicles except those restricted to one of the following types of vehicles: rail vehicles, waterborne vessels, aircraft, space vehicles, hand carts, cycles, animal-drawn vehicles, and sledges, which are covered by the relevant subclasses of . Thus the term "vehicle" includes: vehicular characteristics which are common to more than one of the above-listed types; certain characteristics restricted to automobiles, road or cross-country trailers. The following exceptions to the above should be noted: subclass or embrace all vehicle wheels and tyres, except wheels for roller skates , wheels for model railway vehicles , and special adaptations of wheels or tyres for aircraft ; subclass embraces the connection of valves to inflatable elastic bodies in general, and in this respect it is not limited to vehicles; subclass embraces certain electric equipment of all electrically-propelled vehicles; subclass embraces certain power supply equipment for, but external to, any kind of electrically-propelled vehicle; subclass embraces safety belts or body harnesses used in all types of land vehicles; subclass relates to all kinds of vehicles, except the servicing of rail locomotives , ground equipment for aircraft , or cleaning apparatus peculiar to waterborne vessels , ; subclass includes brake control systems of general applicability, and in this respect it is not limited to vehicles. It also includes rail-vehicle power-brake systems and some other features of rail-vehicle brake systems. In this class, the following expression is used with the meaning indicated: "railway systems" covers: systems in which trains or individual passenger vehicles or load carriers run on, or are guided by, ground or elevated tracks defined by rails, ropes, cables, or other guiding elements for wheels, rollers, or sliding anti-friction devices (permanently attached to a continuous traction element ); systems in which carriers or impellers for persons or loads are attached to, e.g. suspended from, a guided traction rope or cable which determines their path of movement (chain conveyers, scraper conveyers , ); power and free systems of either of the above types in which vehicles, load-carriers, or loads may be selectively coupled to, or uncoupled from, continuous traction members, e.g. cables, chains. This class covers micro-structural devices or systems, including at least one essential element or formation characterised by its very small size, typically within the range of 10^-4 to 10^-7 meters, i.e. its significant features can not, in at least one dimension, be completely discerned without the use of an optical microscope. In this class, the following expressions are used with the meaning indicated: "micro-structural devices" covers: micro-mechanical devices comprising movable, flexible or deformable elements; and three-dimensional structures without movable, flexible or deformable elements, comprising microformations designed to accomplish an essential structural function for interacting with their environment, as opposed to purely electronic or chemical functions, regardless of whether the structures are combined with micro-electronic devices or formed from specific materials; "micro-structural systems" covers: systems of cooperating micro-structural devices; and micro-electro-mechanical or micro-opto-mechanical systems, which combine on a common substrate the specific features of micro-structural devices and electrical or optical components, e.g. for controlling, analysing or signalling the functioning of micro-structural devices.