In biology, the classification of animals and plants, which has sometimes been used as a model for sociological classification, has operated according to two main principles:
While purists may claim that all particular phenomena are different and never absolutely identical, the aim of any classification is to place together all instances of a phenomenon whose similarities, and differences from other types of phenomena, are such as to justify the classification for particular theoretical purposes. See also TAXONOMY.
a system of coordinated concepts (classes of objects) in any area of human knowledge or activity, frequently presented in the form of diagrams (tables) and used as a means for establishing relationships between these concepts or classes of objects, as well as for an exact orientation amid the diversity of concepts or the corresponding objects.
A classification should establish the natural relationships between the classes of objects, with the goal of defining an object’s place in the system; the definition should also indicate the object’s properties. This aspect of classification serves as a means for the storage and retrieval of the information in the classification itself—for example, biological taxonomies, the classification of chemical elements (as in D. I. Mendeleev’s periodic system of elements, the classification of sciences, and the classification of metallurgical processes. Another objective of classification is to provide effective retrieval of information or items kept in special storage (data files, archives, and warehouses); examples include library classification systems, information retrieval languages, and product classifiers.
A truly scientific classification should express the system of laws, inherent to the aspect of reality depicted in it, that determines the properties and relationships of the objects fixed in the classification. The systematization of these laws must take into account that in nature there are no strict delimitations and that transitions from one class to another are an inseparable property of reality. This demand upon classification is reflected in such special procedures as the use in library classifications of cross-references (“see” and “see also”) and the location of the same concept in different places of the classification.
Classification contributes to the advancement of science and the branches of technology from the level of the empirical accumulation of knowledge to the level of theoretical synthesis and the systems approach. Such a transition is possible only with a theoretical analysis of the multiplicity of facts. The practical need for classification encourages the development of the theoretical aspects of science or technology, while the creation of a classificatory system represents a qualitative leap in the development of knowledge. Classification based upon profound scientific principles not only represents a full picture of the state of the science (or technology) or one of its aspects but also makes it possible to draw up valid predictions of still unknown facts or laws. An example is the prediction of properties of still undiscovered elements by the Mendeleev system.
When a classification is a system of coordinated concepts, its structure can sometimes be depicted in the form of an inverted tree. The most general concept corresponds to the node that is the root, the most particular concepts are the leaves, and the names of the remaining classes correspond to the nodes of the branches. The segments that connect all these points express the relationship of subordination that governs the general concepts of varying degrees of abstraction. Lines running from the root to the leaves are called the vertical series of classification; levels equidistant from the general subordinating concept form the horizontal series. Thus, in the Universal Decimal Classification for printed works, the concept of the entire aggregate of printed works corresponds to the root, and the concept is then divided into ten main classes, and so forth.
There are two ways to form the tables of a classification, deductive and inductive. The deductive approach consists in establishing the initial general concepts and the criteria for subdivision. The elucidation of subordinate concepts occurs in the division of the subordinating concept, and the unity of the criteria for subdivision and the stability of classification are guaranteed by the very method of its construction. The inductive approach is based on the concepts of individual objects or aggregates of objects and unites them into classes. The achievement of logical unity and stability of classification becomes more difficult than with the former method. Usually classifications are constructed using both approaches. The more general classes, as a rule, are formed deductively, while the less general ones are formed inductively. Preference is given to deduction in systematizing areas of knowledge and to induction in elaborating the factual material and formulating it in diagrams and tables.
A distinction is made between natural and artificial classifications depending on the degree of essentiality of the criteria for subdivision. If essential features are used as the criteria and if the maximum of derivative features can be derived from these features, so that the classification can serve as the source of knowledge of the objects being classified, then such classification is termed natural (for example, the periodic system of chemical elements). If unessential features are used in the classification, the classification is considered artificial. Artificial classifications include auxiliary classifications (alphabetical subject indexes and name catalogs in libraries). Depending on their breadth, classifications can be encyclopedic (universal), specialized (for one branch of knowledge), or narrow (covering a limited range of uniform phenomena).
At times the term “classification” is used to designate the process of putting objects in classes. Here it would be more correct to use the term “classifying.” The basic principle of this process is the comparing of the designated items with set models or standard representatives of the classes. This principle is used, for example, in biological taxonomies and also lies at the basis of the algorithms for the automatic classification of documents or figures (image recognition).
The problem of the construction and use of classifications has become particularly acute during the modern scientific and technical revolution that has led to the information explosion. The abundance and difficulty of systematization of new concepts and terms, as well as printed and unpublished materials, impede the retrieval and use of the required data, causing an information scarcity that impedes social progress. For this reason the elaboration of an optimal classification becomes not only a scientific but also an economically important problem.
B. V. IAKUSHIN
(in beneficiation of minerals), the separation of fine materials into individual size classes using the differences in the size, shape, and other specific characteristics of the particles being separated. Large pieces (up to 2–4 mm) are divided into classes by screening or sifting. The use of screens for dividing fine materials into individual size classes is difficult, since the fine mesh of the screens becomes clogged, and sifting has low output. The difference in the rate of motion of particles in water or air under the influence of gravity or centrifugal force is most commonly used in classification. In such cases, the speed depends mainly on the size of the particle, but the density and shape are also essential. For example, the rate of fall of a large particle with lower density and a small particle with greater density may be the same. Flat particles fall more slowly than round particles. Therefore, the clearest division of particles by size occurs when the density and shape characteristics are close.
The difficulty of classification increases with decreasing particle size. Very fine particles (less than 10 microns) adhere strongly to each other—that is, they coagulate or flocculate, disrupting the clarity of separation. For the clear separation of fine particles, they must be separated or peptized by adding special peptizing agents, which prevent the cohesion of fine particles. Fine particles descend very slowly in water under the force of gravity, and the classification process becomes unproductive. Devices must be used in which the force of gravity is replaced by centrifugal force, which is hundreds of times stronger than gravity.
Depending on the medium in which the separation of particles by size occurs, a distinction is made between wet (hydraulic) and dry (pneumatic) classification. The advantage of the former is the possibility of classification of wet materials and suspensions, as well as the better separation of cohering particles using peptizing agents. The advantage of dry classification is the elimination of complex drying processes, which increase the cost of classification and sometimes impair the properties of powders.
The theory of classification examines the movement of solid particles in a liquid or gaseous medium on the basis of Stokes’ law, according to which the small particles settle at a speed directly proportional to the square of the cross section and density of the particles and inversely proportional to the viscosity of the medium. When the density of the suspension is relatively high, the rate of fall of the particles is retarded to such a degree that even large particles do not settle out over the time they are in the classifying equipment.
Classification methods have also been developed in which the particles are given a certain electric charge (electric separators); such methods are used for the classification of small quantities of valuable products. Classification is performed in special devices called classifiers. The choice of the classification method depends on the characteristics of the material and the required output, as well as the tasks of the process. Wet classification usually provides optimum sizing of the material before gravity concentration and flotation. Classification sometimes makes possible the output of finished products whose grade is determined by their size (for example, in concentrating kaolin and asbestos and in producing abrasive powders); in such cases various classification methods are possible, including pneumatic classification (in the concentration of asbestos). A special use of classification is to characterize the fineness of powders using sedimentation analysis, which can be performed only under laboratory conditions.
V. I. KLASSEN