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Communication is not a tool used only by humans, plants and animals, but it is also used by microorganisms like bacteria. The process is called quorum sensing. Through quorum sensing, bacteria are able to sense the density of cells, and regulate gene expression accordingly. This can be seen in both gram positive and gram negative bacteria. This was first observed by Fuqua et al. in marine microorganisms like V. harveyi and V. fischeri.

During the 20th century, the study of classics became less common. In England, for instance, Oxford and Cambridge universities stopped requiring students to have qualifications in Greek in 1920, and in Latin at the end of the 1950s. When the National Curriculum was introduced in England, Wales, and Northern Ireland in 1988, it did not mention the classics. By 2003, only about 10% of state schools in Britain offered any classical subjects to their students at all. However, the study of classics has not declined as fast elsewhere in Europe. In 2009, a review of Meeting the Challenge, a collection of conference papers about the teaching of Latin in Europe, noted that though there is opposition to the teaching of Latin in Italy, it is nonetheless still compulsory in most secondary schools.

Ancient Greece was the civilization belonging to the period of Greek history lasting from the Archaic period, beginning in the eighth century BC, to the Roman conquest of Greece after the Battle of Corinth in 146 BC. The Classical period, during the fifth and fourth centuries BC, has traditionally been considered the height of Greek civilisation. The Classical period of Greek history is generally considered to have begun with the first and second Persian invasions of Greece at the start of the Greco-Persian wars, and to have ended with the death of Alexander the Great. Classical Greek culture had a powerful influence on the Roman Empire, which carried a version of it to many parts of the Mediterranean region and Europe; thus Classical Greece is generally considered to be the seminal culture which provided the foundation of Western civilization.

Chemistry is sometimes called the central science because it bridges other natural sciences, including physics, geology and biology. For the differences between chemistry and physics see comparison of chemistry and physics. The history of chemistry can be traced to alchemy, which had been practiced for several millennia in various parts of the world.

A compound is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of its elements. The standard nomenclature of compounds is set by the International Union of Pure and Applied Chemistry (IUPAC). Organic compounds are named according to the organic nomenclature system. The names for Inorganic compounds are created according to the inorganic nomenclature system. When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components. In addition the Chemical Abstracts Service has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as its CAS registry number.