CARBON COMPOUNDS, in chemistry, those compounds which contain the element carbon. These are of two classes, the organic and the inorganic, the former being by far the larger and more important;• so much so that the chemistry of the carbon compounds is practically synonymous with 'organic chem istry.' Until within the past half-century it was thought by many authorities that the com pounds that occur in animals and plants are essentially different in nature from those that are produced in the laboratory, and that they cannot be obtained without the action of the "vital principle.' This idea received its first blow in 1828, when Wader prepared urea from substances that had been previously considered to be inorganic; yet as late as 1849 the great chemist, Berzelius, defined organic chemistry as "the chemistry of compounds formed under the influence of life.' A vast number of sub stances that were formerly classed as organic have now been prepared in the laboratory, and the old classification of chemistry into organic and inorganic branches has broken down, the organic division being now more correctly called the "chemistry of carbon compounds.' The organic carbon compounds form a group of great complexity, and arc apparently unlimited in number. The reasons for this are that carbon is quadrivalent; that it forms multitudes of compounds with hydrogen alone, in many of which more or less of the hydrogen can be replaced by other elements, with the formation of new and altogether different sub stances; that its chemical bonds are apparently powerful; and that it unites with elements of the most widely different nature.
In a general way, the better-known carbon compounds are mostly divided into two great classes, according to the type of the 'graphical' or °structural° formula that must be used in order adequately to represent their chemical re lations. The first class includes all those bodies
whose structural formula: are distinguished by the fact that the atoms (or radicals) that are present form 'open' chains, which do not any where return into one another. The hydrocar bon 'propane," which has the structural formula H H H H —C — C — H H H is an illustration of this class. The ((open chain' compounds are called fatty compounds, and are treated under that heading. The name was originally given because many of the sub stances that are included in the class have long been known in connection with fats and allied bodies; but it would be more logical to call them "methane derivatives,* since they may be considered to be obtainable from the bon methane. CH,. by a process of substitution. The second great class of carbon pounds is guished by the fact that the structural formula' that are quired in order to hibit the chemical properties of its bers return into selves, so as to form 'closed* chains or rings, which (at least in the fundamental forms) contain six carbon atoms. zene is a familiar example. From the fact that many of the first known representatives were balsams, oils and resins, these substances are known collectively as aromatic compounds, and are described under that heading. A better name would be "benzene derivatives,* since all the members of the class are derivable from benzene by substitution. See AROMATIC COM