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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:isomerism:43887f277430",
    "title": "ISOMERISM",
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    "verified_text": "isomerism, in chemistry. when wohler, in 1825, analysed his cyanic acid, and liebig his quite different fulminic acid in 1824, the composition of both compounds proved to be absolutely the same, containing each in round numbers 28% of carbon, 33% of nitrogen, 37% of oxygen and 2% of hydrogen. this fact, inconsistent with the then dominating conception that difference in qualities was due to difference in chemical composition, was soon corroborated by others of analogous nature, and so berzelius introduced the term _isomerism_ (gr. [greek: isomeres], composed of equal parts) to denominate the existence of the property of substances having different qualities, in chemical behaviour as well as physical, notwithstanding identity in chemical composition. these phenomena were quite in accordance with the atomic conception of matter, since a compound containing the same number of atoms of carbon, nitrogen, oxygen and hydrogen as another in the same weight might differ in internal structure by different arrangements of those atoms. even in the time of berzelius the newly introduced conception proved to include two different groups of facts. the one group included those isomers where the identity in composition was accompanied by identity in molecular weight, i.e. the vapour densities of the isomers were the same, as in butylene and isobutylene, to take the most simple case; here the molecular conception admits that the isolated groups in which the atoms are united, i.e. the molecules, are identical, and so the molecule of both butylene and isobutylene is indicated by the same chemical symbol c4h8, expressing that each molecule contains, in both cases, four atoms of carbon (c) and eight of hydrogen (h). this group of isomers was denominated metamers by berzelius, and now often \"isomers\" (in the restricted sense), whereas the term _polymerism_ (gr. [greek: polys], many) was chosen for compounds like butylene, c4h8, and ethylene, c2h4, corresponding to the same composition in weight but differing in molecular formula, and having different densities in gas or vapour, a litre of butylene and isobutylene weighing, for instance, under ordinary temperature and pressure, about 2.5 gr., ethylene only one-half as much, since density is proportional to molecular weight. a further distinction is necessary to a survey of the subdivisions of isomerism regarded in its widest sense. there are subtle and more subtle differences causing isomerism. in the case of metamerism we can imagine that the atoms are differently linked, say in the case of butylene that the atoms of carbon are joined together as a continuous chain, expressed by --c--c--c--c--, _normally_ as it is called, whereas in isobutylene the fourth atom of carbon is not attached to the third but to the second carbon atom, i.e. c-- / --c--c . \\",
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