But the influence of molecular or atomic grouping in determining the specific character of a substance is not confined to compounds. The same phenomenon is observed to occur among the elements. It was conclusively established by Lavoisier that the diamond and charcoal are chemically the same thing—both forms of carbon. Scheele showed that graphite was a third form of carbon. Phosphorus, sulphur, and oxygen were subsequently shown to be each capable of existence in various modifications. Instances of this character were grouped together in 1841 by Berzelius under the term _allotropy_. The recognition of the fact of isomerism exerted a great influence on the development of organic chemistry. It ultimately led to the assumption that particular groups of elements or atomic complexes, so-called _radicals_, were to be found in organic compounds—a conception based originally on Gay Lussac’s discovery of _cyanogen_, a combination of carbon and nitrogen, which was found to behave like a simple substance, such as chlorine, and to give rise to compounds analogous to the corresponding chlorides. This idea of the existence of compound radicals was greatly strengthened by a memorable investigation by Liebig and Wöhler, in 1832, on oil of bitter almonds and its derivatives, in which they showed that these substances might be represented as containing a special group or radical termed _benzoyl_, which behaved like an element. The idea of groups of elements going in and out of combination like a simple substance was not new to chemists: there was not only the case of cyanogen, discovered by Gay Lussac in 1815. The attempt had been made by Dumas and Boullay in 1828 to classify the derivatives of alcohol and ether as compounds containing a common radical _etherin_. Gay Lussac had pointed out that the vapour density of ethyl alcohol seemed to show that it consisted of equal volumes of ethylene and water. Robiquet had also shown that ethyl chloride might be assumed to be a compound of hydrochloric acid and ethylene; and Döbereiner had regarded anhydrous oxalic acid as a combination of carbonic acid with carbonic oxide.