In 1855 Adolph Wurtz had shown that when sodium acted upon alkyl iodides, the alkyl residues combined to form more complex hydrocarbons; Fittig developed this method by showing that a mixture of an aromatic and alkyl haloid, under similar treatment, yielded homologues of benzene.
Upon these follow special methods of induction applicable to quantity, viz., the method of curves, the method of means, the method of least squares and the method of residues, and special methods depending on resemblance (to which the transition is made through the law of continuity), viz.
But its chief technical application depends upon the formation of sulphonic acids when it reacts with aromatic hydrocarbon residues; these compounds being important either as a step towards the preparation of hydroxy-compounds, e.g.
Most of the pyrites consumed in the United Kingdom come from Spain; this Spanish pyrites generally (not always) contains enough copper (say 3 or 4%) to make its extraction from the residues ("cinders) a paying process, and this of course cheapens the price of the sulphur to the acid manufacturer.
In this process, the residues are boiled with a dilute sulphuric acid to which nitric acid and potassium chlorate are added in order to transform the element into selenic acid, H 2 Se0 4, which is then reduced to selenious acid, H 2 Se0 3, by boiling with hydrochloric acid, and finally to selenium by sulphur dioxide.