introduced into the rosaniline molecule to produce dyes of various purple or violet colours, which became known as 'Hofmann's violets'. In 1864, Hofmann confirmed that magenta can only be made by oxidation of commercial aniline in which isomeric orthotoluidine and paratoluidine are present as impurities, not from pure aniline. Other students of Hofmann's who became involved in the British dyestuffs industry include Edward Chambers Nicholson, George Maule, and George Simpson. After his return to Germany, Hofmann continued to experiment with dyestuffs, finally creating quinoline red in 1887. Hofmann studied nitrogen bases, including the development of methods for separating mixtures of amines and the preparation of large numbers of "polyammonias" (diamines and triamines such as ethylenediamine and diethylenediamine). He worked with Auguste Cahours on phosphorus bases between 1855 and 1857. With him, in 1857, Hofmann prepared the first aliphatic unsaturated alcohol, allyl alcohol, C3 H5OH. He also examined its derivative, allyl isothiocyanate (mustard oil), in 1868, and studied various other isocyanates and isonitriles (isocyanides, or carbylamines). Hofmann also developed a method for determining the molecular weights of liquids from vapour densities. Hofmann isolated sorbic acid from rowanberries' oil in 1859, a chemical compound that is widely used as a food preservative. In 1865, inspired by Auguste Laurent, Hofmann suggested a systematic nomenclature for hydrocarbons and their derivatives. It was adopted internationally by the Geneva Congress, with some modifications, in 1892. Molecular models Hofmann was apparently the first to introduce molecular models into organic chemistry, following August Kekule's introduction of the theory of chemical structure in 1858, and Alexander Crum Brown's introduction of printed structural formulas in 1861. At a Friday Evening Discourse at London's Royal Institution on April 7, 1865, he displayed molecular models of simple organic substances such as methane, ethane, and methyl chloride, which he had had constructed from differently colored table croquet balls connected together with thin brass tubes. Hofmann's original colour scheme (carbon = black, hydrogen = white, nitrogen = blue, oxygen = red, chlorine = green, and sulphur = yellow) has evolved into the CPK colour scheme and is in use even today. After 1874, when van't Hoff and Le Bel independently suggested organic molecules can be three-dimensional, molecular models began to assume their modern appearance. Hofmann voltameter The Hofmann voltameter is an apparatus for electrolyzing water, invented by August Wilhelm von Hofmann in 1866. It consists of three joined upright cylinders, usually glass. The inner