Before taking up the investigation of the aniline dyes, Hofmann had been engaged in a line of research, which, though apparently of mere theoretical interest, had especially fitted him for this work, namely, the study of organic ammonias, or amines. In 1849-'50 Hofmann made the discovery that when ammonia was acted upon by certain alcoholic iodides, such as methyl iodide, one, two, or three of the hydrogen-atoms of the ammonia could be replaced by the alcoholic radical. In this way he prepared trimethylamine, a substance which he subsequently found to exist ready formed in herring-pickle, and from which it is still obtained for medicinal purposes. For his investigations on the molecular constitution of the organic bases, he was awarded the Royal Medal in 1854, and in 1887 he received the great prize of the World's Fair at Paris.
Engaged in studies of this sort, the resemblances between aniline oil and ordinary ammonia, and more especially between their respective salts, could not escape his notice. Each contains one atom of nitrogen; the substitution of a certain group of atoms known as the phenyl group for one of hydrogen will convert ammonia into aniline. In the more complex molecule of rosaniline, with its three atoms of nitrogen, he naturally sought for a triple ammonia, but he found the phenyl group alone incompetent to form this base, which led to his discovery of the very important fact that no dyes can be made from pure aniline, an admixture of its homologue, toluidine, being essential to the production of the rosaniline and its derivatives.
Organic bases, containing other elements than nitrogen, have also attracted his attention, and through his labors much has been added to our knowledge of the "phosphines," phosphonium, etc.
Another class of subjects, to which Hofmann has devoted much attention, includes the mustard-oils, both natural and artificial, and the sulpho-cyanides of organic bodies. These researches have resulted in the artificial production or synthesis of many pungent oils and ethers. He has also fearlessly attacked the cyanides themselves, and succeeded in producing some new organic compounds that fairly rival Bunsen's well-known cacodyle in their repulsive odors.
Among the analytical processes introduced by Dr. Hofmann are several of importance, including the separations of arsenic from antimony, and of copper from cadmium, and the detection and estimation of carbon disulphide. Hofmann's method of determining the specific gravity of vapors is as remarkable for its simplicity as for its accuracy.
Although a fertile writer, Professor Hofmann is not given to writing books. He has, however, contributed a great many original papers to various chemical journals, of which the "Journal of the London Chemical Society" contains more than ninety, and nearly two hundred more are to be found in the "Berichte" of the Berlin Chemical Society. He was for a time one of the editors of Fowne's "Manual of Chemis-
