Although Hofmann excels as a lecturer and teacher, his reputation rests chiefly on his valuable and numerous contributions to the science of organic chemistry, foremost among which are his investigations on the coal-tar colors.
He first began the study of the bases in coal-tar under the direction of Liebig, and in 1843 we find him publishing his first original paper on this subject. One of these bases, then known as "cyanol," attracted his special attention, and by working over half a ton of coal-tar he succeeded in obtaining this rare base in sufficient quantity to investigate its properties, which he found to be the same as those of "benzidam." Further investigation also enabled him to prove that "aniline," the name then given to a substance that had only been obtained from indigo by distillation, was identical with both cyanol and benzidam. Here, then, were three sources for obtaining this rare material. Evidently there could not be much of it in coal-tar, when only three pounds could be separated from half a ton of tar indigo, too, was an expensive source; hence it was a fortunate circumstance that Zinin had discovered another method of making it, and that too from a far more abundant constituent of coal-tar, namely, benzol; it is from that all the aniline of the present day is prepared.
Hofmann, it is said, noticed that aniline gave rise, under certain conditions, to the production of a red color, but he failed to publish the fact, and to Perkin belongs the credit of having discovered the first aniline dye mauvine. This took place in 1856, and two years later Hofmann discovered a red dye, then called Hofmann's red, which was formed by the action of chloride of carbon upon aniline. Aniline was beginning to attract the attention of manufacturers as well as of chemists, and many different methods were devised for making what seemed to be the same substance, a fine red dye variously known as magenta, solferino, fuchsine, and aniline red. Hofmann undertook a careful investigation of the dye, which resulted in his discovery of the surprising fact that the red dye was in reality the salt of an organic base, like an alkaloid, and that this base, to which he gave the name of "rosaniline," was colorless. From this base he prepared another which he called "leucaniline" by reducing it with zinc. Turning his attention to the blues, greens, and purples, he found them to be derivatives from this same base, but of more complex construction. The importance of these investigations can scarcely be overestimated. The production of dyes from aniline was no longer a matter of blind experimentation; empirical methods gave place to scientific ones, and the process of making dyes has gone on to the present day nearly in the same direction. One of the earliest practical results of this discovery was the invention of a series of most beautiful purples which still bear the name of Hofmann. Like Leverrier's discovery of Neptune, their elements had been calculated beforehand, their existence foretold, and they needed only to be made.