Now one of the men who was worried over this benzene puzzle was the German chemist, Kekule. One evening after working over the problem all day he was sitting by the fire trying to rest, but he could not throw it off his mind. The carbon and the hydrogen atoms danced like imps on the carpet and as he watched them through his half-closed eyes he suddenly saw that the chain of six carbon atoms had joined at the ends and formed a ring while the six hydrogen atoms were holding on to the outside hands, in this fashion: H | C / \\ H-C C-H || | H-C C-H \ // C | H Professor Kekule saw at once that the demons of his subconscious self had furnished him with a clue to the labyrinth, and so it proved. We need not suppose that the benzene molecule if we could see it would look anything like this diagram of it, but the theory works and that is all the scientist asks of any theory. By its use thousands of new compounds have been constructed which have proved of inestimable value to man. The modern chemist is not a discoverer, he is an inventor. He sits down at his desk and draws a "Kekule ring" or rather hexagon. Then he rubs out an H and hooks a nitro group (NO_{2}) on to the carbon in place of it; next he rubs out the O_{2} of the nitro group and puts in H_{2}; then he hitches on such other elements, or carbon chains and rings as he likes. He works like an architect designing a house and when he gets a picture of the proposed compounds to suit him he goes into the laboratory to make it. First he takes down the bottle of benzene and boils up some of this with nitric acid and sulfuric acid. This he puts in the nitro group and makes nitro-benzene, C_{6}H_{5}NO_{2}. He treats this with hydrogen, which displaces the oxygen and gives C_{6}H_{5}NH_{2} or aniline, which is the basis of so many of these compounds that they are all commonly called "the aniline dyes." But aniline itself is not a dye. It is a colorless or brownish oil. It is not necessary to follow our chemist any farther now that we have seen how he works, but before we pass on we will just look at one of his products, not one of the most complicated but still complicated enough. [Illustration: A molecule of a coal-tar dye] The name of this is sodium ditolyl-disazo-beta-naphthylamine- 6-sulfonic-beta-naphthylamine-3.6-disulfonate.