It was early recognized that the alkaloids were complex amines or ammonia derivatives. The more or less strongly marked basic character of these bodies, the presence of nitrogen as an essential element, and, above all, the analogy shown to ammonia in the way these bases united with acids to form salts, not by replacement of the hydrogen of the acid, but by direct addition of acid and base, pointed unmistakably to this constitution. But with this granted, the simplest alkaloid formulas, those of conine, C_{8}H_{17}N, and nicotine, C_{10}H_{14}N_{2}, still showed that the amine molecule contained quite complex groups of carbon and hydrogen atoms, and the great majority of the alkaloids--the non-volatile ones--contained groups in which the three elements, carbon, hydrogen, and oxygen, all entered. Hence the difficulty in acquiring a knowledge of the molecular structure of those alkaloids at all comparable with that attained in the case of other organic compounds. Of course synthesis could not be applied until analysis had revealed something of the molecular grouping of these compounds, so the action of different classes of reagents was tried upon the alkaloids. Before summarizing the results of this study of the decomposition and alteration products of the alkaloids, a brief reference to a related class of organic compounds will be of assistance to those unfamiliar with recent researches in this field. It is well known that in coal-tar is found a series of ammonia-like bases, aniline or amido-benzol, toluidine or amido-toluol, and xylidine or amido-xylol, which are utilized practically in the manufacture of the so-called aniline dye-colors. It is perhaps not so well known that there are other series of bases found there too. The first of these is the pyridine series, including _pyridine_, C_{5}H_{5}N, _picoline_ (methyl-pyridine), C_{5}H_{4}N(CH_{3}), _lutidine_ (dimethyl-pyridine), C_{5}H_{5}N(CH_{3})_{2}, and _collidine_ (trimethyl-pyridine), C_{5}H_{2}N(CH_{3})_{3}. This series is also found in relatively larger proportion in what is known as Dippel's oil, the product of the dry distillation of bones.