In recent years, several substances have been obtained which resemble one another in the fact that all of them contain in their molecules phosphoric acid and a sugar, but which differ one from another in the number and in the character of the nitrogenous radicals contained in their molecules. To this group of substances belong: (1) glucophosphoric acid; (2) inosinic acid and guanylic acid; (3) yeast nucleic and triticonucleic acid; and (4) thymonucleic acid.All these substances may be classified as nucleic acids:1. The first substance is a glucophosphoric acid proper.2. Inosinic and guanylic acids are monopurin-glucophosphoric acids. Each of them contains in its molecule only one purin base besides the glucophosphoric acid.3. Yeast and triticonucleic acids each contains two purin and one pyrimidin radical in its molecule and may be regarded as dipurin-monopyrimidin-glycophosphoric acid.4. Finally, thymonucleic acid is a dipurin-dipyrimidin-gluco-phosphoric acid.Conclusions as to the nature and existence o...