residue of the alcoholic extract was too high for the body to be protagon, as had previously been supposed. He put his pupils Manassien and Diakonow on this problem. The former quickly found the substance to he not protagon, hut lecithin, a peculiar nitrogenous phosphorized fat, and the latter laid bare the chemical nature of this substance. Hoppe-Seyler himself proceeded to examine other cells for the presence of lecithin and cholesterin, and soon recognized that these substances, as well as the albumins, potassium phosphate, and glycogen, must he considered ever-present constituents of protoplasm. Hoppe-Seyler further showed that the lecithin generally existed in the cell, not free, hut in combination with albumin. This fact drew Hoppe-Seyler's attention to similar compounds which are formed by albumin combined with other substances, and he proposed for such bodies the name "proteids," a name which they have subsequently borne.
The discovery of lecithin drew attention to the organic compounds of phosphorus in protoplasm. There were many indications that the nuclei contained such a substance, although the difficulties in its isolation had hitherto been insurmountable. His pupil Miescher was placed on this problem, and discovered the "nucleins," probably the most important constituents of protoplasm thus far recognized. Hoppe-Seyler himself discovered "vitellin," the first of the pseudo-nucleins, and casein was soon shown to have a similar phosphorized radicle.
In the chemistry of respiration Hoppe-Seyler was personally active, and formulated a hypothesis of the chemical nature of respiration and living matter which is practically the only well-grounded theory of its kind extant, and one which has seldom been surpassed for brilliancy. by a stroke of genius he correlated the newly discovered reducing powers of protoplasm with its oxidizing powers, and recognized the essential identity of the processes of life and putrefaction. This idea threw light on many phases of cellular metabolism. It was first briefly mentioned in the Medicinische-chemische Untersuchungen in 1871. It was clearly stated in volume xii of Pflüger's Archiv für die gesammte Physiologie some years later, and forms the kernel of most of his subsequent work.
The oxygen of the air, as is well known, is in a molecular or inert state, and not in an atomic form. It is unable, in this form, to carry out the oxidations of protoplasm. To do this it must he first made "active," either by conversion into ozone or by having its molecule split. Hoppe-Seyler's theory of respiration turns on the fact that in protoplasm and in putrefying masses, reducing substances are formed which are able to split the oxygen molecule, setting free atomic oxygen. In this manner, if only air
