Despite V. Mach’s revelation, the origin of uric acid from nucleic acid was still to seek. In the year following (1889) Horbaczewski traced it to this source, and in the following manner. Mixed with water, the pulp of the calf’s spleen was put to digest at 50° until putrefaction began. The fluid was then sterilised with a solution of lead acetate, and arterial blood being added it was kept at 50°, a current of air meanwhile being passed slowly through the mixture. Subsequently the fluid was found to contain _uric acid_; but the experiment being repeated, without the passage of air, _xanthine and hypoxanthine_ and not uric acid resulted. While Horbaczewski’s experimental findings were amply confirmed, some of his deductions therefrom were subsequently proved faulty. (Thus, he thought putrefaction an essential factor; also he believed that the formation of uric acid ensued _before_ the purin groups were disengaged from the nucleic acid, and definitely affirmed that the uric acid was not produced by the oxidation of _free_ xanthine or hypoxanthine.) But, nevertheless, this pioneer established that in both _man_ and rabbits _uric acid_ was derived from _nucleic acid_. Also, having observed that when after starvation the food intake was resumed, a _leucocytosis_ occurred, he announced his belief in the following theory. Thus, he noted that _leukæmics_, whose blood showed a high leucocyte count, excreted an unusually large amount of uric acid; consequently, he came to the conclusion that _uric acid_ was formed from _defunct leucocytes_. Also that nuclein-rich food, when ingested, contributed to the formation of uric acid only in so far as it induced leucocytosis. Hence the origin of the increased uric acid excretion which occurs when feeding is resumed after starvation. This increased excretion of uric acid after the ingestion of food rich in _nucleic acid_ has been amply confirmed; but all the earlier attempts to achieve an increased excretion by the ingestion of _free_ purin bases, as opposed to the _combined_ purin bases, existing as such in _nucleic acid_, failed, although tried repeatedly. So much for the various stages by which our knowledge of the purin derivatives of nucleic acid has been gradually acquired, for though _purin bases_ had, from early times, been known to exist in _animal_ tissues, their presence there could not be rationally accounted for prior to the discovery of _nucleic acid_. It still remains for us to deal in detail with the further developments of our knowledge which concern the _disruption of nucleic acid in the body_ and the process by which _uric acid_ is derived therefrom. But before proceeding to consider in detail the complex series or _enzymatic_ transformation that this entails, it will, I think, be wiser to deal first with the _chemistry_ of uric acid, its _solubilities_, and its _sources_, whether exogenous, endogenous or synthetic. THE CHEMISTRY OF URIC ACID AND THE PURIN BODIES