Much of the vague philosophy of disease in past times may fairly be attributed to the complexity and mystery of action inherent in living matter. The subjects of physics, chemistry and biology, in their wider acceptation, were unevolved, and scientific pathology, the offspring of this ancestry, was yet unborn. How much we owe to physics, chemistry, and biology, those handmaids of medicine, is inestimable! But something at least of our debt thereto will be revealed in the following pages. Of the purins in human urine, the most important is _uric acid_, and far behind comes xanthine, while traces of hypoxanthine, guanine, and adenine are also detectable. Some years ago the current view was that the metabolism of any _protein_ gave rise to _uric acid_. This assumption has now proved to be erroneous, for it is known that only certain foodstuffs lead to an increase in the uric acid excretion; in other words, on a diet rich in _purin_ the output thereof is considerably higher than on a purin-free diet, this being due to the large amount of _nuclein_ and purin bases in flesh foods, especially those containing glandular substances. Under ordinary conditions the excretion of uric acid ranges from 0·3-1·2 gm. per diem, or 0·02-0·10 per cent. The oscillations in output vary with the state of health, diet, and personal idiosyncrasy. CHEMICAL CONSTITUTION The empirical formula of the uric acid molecule, C₅H₄N₄O₃, has for long been known, but it was reserved for Emil Fischer to reveal the chemical structure thereof. Through his labours we now know that uric acid is one of a group of substances which owe their kinship to their possession in common of the heterocyclic ring termed by Fischer the “purin nucleus” (1898). The intimate relations of the purins of bio-chemical interest to the purin nucleus, and alike to each other, will be rendered more intelligible by examination of their structural formulæ as hereafter given. All, as will be seen, are derivatives of a synthetically formed body _purin_ which, though unimportant in itself, is yet interesting in that it is the basic substance from which the following take origin:— Purin C₅H₄N₄ Hypoxanthine C₅H₄N₄O Monoxy-purin } Adenine C₅H₃N₄NH₂ Amino-purin } Xanthine C₅H₄N₄O₂ Dioxy-purin } Purin Bases. Guanine C₅H₃N₄ONH₂ Aminooxy-purin } Uric acid C₅H₄N₄O₃ Trioxy-purin } It now devolves upon us to note the arrangement of the atoms in the purin nucleus. To each atom is affixed a number indicating the exact location of the various atoms and groups attached to the said nucleus. The manner in which the various purin bodies are built up around the _purin nucleus_ C₅N₄ will become apparent from a study of the following structural formulæ culled from Wells’ “Chemical Pathology”:— STRUCTURAL FORMULÆ