EDITOR'S LETTER BOX. [BRITISH MEDICAL JOURNAL. in Thames water, but still in as large quantity as in a water impregnated witlh one part in ten of sewage matter. It exists in larger quantitieslin surface wells, and is absent in deep well water. It evidently arises from the decomposition of animal and vegetable mnatter, and indicates the impurity of the water in which it is found. By oxidation, ammonia becomes converted into nitric acid, and this acid, combined with potash,