CERTAIN QUANTITATIVE RELATIONS OF NITRATE AND NITRITE METABOLISM IN FARM ANIMALS WITH SPECIAL REGARD TO THE ORIGIN AND DEVELOPMENT OF METHAEMOGLOBINEMIA CAUSED BY NITRITES AND THE DIAGNOSIS OF POISONINGS : I. METHODS FOR DETERMINING NITRATE, NITRITE AND METHAEMOGLOBIN IN THE BLOOD. NORMAL METHAEMOGLOBIN CONTENT IN THE BLOOD OF FARM ANIMALS