Book excerpt: The Collected Papers of Paul Ehrlich, Volume I covers topics on histology, biochemistry, and pathology. The book presents topics on practice and theory of staining in histology; cytology of the blood in health and disease; and the staining of bacteria and its application to diagnosis. The text also includes topics on the requirement of the organism for oxygen and other allied biochemical studies; pharmacology, therapeutics and toxicology; and the diazobenzenesulphonic acid and dimethylaminobenzaldehyde reactions.