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It is not the purpose of this book to deal with every phase of bioluminescence, the production of light by animals and plants. Volumes could be written on the evolutionary side of the problem and the structure and uses of luminous organs. These questions can only be touched upon. Neither is it the purpose to discuss the ultimate cause of light, whether due to vibration of electrons or to other causes. That problem must be left to the physicist, although it is highly probably that a study of animal light will give important information regarding the nature of light in general, and no theory of light can be adequate which fails to take into account the extraordinary powers of luminous animals. This work is concerned largely with the physical characteristics of animal light and the chemical processes underlying its production. CONTENTS (Originally published in 1920.) I. Light-Producing Organisms Early records and theories. "Shining fish and flesh." "Burning of the sea." Distribution of luminous organisms in plant and animal kingdoms. Secondary luminosity. False luminosity. St. Elmo's fire. Ignis fatuus. Flashing of flowers. Luminosity in man. Use to man of photogenic organisms. II. Luminescence and Incandescence The complete spectrum. Radiation and temperature. "Cold light." Thermoluminescence. Phosphorescence and fluorescence. Triboluminescence and piezoluminescence. Crystalloluminescence. Chemiluminescence. III. Physical Nature of Animal Light Purkinje phenomenon. Color and spectra of animal light. Polarization. Efficiency of animal light. Infra-red radiation. Ultra-violet radiation. Luminous efficiency and visual sensibility. Production of radiation penetrating opaque objects. Intensity of animal light. Summary. IV. Structure of Luminous Organs Photochemical and chemiphotic changes. The eye and the luminous organ. Intracellular and extracellular luminescence. Continuous and intermittent luminescence. Periodicity of luminescence. Luminous bacteria. Noctiluca and photogenic granules. Chætopterus and luminous gland cells. Cypridina. Luminous glands. The firefly. Luminous organs (photophores) with lenses, reflectors, opaque and color screens. Uses and purpose of animal light. V. The Chemistry of Light Production, Part I Boyle's and Spallanzani's experiments. Shining wood and burning coal. Oxygen and luminescence. Carbon dioxide and luminescence. Heat production during luminescence. Luminescence and respiration. Water and luminescence. Phipson's noctilucin. Luciferin and luciferase. Photogenin and photophelein. Proluciferin. Oxyluciferin. Pyrophorin or luciferescein. Chemiluminescent reactions. "Biozymoöxyluminescence." VI. The Chemistry of Light Production, Part II Pyrophorus luciferin and luciferase. Pholas luciferin and luciferase. Cypridina luciferin; stability, hydrolysis by acid and enzymes, adsorption, precipitation, salting out, solubility, distribution. Cypridina luciferin a proteose? Cypridina luciferase and properties. Cypridina luciferase an albumin. Specificity of luciferase. Action of fat solvent anæsthetics. Action of cyanides. Oxyluciferin. Nature of oxidative reaction. VII. Dynamics of Luminescence Minute amounts of material for luminescence. Reaction velocity and chemiluminescence. Temperature and chemiluminescence. Oxidation in steps. Concentration and bioluminescence. Temperature and bioluminescence. Oxidation with and without luciferase. Reaction velocity and color of bioluminescence.