A photon counter which measures weak light in the wavelength range 2,000 to 6,000 Å has been constructed. This device counts the amplified photoelectron pulses released in a photomultiplier tube. It was shown both experimentally and theoretically that the measured pulse rate r, is related to the light intensity I by r∞Ik where k is unity at low discriminator heights but increases as the discriminator height is raised. The photon counter was used to investigate the following light emitting reactions between liquids. It was observed that solutions which generate di-imide cause alkaline solutions of luminol [Diagram] to emit light even when hydrogen peroxide is not present. Kinetic measurements were consistent with the view that in the usual light emitting reactions of luminol, H₂O₂ first oxidises a molecule of luminol to di-imide which collides with and excites another luminol molecule. Weak light was detected when aqueous solutions of inorganic acids and bases were reacted in a flow system at 0.066 moles/second. The intensity of the light from the reaction between analytical grade NaOH and H₂SO₄ was unaffected by displacing dissolved oxygen from the solutions but was reduced 3/4 when the acid and alkali were strongly heated to remove organic impurities. The quantum yield after purification was 1 photon per 10¹⁹ reacting molecules. No light (mitogenetic radiation) could be detected from rapidly dividing cultures of various yeasts and bacteria. If light is emitted, it must be less than 1/8 as intense as the light from a reference solution containing 0.01M KMnO₄ and 0.01M oxalic acid. The literature relating to the above studies has been reviewed and computer programs for curve fitting, graph plotting, the generation of random numbers and the calculation of pulse overlap have been written.