PHOTODYNAMIC ACTION.

H. von Tappeiner originated the term photodynamic action to designate the toxic effects exerted by fluorescent substances in the light. He first worked with certain chemical substances, the chinin derivatives, and showed that the toxic action was due to a definite nucleus in the molecule. Grethe and later Raab studied the marked toxic effect that certain fluorescent substances have on the paramecium; and Raab traced this to the action of light on these chemicals since in the dark they were quite ineffective. The sunlight does not affect non-fluorescent poisons in this way. The fluorescent substance must be in solution and in order to obtain the toxic effect the living cells must be immersed in the fluid as fluorescent light itself has no effect; further, light passing through one fluorescent solution has lost its power to cause a second solution to fluoresce. Photodynamic action and absorption of light are not identical,