reaction (re-ak's hun). 1

An action or motion resulting from and opposed to another action or motion. In Newton's third law of motion, the word r. designates the effect produced upon a moving body by its own motion as distinguished from "action," the effect which it produces upon other bodies. The word hence denotes mutual action, interaction. 2. In chemistry, a physical phenomenon occurring on the application of a chemical test, and hence indicative of the po e ion of a special chemical property; also, a physical phenomenon indicative of a change in the chemical state of a body; hence the change of state itself, whether this is brought about by the molecular interaction of two or more substances or the application of a physical force. 3. In electrophysiology, a physical phenomenon occurring upon the application of electricity and indicating therefore the po e ion of certain electrical properties or some change in electrical condition. 4. In psycho-analysis the psychical response to an emotional situation. 5. The proce of recovery from "shock." 6. The development of fever as the result of an injury, a surgical operation, or the like, acid r. 1. The reddening of blue litmus paper to test the presence of acids. Adamkiewicz's r. See Adamkiewicz- Hopkins r. agglutination r. See Widal's agglutination r., under Widal. alkaline r. A r. indicative of alkaline properties, such as the turning of red litmus paper blue by an alkali, amphicroitic r., amphigenous r., amphoteric r., amphoterous r. A combination of acid and alkaline r. shown under certain conditions, when substances stain red litmus paper blue and blue litmus paper red. biuret r. A r. by which a trace of a solution of copper sulphate and an exce of caustic soda produces a reddish or deep violet color when brought into the presence of biuret, of proteins, of their first decomposition products, the proteoses and peptones, chemical r. See r. (2d def.). diazo-r. The dark red r. obtained with certain pathological urines when treated with Ehrlich's reagent, which is diazo benzene sulphuric acid.