reaction } (?). [After Fernand
Widal (b. 1862), French
physician.] (Med.) A test for typhoid fever based on the
fact that blood serum of one affected, in a bouillon culture of typhoid bacilli, causes the
bacilli to agglutinate and lose their motility.
Re*ac"tion , n.
(Psycophysics) A regular or characteristic response to a
stimulation of the nerves.
Re*ac"tion (r&esl;*ăk"shŭn), n. [Cf. F. réaction.]
1. Any action in resisting other action or force; counter tendency; movement in a contrary
direction; reverse action.
2. (Chem.)
The mutual or reciprocal action of chemical agents upon each other, or the action
upon such chemical agents of some form of
energy, as heat, light, or electricity, resulting in a chemical
change in one or more of these agents, with the production of new compounds or the manifestation of distinctive characters. See Blowpipe reaction, Flame reaction, under Blowpipe, and Flame.
3.
(Med.) An action
induced by vital resistance to some other action; depression or exhaustion of vital force consequent on overexertion or overstimulation; heightened activity and overaction succeeding
depression or shock.
4. (Mech.)
The force which a body
subjected to the action of
a force from another body exerts upon the latter body in the opposite direction.
Reaction is always equal and opposite to action, that is to say, the actions of two bodies upon each other are always equal and in opposite directions.
Sir I. Newton (3d Law of Motion). 5. (Politics) Backward tendency or movement after revolution, reform, or great progress in any direction.
The new king
had, at the
very moment at which his
fame and fortune reached the highest point, predicted the coming reaction.
Macaulay.
Reaction time
(Physiol.), in nerve
physiology, the interval between the application of a stimulus
to an end organ of sense and the
reaction or resulting movement; -- called also physiological time. -- Reaction
wheel (Mech.), a
water wheel driven by the
reaction of water, usually one in which the
water, entering it centrally, escapes at its periphery in a direction opposed to that of its motion by
orifices at right angles, or inclined, to its radii.
Hear it pronounced