化学反応式 (Japanese Wikipedia)

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The notation $\rightleftarrows$ was proposed in 1884 by the Dutch chemist Jacobus Henricus van 't Hoff. See: van 't Hoff, J.H. (1884) (フランス語). Études de Dynamique Chemique [Studies of chemical dynamics]. Amsterdam, Netherlands: Frederik Muller & Co.. pp. 4–5 Van 't Hoff called reactions that didn't proceed to completion "limited reactions". From pp. 4–5: "Or M. Pfaundler a relié ces deux phénomênes … s'accomplit en même temps dans deux sens opposés." (Now Mr. Pfaundler has joined these two phenomena in a single concept by considering the observed limit as the result of two opposing reactions, driving the one in the example cited to the formation of sea salt [i.e., NaCl] and nitric acid, [and] the other to hydrochloric acid and sodium nitrate. This consideration, which experiment validates, justifies the expression "chemical equilibrium", which is used to characterize the final state of limited reactions. I would propose to translate this expression by the following symbol: HCl + NO₃ Na $\rightleftarrows$ NO₃ H + Cl Na. I thus replace, in this case, the = sign in the chemical equation by the sign $\rightleftarrows$ , which in reality doesn't express just equality but shows also the direction of the reaction. This clearly expresses that a chemical action occurs simultaneously in two opposing directions.)

Crosland, M.P. (1959). “The use of diagrams as chemical 'equations' in the lectures of William Cullen and Joseph Black”. Annals of Science 15 (2): 75–90. doi:10.1080/00033795900200088.
The notation ${\ce {<=>}}$ was suggested by Hugh Marshall in 1902. See: Marshall, Hugh (1902). “Suggested Modifications of the Sign of Equality for Use in Chemical Notation”. Proceedings of the Royal Society of Edinburgh 24: 85–87. doi:10.1017/S0370164600007720.