  :A search in Google Books turned up a short article in Nature (July 23, 1891, p. 280). The Palace of Westminster was destroyed in a fire in 1834. A number of standards of weights and measures were thought to have been damaged or destroyed; some of the standard brass yards had gold plugs near the ends, and the length of the yard was indicated by marks on the plugs, which might have given rise to the idea of gold standards. Several of the standards were not actually destroyed, but were rescued from obscurity in 1891. In any case, a fire in 1834 is too late to account for the French adoption of the metric system.  : I'm not sure if this is the fire you are thinking of, or if there was a different fire that destroyed important standards at an earlier date  