Two years on the trail and no drugstores? Better remember to pack a first-aid kit. Meriwether Lewis consults Dr. Benjamin Rush, who has just the prescription: Peruvian bark, Glauber salts, niter, opium, laudanum, tartar emetic, and (among other cures) 600 of Rush’s own patented purging pills—called “thunderclappers” for their rather explosive effect.