To Jacques Barbeu-Dubourg
London, July 28, 1768.
I greatly approve the epithet, which you give in your letter of the 8th of June, to the new method of treating the small-pox, which you call the tonic or bracing method. I will take occasion from it, to mention a practice to which I have accustomed myself. You know the cold bath has long been in vogue here as a tonic; but the shock of the cold water has always appeared to me, generally speaking, as too violent: and I have found it much more agreeable to my constitution, to bathe in another element, I mean, cold air. With this view I rise early almost every morning, and sit in my chamber, without any clothes whatever, half an hour or an hour, according to the season, either reading or writing. This practice is not in the least painful, but on the contrary, agreeable; and if I return to bed afterwards, before I dress myself, as sometimes happens, I make a supplement to my night’s rest, of one or two hours of the most pleasing sleep that can be imagined. I find no ill consequences whatever resulting from it, and that at least it does not injure my health, if it does not in fact contribute much to its preservation. I shall therefore call it for the future a bracing or tonic bath.
Image source: http://blog.servicesource.com/2013/02/ben-franklin/