Be so kind as to tell me at your leisure [he wrote to Barbeu Dubourg], whether in France, you have a general Belief that moist Air, and cold Air, and damp Shirts or Sheets, and wet Floors, and Beds that have not lately been used, and Clothes that have not been lately worn, and going out of a warm Room into the Air, and leaving off a long-worn Wastecoat, and wearing leaky Shoes, and sitting near an Open Window, or Door, or in a Coach with both Glasses down, are all or any of them capable of giving the Distemper we call a Cold, and you a Rheum, or Catarrh? Or are these merely English ideas?
His views on the wholesomeness of fresh air were far in advance of the general intelligence of his time, and were expressed in spirited terms. After stating in a letter to Jean Baptiste Le Roy that he had become convinced that the idea that perspiration is checked by cold was an error as well as the idea that rheum is occasioned by cold, he added:
But as this is Heresy here, and perhaps may be so with you, I only whisper it, and expect you will keep my Secret. Our Physicians have begun to discover that fresh Air is good for People in the Small-pox & other Fevers. I hope in time they will find out that it does no harm to People in Health.
At times his language on what he called aerophobia grew highly animated.
What Caution against Air [he said in a letter to Thomas Percival], what stopping of Crevices, what wrapping up in warm Clothes, what shutting of Doors and Windows! even in the midst of Summer! Many London Families go out once a day to take the Air; three or four Persons in a Coach, one perhaps Sick; these go three or four Miles, or as many Turns in Hide Park, with the Glasses both up close, all breathing over & over again the same Air they brought out of Town with them in the Coach with the least change possible, and render'd worse and worse every moment. And this they call taking the Air.
Indeed, there is at times something just a little ludicrous in the uncompromising fervor with which Franklin insisted upon his proposition. It seemed strange he said, in the letter from which we have just quoted, that a man whose body was composed in great part of moist fluids, whose blood and juices were so watery, and who could swallow quantities of water and small beer daily without inconvenience, should fancy that a little more or less moisture in the air should be of such importance; but we abound in absurdity and inconsistency.
It is a delightful account that John Adams gives us of a night which he spent in the same bed with Franklin at New Brunswick, on their way to the conference with Lord Howe:
The chamber [Adams tells us] was little larger than the bed, without a chimney, and with only one small window. The window was open, and I, who was an invalid, and afraid of the air in the night, shut it close. "Oh!" says Franklin, "don't shut the window, we shall be suffocated." I answered I was afraid of the evening air. Dr. Franklin replied, "The air within this chamber will soon be, and indeed is now, worse than that without doors. Come, open the window and come to bed, and I will convince you. I believe you are not acquainted with my theory of colds." Opening the window and leaping into bed, I said I had read his letters to Dr. Cooper, in which he had advanced that nobody ever got cold by going into a cold church or any other cold air, but the theory was so little consistent with my experience, that I thought it a paradox. However, I had so much curiosity to hear his reasons, that I would run the risk of a cold. The Doctor then began a harangue upon air and cold, and respiration and perspiration, with which I was so much amused that I soon fell asleep, and left him and his philosophy together; but I believe they were equally sound and insensible within a few minutes after me, for the last words I heard were pronounced as if he was more than half asleep. I remember little of the lecture, except that the human body, by respiration and perspiration, destroys a gallon of air in a minute; that two such persons as we were now in that chamber would consume all the air in it in an hour or two; that by breathing over again the matter thrown off by the lungs and the skin, we should imbibe the real cause of colds, not from abroad, but from within.
At times Franklin merely gave hints to brother philosophers and left them to run the hints down. For instance, he suggested to M. De Saussure, of Geneva, who succeeded in ascending Mont Blanc, the idea of ascertaining the lateral attraction of the Jura Mountains for the purpose of discovering the mean density of the earth upon the Newtonian theory of gravitation. This was subsequently done with complete success by Nevil Maskelyne on Mt. Schehallion in Perthshire. To Ingenhousz he suggested the idea of "hanging a weight on a spiral spring, to discover if bodies gravitated differently to the earth during the conjunctions of the sun and moon, compared with other times."