“Surely You’re Joking, Mr. Feynman! § Testing Bloodhounds”, 1985 ():
I read an article in Science [sic] about bloodhounds, and how they could smell so very well. The authors described the various experiments that they did—the bloodhounds could identify which items had been touched by people, and so on.
[Feynman’s citation here is probably wrong. There appears to be no such bloodhound article in Science prior to the terminus ante quem of 1945, when his wife died & he left Los Alamos (having arrived in 1943). The closest I found is a Popular Science magazine article in June 1942: “Training Bloodhounds”. While informal and not really recording “experiments”, it does otherwise match his description in a magazine widely read in Los Alamos at the right time.]
…When the time came that I could visit my wife, I went to see her, and I said, “We’re gonna do an experiment. Those Coke bottles over there (she had a 6-pack of empty Coke bottles that she was saving to send out)–now you haven’t touched them in a couple of days, right?” “That’s right.” I took the 6-pack over to her without touching the bottles, and said, “OK. Now I’ll go out, and you take out one of the bottles, handle it for about two minutes, and then put it back. Then I’ll come in, and try to tell which bottle it was.”
So I went out, and she took out one of the bottles and handled it for quite a while—lots of time, because I’m no bloodhound! According to the article, they could tell if you just touched it. Then I came back, and it was absolutely obvious! I didn’t even have to smell the d—n thing, because, of course, the temperature was different. And it was also obvious from the smell. As soon as you put it up near your face, you could smell it was dampish and warmer. So that experiment didn’t work because it was too obvious.
…So I went out again, she took a book, opened it and closed it, and put it back. I came in—and nothing to it! It was easy. You just smell the books. It’s hard to explain, because we’re not used to saying things about it. You put each book up to your nose and sniff a few times, and you can tell. It’s very different. A book that’s been standing there a while has a dry uninteresting kind of smell. But when a hand has touched it, there’s a dampness and a smell that’s very distinct.
…People’s hands smell very different—that’s why dogs can identify people; you have to try it! All hands have a sort of moist smell, and a person who smokes has a very different smell on his hands from a person who doesn’t; ladies often have different kinds of perfumes, and so on. If somebody happened to have some coins in his pocket and happened to be handling them, you can smell that.
…(I’ve noticed that my dog can correctly tell which way I’ve gone in the house, especially if I’m barefoot, by smelling my footprints. So I tried to do that: I crawled around the rug on my hands and knees, sniffing, to see if I could tell the difference between where I walked and where I didn’t, and I found it impossible. So the dog is much better than I am.)