What could be, according to you, the meaning of "turned septic" in this extract of Chapter Six of The Great Gatsby? Does it mean that the the same kind of conversation which had formerly been enjoyable now seemed virulent, or poisonous, even while the words were in the air, and this immediately, at the very first impression, as if they became poisonous while passing from one person's lips to the other's ears, "on the air" meaning in this case "on contact with air"?
Tom appeared from his oblivion as we were sitting down to supper together. “Do you mind if I eat with some people over here?” he said. “A fellow’s getting off some funny stuff.”
“Go ahead,” answered Daisy genially. “And if you want to take down any addresses here’s my little gold pencil.”... She looked around after a moment and told me the girl was “common but pretty,” and I knew that except for the half hour she’d been alone with Gatsby she wasn’t having a good time.
We were at a particularly tipsy table. That was my fault—Gatsby had been called to the phone and I’d enjoyed these same people only two weeks before. But what had amused me then turned septic on the air now.
Nick recalls a party two weeks ago when these people and the atmosphere of the party were amusing to him. Now, the same people and their inane discussions about getting too drunk are annoying to Nick. It also seems that Nick is beginning to feel more disgusted with Gatsby's party guests because the majority (or all) of the guests seem shallow and they have no idea who Gatsby is. Over the course of the novel, Nick is increasingly more annoyed that the party-goers are not really Gatsby's friends; they just use him and his house as a party spot.
Nick says it is his fault that they were "at a particularly tipsy table" because he remembered these people from a previous party and perhaps he mingled with them in hopes of starting a decent conversation. This, of course, didn't happen and Nick becomes annoyed with their useless discussion. Their discussions and their general demeanor no longer amused Nick. It wasn't so much that their words instantaneously "turned septic on the air." Rather, Fitzgerald is saying that, to Nick, the general atmosphere/mood of these people's discussions do not add fun and exuberance to the party as they did at the previous party, two weeks prior. At this current party, in Chapter Six, Nick interprets their presence as infecting (septic) the atmosphere with pointlessness. Using the phrase it "turned septic" implies that it changed right then and there, like casting a spell. But Nick is describing the difference between how he interpreted these people two weeks ago to how he interprets their behavior during the current moment in Chapter Six. So, Nick has a change of heart here. What he thought was amusing at the earlier party, he now thinks is pointless. His perception changes. It is misleading to say that their behavior turned septic "on the air now" because their behavior turned septic in Nick's mind (from his perspective, not "on the air") and it occurred, in a way, over a period of time, as Nick compared his state of mind at the earlier party to his more annoyed state of mind at the current one.
When Fitzgerald uses "on the air," he means to imply that, again from Nick's perspective, the general mood or the atmosphere (metaphorically) is now infected by the guests' shallow attitudes. This might also suggest a sense of foreboding because Nick sees how this part of Gatsby's persona (the whole party atmosphere) is fake; not genuine. These people have no idea who he is and he doesn't know most of them.