This is based off of two appearances, but it seems worth mentioning, if only because I missed both of them.
Kevin Delaney is Director of Visitor Experience at the Museum of Discovery in Little Rock, Arkansas. Last Friday’s segment marked his second appearance on The Tonight Show this year (the first was in May). As the museum notes on its website, Delaney conducted the same kind of experiment he organizes at the museum for the “Awesome Science” program.
Delaney was approached by the program shortly before Fallon moved to the 11:30 slot in February. Staff have apparently approached many different science museums and organizations to find people like Kevin Delaney. So while Delaney appears to be the first regular guest to conduct experiments, he may not be the only one. (I’m not counting Fallon’s occasional segments with kids.) I know I’ll keep a closer watch in order to note any additions.