*According to multiple sources it was Hydrochloric acid.
This famous photograph by Horace Cort shows a group of white and black integrationists in the former Monson Motor Lodge swimming pool on June 18, 1964. The photo was connected to the St. Augustine Movement, named for the town in Florida where it took place.
The photo was taken right after anti-segregation protesters had jumped into the whites only pool and the manager thought this acid would drive the protesters out. It doesn’t matter that the acid didn’t harm them, his intent was to terrorize them. The protesters were arrested just a few minutes later. This photo made global news at the time and was a major embarrassment for the U.S.