The vomited substance... was stomach contents.
A turbid liquid, like rice water, mixed with undigested food remnants and some bile. Its appearance was somewhat similar to the so-called "rice water stool." However, due to individual differences, descriptions of such vomit often vary significantly. Clinically, these can only serve as auxiliary diagnostic criteria; a definitive diagnosis requires stool or vomit culture.
Obviously, Lister didn't have those conditions now, so he could only rely on the empirical rules summarized by ancient sages and his own observations to make a diagnosis.
