Sulfurina is the name of the embodiment of the Emerald Green Spring, a hot spring that emits a deadly poisonous gas consisting of hydrogen sulfide and sulfur dioxide. The group has to tread to retrieve sulfuric acid from the spring to make sulfanilamide to cure Ruri of her illness. Silver can be used to detect hydrogen sulfide to see where the poison gas is and to avoid it.
Senku nicknames her Ryuu-san because in Japanese the word for sulfuric acid is ryuusan (硫酸).
(Source: Dr. Stone Fandom)
