Litmus paper is a type of filter paper that has been treated with a natural water-soluble dye obtained from lichens. It is used as a pH indicator to test whether a solution is acidic or basic. The color transition occurs over the pH range 4.5–8.3 at 25 °C (77 °F), with blue litmus paper turning red under acidic conditions and red litmus paper turning blue under basic or alkaline conditions. Neutral litmus paper is purple. Litmus paper can also be used to test for water-soluble gases that affect acidity or basicity, as the gas dissolves in the water and the resulting solution colors the litmus paper. The use of litmus paper allows an inspector to determine only whether a solution is acidic or basic, but more precise pH determinations can be found by using pH test paper, which compares subtle color differences to a color chart to indicate pH level. Litmus paper was first used by the Spanish physician Arnaldus de Villa Nova around 1300, and the blue dye was extracted from some lichens from the 16th century onwards, especially in the Netherlands.