It is named after Leopold Gmelin, who introduced the test.
Different coloured rings between the two layers are visible if bile pigments are present as they are oxidised to various chemical products.
[4][5] Nitric acid is used as the oxidising agent.
[6] Blue, green and violet rings are seen if bilirubin is present.
[7] Gmelin's test is not sensitive, so a positive result always indicates the presence of bile pigments, but a negative result does not exclude the presence of small quantities of bile pigments.