Portrait of the German organic chemist Hermann Christian von Fehling, 1812-1885. Fehling was a pupil of Liebig, an eminent German chemist, and later taught in Stuttgart. He is best known for the test reagent Fehling's solution, which contains a deep blue copper(II) complex in aqueous alkaline solution. If this gives a brick-red copper(I) oxide on boiling with the organic test sample the presence of an organic aldehyde or reducing sugar is indicated. Some other organic substances also give a positive result, however.