Missing Formula, The by BENSON, Mildred A. Wirt

Madge Sterling is an out-of-doors girl with a keen alert mind, who finds her talent for solving baffling mysteries put to the test.
Aided by Jack French, a young forest ranger, Madge and her orphan friend, Anne Faraday, trace down many a hidden clue- in their search for a valuable formula which has been cunningly secreted by a famous chemist.
Each story in the Marge Sterling series is an exciting tale of adventure and mystery – Summary by https://www.goodreads.com/series/54071-madge-sterling
This title is avalable for free download at: www.librivox.org.