Chemist

As a chemist and a scientist, Mitchell's work covered a wide range of topics. He became a fellow of the Royal Institute of Chemistry in 1897 and of the Chemical Society in 1916.^[1][4] In 1907 he advocated using a pinhole camera to photograph the sun in an article in Knowledge and Scientific News, a method which he notes was known as early as 1615 but seemed to have been forgotten.^[5][6] In 1911, he advocated the switch from lead to copper pipes for drinking water. His reasoning was that copper would better be able to alert the drinker to an excess of poisonous sulphates in the water.^[7] In 1920, he became editor of The Analyst, a journal of the Chemical Society, and under 25 years of his editorship the journal expanded in scope and increased in reputation.^[8]

Criminologist

In 1911 he was head of the inspection bureau of Scotland Yard.^[9] He frequently served as an expert witness. In 1915, he gave testimony about the invisible ink used in the case of German spy, Anton Kuepferle.^[3]

In 1925 he analyzed documents and seals of Mary, Queen of Scots and claimed to show that Mary was innocent of conspiracy to assassinate Queen Elizabeth I of England. He said that William Maitland, Mary's secretary, forged Mary's hand in the documents which led to Mary's execution.^[10] In 1929, he was an advocate of the use of fingerprints to determine identity, a method which he traced to Sir William Herschell in 1853, who saw the method used to document an individual's identity in India, where he was a commissioner.^[11] He showed that monkeys may be identified by fingerprints and that the same could be done with the markings on a cow's nose.^[12]

He was hired by Ruth White to analyze the Will and Testament of `Abdu'l-Bahá. White opposed parts of the will that suggested the establishment of a hierarchy in the Baháʼí Faith. His report concluded in agreement with White, that the document was a forgery.^[13] White placed Mitchell's signed report on the writing shown on the photographs of the document with the U.S. Library of Congress in 1930.^{[clarification needed][14][better source needed]}