[2] Mercer pioneered research into antimicrobials, preventing the spread of cholera in Sykeside(now part of Haslingden) in 1847 with chloride of lime, or "calcium hypochlorite", which is today used to disinfect public swimming pools and drinking water.

His wife died in 1859 and he afterwards became a juror to the second Great Exhibition in 1862, and a justice of the peace in Lancashire, continuing to give lectures at Clayton-le-Moors and supporting local Anglican and Methodist churches.

[2] The 1861 census records him as a 70-year-old "Chymist", living with his son John and 12 others at 29 Burlington Hotel, London (Florence Nightingale was next door, at No.

Mercer died at home in 1866 and was buried in St Bartholomew's Church, Great Harwood.

Funds for his commemoration were provided by his daughter Maria, and a clock tower was unveiled in Great Harwood in 1903, as well as the Mercer Hall.