John Laing Leal (May 5, 1858 – March 13, 1914) was an American physician and water treatment expert who, in 1908, was responsible for conceiving and implementing the first disinfection of a U.S. drinking water supply using chlorine. He was one of the principal expert witnesses at two trials which examined the quality of the water supply in Jersey City, New Jersey, and which evaluated the safety and utility of chlorine for production of "pure and wholesome" drinking water. The second trial verdict approved the use of chlorine to disinfect drinking water which led to an explosion of its use in water supplies across the U.S. Early life and education John L. Leal was born in Andes, New York, in 1858. In 1862, his father, John R. Leal who was a physician, joined the 144th New York Volunteer Infantry Regiment. John R. Leal saw service in a number of areas during the Civil War including Folly Island during the Siege of Charleston, South Carolina. He contracted a chronic case of amoebic dysentery (most likely from contaminated drinking water) at Folly Island from which he suffered for the next 19 years before the disease caused his death in 1882. In 1867, Dr. John R. Leal moved his family from Andes to the rapidly growing industrial city of Paterson, New Jersey. John L. Leal received his primary education at the Paterson Seminary. He attended Princeton College (now Princeton University) from 1876 to 1880. John L. Leal attended medical school at the Columbia College of Physicians and Surgeons from 1880 to 1884 where he received his medical degree. Career After obtaining his medical degree, Leal opened a medical practice in Paterson, New Jersey, and was appointed City Physician in 1886. Along with other physicians, he founded the outpatient clinic at Paterson General Hospital in 1887 and worked there until 1892. In 1888, he married Amy Arrowsmith and their only son, Graham, was born within the year. Leal's career in Paterson city government continued with his appointment as Health Inspector in 1891 and Health Officer in 1892. As Health Officer, Leal was responsible for the identification of epidemics of communicable diseases and for the disinfection of the homes of the afflicted. He also oversaw the public water supply and was responsible for constructing the growing network of sewers to remove domestic and industrial wastes from the City. To prevent the spread of contagious diseases,