One day he picked up a New York paper and saw that the Health Department reported that during the past two weeks there had been a “trace of nitrates” in the city water. For months cholera, the dreaded scourge that comes from impure water and has, in the life of the world, killed more people than all the battles, had threatened the city. Riis investigated. The water supplying two million people must be pure. “Nitrates” were a sign of sewage contamination. The city was threatened, and no one seemed to realize the danger. The humble investigations of the newspaper reporter were to produce far-reaching results. Riis wrote an article that day for the _Evening Sun_, and advised the people to boil the water before drinking it. Then, with a camera in his hand, he spent a week following to its source every stream that discharged into the Croton River. He found evidence enough: town after town discharged its sewage into the water which supplied New York City; people bathed in it; cities dumped refuse into it. With exact details, and the evidence illustrated with photographs, he returned. The city was saved. So strong was the evidence that over a million dollars was promptly spent by the city to guard the water-supply. The real public-service life of Riis had begun. In 1884, an awakened interest in the housing conditions in the tenement districts came to a head with the establishment of the Tenement-House Commission. It brought out the fact that the people living in the tenements were “better than the houses.” Riis worked heart and soul for the cause. Four years later the reform was assured, with the backing of such men as Dr. Felix Adler and Alfred T. White. The Adler Tenement-House Commission was formed. Now Riis turned to other conditions which cried aloud for reform. The “Bend” in Mulberry Street and the Police Lodging-Room system needed him. The “Bend” was a crowded slum that for a half-century had been the centre of vice and misery. Riis aroused the community, and in 1888 a bill was introduced in the Legislature to wipe it out bodily. To-day, a city park, a breathing-place and playground marks the site. The Police Lodging-Room was another civic disgrace and a breeder of crime. In these vile and filthy rooms the Police Department gave night lodging to thousands of vagrants. The idea sounds well enough, but the system was wrong, and Riis realized it. In these foul quarters young men lodged with professional beggars and criminals, and learned the ways of vice; in these rooms was bred disease, physical as well as moral. Riis warned the city through his paper. Then his prophecies came true: typhus broke out in the Police Lodging-Rooms.