The first outbreak of cholera in the United States occurred in 1832. In one special hospital in New York city, 2030 patients were received in the nine weeks from July 1 to September 1, and of these 850 died. An eye-witness, who was personally known to the writer, one not given to exaggeration, said that the state of dread and alarm had been increasing until, when the disease first made its appearance in New York, fully one half of the population had left the city, many of the physicians fleeing with the rest. There was no efficient health department, and no organized system for the protection of the public health. This gentleman was a city missionary, and, in the performance of his duties, visited many of the houses. He mentioned visiting one of these on a morning when the fifteenth body had been carried out. It was the time of the rumble of the dead cart and the indiscriminate burial in public trenches. Contrast the horrors of this scene with the last attempt of cholera to invade the United States, in 1893, when, notwithstanding its presence at the quarantine station in New York harbor, and the actual presence of a few well-authenticated cases in the city itself, _not one of these cases proved a focus for the spread of the disease_. The opinion that water in some way acts as a conveyer of disease can be generalized after a very little observation. To explain how it does this is a problem that was attempted to be solved by the chemist. He added vastly to our knowledge, but it was not until the biologist showed the presence of the disease-producing bacteria in water that a full explanation was possible. But the biologist has done more: it has been found, and notably in the very complete series of experiments carried on by the Massachusetts Board of Health, that even an effluent of a sewer, if filtered through a bed of sand, is purified to such an extent that the filtrate is a perfectly safe water to drink. The dangerous organic matter disappears, and ninety-eight per cent of the bacteria is removed. And it is pleasing to note, when one has so much to say of the dangers of bacteria, that the purification is entirely brought about by the action of bacteria working for the good of man. A sand filter bed does not purify water properly until it has been in operation for a few days, when the top of the bed is covered with a slime in which the bacteria act upon the organic matter in the water and purify it. The fact of the purification was known before the manner in which it was done was understood; and in those cities where the authorities have acted upon this knowledge and have purified their water supply, the influence upon the death-rate of typhoid fever is almost as marked as those already quoted for typhus fever, while the scourge of cholera has been almost entirely removed from their borders, as many an instance during the late outbreak in Europe could illustrate. It does not contribute to our self-esteem to know that most