Companies, or in any of the physical conditions with which they are surrounded, it is obvious that no experiment could have been devised which would more thoroughly test the effect of water supply on the progress of Cholera than this, which circumstances placed ready made before the observer. The experiment too, was on the grandest scale. No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentlefolks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied water containing the sewage of London, and amongst it, whatever might have come from the cholera patients, the other group having water quite free from such impurity. Snow went on to study the water contents from each home through a test performed on each sample. In this way, it could be deduced from which supplier the home was receiving their water. He concluded that it was indeed impure water from the big companies that allowed the spread of cholera to progress rapidly. He went on to prove his theory through the observation of prisons in London, finding that cholera ceased in these places only a few days after switching to cleaner water sources. Broad Street outbreak On 31 August 1854, after several other outbreaks had occurred elsewhere in the city, a major outbreak of cholera occurred in Soho. Snow later called it "the most terrible outbreak of cholera which ever occurred in this kingdom." Over the next three days, 127 people on or near Broad Street died. During the next week, three quarters of the residents had fled the area. By 10 September, 500 people had died and the mortality rate was 12.8 per cent in some parts of the city. By the end of the outbreak, 616 people had died. Many of the victims were taken to the Middlesex Hospital, where their treatment was superintended by Florence Nightingale, who briefly joined the hospital in early September in order to help with the outbreak. According to a letter from Elizabeth Gaskell, "She herself [Nightingale] was up night and day from Friday afternoon (Sept. 1) to Sunday afternoon, receiving the poor creatures (chiefly fallen women of that neighbourhood - they had it the worst) who were being constantly brought in - - undressing them - putting on turpentine