of water to Londoners, and by 1858 many of the city's medieval wooden water pipes were being replaced with iron ones. This, combined with the introduction of flushing toilets and the rising of the city's population from just under one million to three million, led to more water being flushed into the sewers, along with the associated effluent. The outfalls from factories, slaughterhouses and other industrial activities put further strain on the already failing system. Much of this outflow either overflowed, or discharged directly, into the Thames. The scientist Michael Faraday described the situation in a letter to The Times in July 1855: shocked at the state of the Thames, he dropped pieces of white paper into the river to "test the degree of opacity". His conclusion was that "Near the bridges the feculence rolled up in clouds so dense that they were visible at the surface, even in water of this kind. ... The smell was very bad, and common to the whole of the water; it was the same as that which now comes up from the gully-holes in the streets; the whole river was for the time a real sewer." The smell from the river was so bad that in 1857 the government poured chalk lime, chloride of lime and carbolic acid into the river to ease the stench. The prevailing thought in Victorian healthcare concerning the transmission of contagious diseases was the miasma theory, which held that most communicable diseases were caused by the inhalation of contaminated air. This contamination could take the form of the odour of rotting corpses or sewage, but also rotting vegetation, or the exhaled breath of someone already diseased. Miasma was believed by most to be the vector of transmission of cholera, which was on the rise in 19th-century Europe. The disease was deeply feared by all, because of the speed with which it could spread, and its high fatality rates. London's first major cholera epidemic struck in 1831 when the disease claimed 6,536 victims. In 1848–49 there was a second outbreak in which 14,137 London residents died, and this was followed by a further outbreak in 1853–54 in which 10,738 died. During the second outbreak, John Snow, a London-based physician, noticed that the rates of death were higher in those areas supplied by the Lambeth and the Southwark and Vauxhall water companies. In 1849 he published a paper, On the