The nineteen-arch London Bridge was even more congested. The better-off used hackney carriages and sedan chairs to get to their destinations without getting filthy. The poor walked, and might be splashed by the wheeled vehicles and drenched by slops being thrown out and water falling from the overhanging roofs. Another hazard was the choking black smoke belching forth from factories which made soap, from breweries and iron smelters and from about 15,000 houses burning coal. Outside the city walls, suburbs had sprung up providing homes for the craftsmen and tradespeople who flocked to the already overcrowded city. These were shanty towns with wooden shacks and no sanitation. The government had tried to control this development but had failed and over a quarter of a million people lived here. Other immigrants had taken over fine town houses, vacated by Royalists who had fled the country during the Commonwealth, converting them into tenements with different families in every room. These properties were soon vandalised and became rat-infested slums. The administration of the City of London was organised by the Lord Mayor, Aldermen and common councillors, but not all of the inhabited area generally comprising London was legally part of the City. Both inside the City and outside its boundaries there were also Liberties, which were areas of varying sizes which historically had been granted rights to self-government. Many had been associated with religious institutions, and when these were abolished in the Dissolution of the Monasteries, their historic rights were transferred along with their property to new owners. The walled City was surrounded by a ring of Liberties which had come under its authority, called 'the City and Liberties', but these were surrounded by further suburbs with varying administrations. Westminster was an independent town with its own liberties, joined to London by urban development. The Tower of London was an independent liberty, as were others. Areas north of the river not part of one of these administrations came under the authority of the county of Middlesex, and south of the river under Surrey. At that time, bubonic plague was a much feared disease but its cause was not understood. The credulous blamed emanations from the earth, "pestilential effluviums", unusual weather, sickness in livestock, abnormal behaviour of animals or an increase in the numbers of moles, frogs, mice or flies. It was not until 1894 that the identification by Alexandre Yersin of its causal agent "Yersinia pestis" was made and the transmission of the bacterium by rat fleas became known.