The second edition of Snow's book on cholera, with 162pp. compared to 31pp. in the first edition, incorporated the results of five more years of research, and contained so much additional material that it was essentially a new work. Snow set out his views that cholera was caused by a living organism, a belief later confirmed by Koch's discovery of the cholera vibrio in 1883. Snow included statistical surveys made during the great cholera epidemic of 1854, demonstarting that the number of cholera deaths in each area of southern London corresponded to the degree of contamination of the local drinking water. In this edition he told the famous story of the Broad Street pump for the first time, and included the famous spot map of the district showing the location of each pump and the fatal cholera cases. This was the first use of a spot map in epidemiology. Digital facsimile of the 1855 second edition from Google Books at this link.