What was the scientific contribution of John Snow?

Which three gases make up over 99.9 percent of the air? Is oxygen a non metal or a noble gas? When After refrigerant liquid has been recovered from appliance any remaining vapor is? Is it true during fibrosis fibroblasts in the granulation tissue lays down keratin fibers? When was the last time it snowed on Halloween in Maryland? What elements make up the compound C8H18? How many tablespoon of butter is equal to 175gm? What are the four main resivoirs of carbon? Is 12pm noon time? Can any one study history without knowing its geography? What is the average birth rate of tigers? What is the difference between the tarsals and the metatarsals? What rocks are in crust mantle outer core and inner core? How does seismic waves determine the location of petroleum far beneath the surface? What happens when you type more then one line of text into a cell? Since Comets have tails after millions of years why wouldn't the comet it self dissipate into one large tail? Who were two or three scientists who worked on or discovered metals? How do you embellish glass? The major difference between a topographic map and other maps is that a topographic map shows .? What is 2 inches in the metric system?

What was the scientific contribution of John Snow?