falls below the intake, allowing air or other surrounding gas to break the siphon, or until the outlet of the siphon equals the level of the reservoir, whichever comes first.
In addition to atmospheric pressure, the density of the liquid, and gravity, the maximum height of the crest in practical siphons is limited by the vapour pressure of the liquid. When the pressure within the liquid drops to below the liquid's vapor pressure, tiny vapor bubbles can begin to form at the high point and the siphon effect will end. This effect depends on how efficiently the liquid can nucleate bubbles; in the absence of impurities or rough surfaces to act as easy nucleation sites for bubbles, siphons can temporarily exceed their standard maximum height during the extended time it takes bubbles to nucleate. One siphon of degassed water was demonstrated to 24 meters for an extended period of time. For water at standard atmospheric pressure, the maximum siphon height is approximately 10 m (32 feet); for mercury it is 76 cm (30 inches), which is the definition of standard pressure. This equals the maximum height of a suction pump, which operates by the same principle. The ratio of heights (about 13.6) equals the ratio of densities of water and mercury (at a given temperature), since the column of water (resp. mercury) is balancing with the column of air yielding atmospheric pressure, and indeed maximum height is (neglecting vapor pressure and velocity of liquid) inversely proportional to density of liquid.
Modern research into the operation of the siphon
In 1948, Malcolm Nokes investigated siphons working in both air pressure and in a partial vacuum, for siphons in vacuum he concluded that: "The gravitational force on the column of liquid in the downtake tube less the gravitational force in the uptake tube causes the liquid to move. The liquid is therefore in tension and sustains a longitudinal strain which, in the absence of disturbing factors, is insufficient to break the column of liquid". But for siphons of small uptake height working at atmospheric pressure, he concluded that: "... the tension of the liquid column is neutralized and reversed by the compressive effect of the atmosphere on the opposite ends of the liquid column."
Potter and Barnes at the University of Edinburgh revisited siphons in 1971. They re-examined the theories of the siphon and ran experiments on siphons in air pressure. Their conclusion was that; "By now it should be clear that, despite a wealth of tradition, the basic mechanism of a siphon does not depend upon atmospheric pressure." 
Gravity, pressure and molecular cohesion were the focus of work in 2010 by Hughes at the Queensland University of Technology. He used siphons at air pressure and his conclusion was that: "The flow of water out of the bottom of a siphon depends on the difference in height between the inflow and outflow, and therefore cannot be dependent on atmospheric pressure…" Hughes did further work on siphons at air pressure in 2011 and concluded that: "The experiments described above demonstrate that ordinary siphons at atmospheric pressure operate through gravity and not atmospheric pressure".
The father and son researchers, Ramette and Ramette, successfully siphoned carbon dioxide under air pressure in 2011 and concluded that molecular cohesion is not required for the operation of a siphon but that: "The basic explanation of siphon action is that, once the tube is filled, the flow is initiated by the greater pull of gravity on the fluid on the longer side compared with that on the short side. This creates a pressure drop throughout the siphon tube, in the same sense that 'sucking' on a straw reduces the pressure along its length all the way to the intake point. The ambient atmospheric pressure at the intake point responds to the reduced pressure by forcing the fluid upwards, sustaining the flow, just as in a steadily sucked straw in a milkshake."
Again in 2011, Richert and Binder (at the University of Hawaii) examined the siphon and concluded that molecular cohesion is not required for the oper