Liquid helium physicist Richard Feynman. Liquid helium At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temperature of −270 °C (about 4 K or −452.2 °F). Its boiling point and critical point depend on which isotope of helium is present: the common isotope helium-4 or the rare isotope helium-3. These are the only two stable isotopes of helium. See the table below for the values of these physical quantities. The density of liquid helium-4 at its boiling point and a pressure of one atmosphere (101.3 kilopascals) is about 0.125 grams per cm, or about
Lowest bridging point often was at a location that allowed seagoing traffic to approach it from a downstream direction. Examples of lowest bridging points in Britain include London, the lowest bridging point on the Thames; Lancaster, the lowest bridging point on the Lune; York, the lowest bridging point on the Ouse; and Gloucester, the lowest bridging point on the Severn. Glasgow grew up at the lowest bridging point on the Clyde which was about twelve miles upstream from its lowest fording point at Dumbuck. Mathew Paris’s map of 1247 appears to show only one bridge in the whole of Britain: at Stirling, the