Liquid nitrogen is nitrogen in a liquid state at an extremely low temperature. It is a colorless clear liquid with a density of 0.807 g/ml at its boiling point (−195.79 °C (77 K; −320 °F)) and a dielectric constant of 1.43. Nitrogen was first liquefied at the Jagiellonian University on 15 April 1883 by Polish physicists, Zygmunt Wróblewski and Karol Olszewski. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is often referred to by the abbreviation, LN2 or "LIN" or "LN" and has the UN number 1977. Liquid nitrogen is a diatomic liquid, which means that the diatomic character of the covalent N bonding in N2 gas is retained after liquefaction.Liquid nitrogen is a cryogenic fluid that can cause rapid freezing on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. The temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks. The development of pressurised super-insulated vacuum vessels has enabled liquefied nitrogen to be stored and transported over longer time periods with losses reduced to 2% per day or less.The temperature of liquid nitrogen can readily be reduced to its freezing point 63 K (−210 °C; −346 °F) by placing it in a vacuum chamber pumped by a vacuum pump. Liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, applies to any liquid in contact with an object significantly hotter than its boiling point. Faster cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen rather than liquid nitrogen alone.