Liquid helium of liquid water. Helium was first liquefied on July 10, 1908, by the Dutch physicist Heike Kamerlingh Onnes at the University of Leiden in the Netherlands. At that time, helium-3 was unknown because the mass spectrometer had not yet been invented. In more recent decades, liquid helium has been used as a cryogenic refrigerant, and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), Magnetoencephalography (MEG), and experiments in physics, such as low temperature Mössbauer spectroscopy. The temperature required to produce liquid helium is low because
Johannes Diderik van der Waals the van der Waals equation of state can be expressed as a simple function of the critical pressure, critical volume, and critical temperature. This general form is applicable to all substances (see van der Waals equation.) The compound-specific constants "a" and "b" in the original equation are replaced by universal (compound-independent) quantities. It was this law which served as a guide during experiments which ultimately led to the liquefaction of hydrogen by James Dewar in 1898 and of helium by Heike Kamerlingh Onnes in 1908. In 1890, van der Waals published a treatise on the "Theory of Binary Solutions" in