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The hydrogen molecule consists of two protons and two electrons. There are two different ways in which the protons can be arranged, viz., with their spins in the same or in opposite senses, and two different kinds of hydrogen molecule result, known as ortho-hydrogen and para-hydrogen, respectively. It had been shown by Denmson (1927) that the curves representing the specific heat of hydrogen at low temperatures, which previously offered great theoretical difficulties, could be explained on the assumption that ordinary hydrogen is a mixture of these two kinds of molecules in the ratio of 3 to 1. Evidence of the existence of these in liquid hydrogen was found by an optical method by McLennan and McLeod early in 1929. In 1929 Bonhoeffer and Harteck found that when ordinary hydrogen is cooled and compressed, conversion of ortho-hydrogen into para-hydrogen occurs. On adsorbing ordinary hydrogen on charcoal at the temperature of liquid hydrogen, there is practically complete catalytic conversion into para-hydrogen, which may be pumped off as gas. It has been shown that para-hydrogen has a slightly lower boiling point than normal hydrogen.
By mixing para-hydrogen (opposite spins) with atomic hydrogen (produced by an electric discharge in hydrogen), it is slowly converted into ortho-hydrogen (parallel spins):
The isotopes of hydrogen (deuterium and tritium) are described later.
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