When matter is cooled the heat agitation of the particles which make it up becomes less vigorous and this produces many characteristic effects. For instance, a great part of the electrical resistance of a metal is caused by the obstructing influence of the heat motion of the metal atoms on the electrons' motions, and so when the temperature is reduced the resistance falls. Similarly the ability of a magnetic field to line up the magnetic ions of a paramagnetic salt becomes greater as the temperature is lowered and the magnetic susceptibility rises accordingly. In the early years of this century, and particularly after the quantum theory was developed, the atomic theory of matter was applied to many such problems, and on the whole very satisfactory explanations were found for the observed temperature variation of most physical phenomena down to the lowest temperatures then available (in the region of ten to twenty degrees absolute). There were, it is true, many slight discrepancies of detail, but it looked as if these difficulties would disappear if suitable refinements were made to the theory.

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