Four series of observations carried out in the Kamerlingh Onnes Laboratory on Cu${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O crystals are reviewed. The magnetization curves confirm the general picture given by the theoretical analysis of Gorter and Haantjes. At the lower temperature a sharp threshold field is found in the crystallographic $\mathrm{ac}$ plane with a minimum value of 6500 φ; at higher temperature its value increases somewhat.

Specific heat measurements reveal the existence of a short-range order tail above the sharp Néel temperature at which antiferromagnetism vanishes.

In the investigations on proton magnetic resonance a sharp transition is found between a nonsymmetrical paramagnetic resonance diagram and a symmetrical resonance diagram characteristic for antiferromagnetism. In this way it is found that the Néel temperature depends on the direction of the magnetizations.

It is possible to follow the turn-over of the magnetizations near the threshold field. The position of the protons and the nature of the sublattices have been determined. The sublattices do not exchange position within ${10}^{-4\mathrm{}}$ sec. Microwave resonances have been observed at 9400 and at 4000 megacycles/sec. The absorption bands are confined to the neighborhood of the turn-over fields. General agreement with Ubbink's theoretical treatment is found.

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