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Essence of Materia Medica – page 124

Mercurius patient has a narrow range of tolerance to everything; for example, such a patient will be comfortable between only a very narrow range of temperatures, becoming uncomfortable from even slight heat or rold.
The intolerance to heat and cold illustrates the instability which characterises the particular Mercurius weakness. As mentioned by Kent, the patient is a living "thermometer". At one moment, he is suffering from cold and seeks warmth, but once warm he becomes aggravated by the warmth also. This is true not only during a fever, but also chronically. There is also a weakness and instability in emotional expression; weeping alternating with laughing. Unlike Ignatia, in which this symptom is a manifes¬tation of a hysterical state from uncontrolled emotion, the Mer¬curius weeping/laughing is more of a mechanical instability. Once weeping, Mercurius feels a kind of mood come upon her which results in a swing to the opposite extreme of laughing; mechan¬ically, laughing and weeping can often be quite similar, and so the Mercurius instability causes the patient to waver from one state to the other quite readily.
The Mercurius instability, its inefficiency in function, can be illu¬strated nicely by reflecting on the physical state of Mercury. If you break a Mercury thermometer, you discover that Mercury seems to exist in a state somewhere between a liquid and a solid. It flows like a liquid, yet it tends to retain its own shape to some extent like a solid. If you try to pick it up with your fingers, it seems to elude you; it does not allow itself to be grasped like a solid, and it does not stick to your skin like a liquid. In its physical form, Mercurius is erratic in function, just as it is unsta¬ble and inefficient in its pathological state.
Thus we can see readily that the Mercurius weakness is not like that of other remedies. Arsenicutn can exhibit a prostrating kind of weakness, but it is quite unlike the instability of Mercurius.
Arsenicum, of course, shares many similar specific pathological symptoms, but the Arsenicum patient’s cold intolerance is relieved by warmth; it is also true, of course, that mentally Arsenicum shows much more reactive power—anxiety, restless activity of mind, a shrewdness. Stannum, Helonias, and Baptisia are other remedies with severe weakness of reactive power, but not with the instability and inefficiency of Mercurius.