Thirst is probably the effect of a preternatural excitement of the vessels of the fauces. It is by no means a uniform symptom of fever. We sometimes observe it, in the highest degree, in the last stage of diseases, induced by the retreat of the last remains of excitement from every part of the body, to the throat. The white tongue is produced by a change in the secretion which takes place in that organ. Its yellow colour is the effect of bile; its dryness is occasioned by an obstruction of secretion, or by the want of action in the absorbents; and its dark and black colour, by a tendency to mortification. It will be difficult to account for the variety in the degrees and locality of _heat_ in the body in a fever, until we know more of the cause of animal heat. From whatever cause it be derived, its excess and deficiency, as well as all its intermediate degrees, are intimately connected with more or less excitement in the arterial system. It is not necessary that this excitement should exist only in the large blood-vessels. It will be sufficient for the purpose of creating great heat, if it occur only in the cutaneous vessels; hence we find a hot skin in some cases of malignant fever in which there is an absence of pulse. Eruptions seem to depend upon effusions of serum, lymph, or red blood upon the skin, with or without inflammation, in the cutaneous vessels. I decline taking notice in this place of the symptoms which are produced by the debility from action and abstraction, and by the depression of the system. They appear not only in the temperature of the body, but in all the different symptoms of fever. It is of importance to know when they originate from the former, and when from the latter causes, as they sometimes require very different and opposite remedies to remove them. It remains only to explain the cause why excess in the force or frequency of the action of the blood-vessels should succeed debility in a part, or in the whole of the body, and be connected for days and weeks with depression and preternatural debility in the nerves, brain, muscles, and alimentary canal. I shall attempt the explanation of this phenomenon by directing the attention of the reader to the operations of nature in other parts of her works. 1. A calm may be considered as a state of debility in the atmosphere. It predisposes to a current of air. But is this current proportioned to the loss of the equilibrium of the air? By no means. It is excessive in its force, and tends thereby to destroy the works both of nature and art.