EV. Why tremble for the mortal sleep of the just and good, who will feel, with William Hunter, on their death-bed, “how _pleasant_ and easy it is to die;” and with another moralist,— “Oh what a wonder seems the fear of death, Seeing how gladly we all sink to sleep; Night following night!” Fear not, Castaly; I do not term _slumber_ and _gentle_ sleep disease, but signs of health. Not so, however, many a _profound_ sleep, and its advances towards _coma_; those results of exhaustion from excess, or from intense and direct narcotics, as opium sleep, and the paralyzing senselessness from extreme cold, as in the story of Sir Joseph Banks and Dr. Solander in the antarctic regions. You are aware that many _remedies_ in medicine may be so intense as to cause fatality: inflammation, too, is the restorative process of wounds, but if in _excess_ it is fatal. Appetite also, to a certain degree, is healthy; but craving and thirst, its extremes, are proved, by _suffering_, to be morbid. If the mind is composed to perfect rest, it is lulled to senselessness; then metaphysically we are said to sleep: the mind is not excited by thought, and, in consequence, its supply of _arterial_ blood is less, the more _rapid_ flow of which would be the cause of waking. Within certain limits sleep is a _remedy_; but it becomes perilous when intense, or too much indulged. One eccentric physician, as we read in the learned Boërhave, even fancied sleep the _natural_ condition of man, and was wont to yield to its influence during eighteen of the twenty-four hours; but apoplexy soon finished his experiment. This _negative_ quiescence (for sleep is not a _positive_ state) allows the restoration of energy, and then we wake. Even the senses accumulate their power in sleep; the eye is dazzled by the light when we wake, from the sensitiveness imparted by this accumulation. The conceits regarding the cause of sleep are so various, that if I were to discuss their merits I should only weary your patience, as I perceive I have already done. Some have thought that sleep arose from certain conditions of the blood in the vessels and nerves of the brain; its congestion in the _sinuses_; or a _reflux_ of a great portion of it towards the heart: the result of depressed nervous energy—exhaustion, fatigue, cold, and the influence of powerful narcotics, or the combustion of charcoal. Others, that sleep arises from the deposition of fresh matter on the brain, and its sudden pressure. Then we have the cerebral collapse of Cullen, and of Richerand; the deficiency of animal spirits of Haller; the diminished afflux of blood to the brain of Blumenbach; and the exhausted irritability of the Brunonian theory adopted by Darwin. Where the truth lies I presume not to decide, but it is clear there is a necessity for the occasional repose of the mental organ: “Non semper arcum Tendit Apollo.”