Confined air, when saturated with perspirable matter,[1] will not receive more; and that matter must remain in our bodies and occasion diseases; but it gives some previous notice of its being about to be hurtful, by producing certain uneasiness, slight indeed at first, such as with regard to the lungs is a trifling sensation, and to the pores of the skin a kind of restlessness, which is difficult to describe, and few that feel it know the cause of it. But we may recollect that sometimes, on waking in the night, we have, if warmly covered, found it difficult to get asleep again. We turn often, without finding repose in any position. This fidgetiness (to use a vulgar expression for want of a better) is occasioned wholly by an uneasiness in the skin, owing to the retention of the perspirable matter, the bedclothes having received their quantity, and, being saturated, refusing to take any more. To become sensible of this by an experiment, let a person keep his position in the bed, but throw off the bedclothes, and suffer fresh air to approach the part uncovered of his body; he will then feel that part suddenly refreshed; for the air will immediately relieve the skin, by receiving, licking up, and carrying off, the load of perspirable matter that incommoded it. For every portion of cool air that approaches the warm skin, in receiving its portion of that vapour, receives therewith a degree of heat that rarefies and renders it lighter, when it will be pushed away, with its burden, by cooler and, therefore, heavier fresh air, which for a moment supplies its place, and then, being likewise changed and warmed, gives way to a succeeding quantity. This is the order of nature, to prevent animals being infected by their own perspiration. He will now be sensible of the difference between the part exposed to the air and that which, remaining sunk in the bed, denies the air access; for this part now manifests its uneasiness more distinctly by the comparison, and the seat of the uneasiness is more plainly perceived than when the whole surface of the body was affected by it. [1] What physicians call perspirable matter is that vapour which passes off from our bodies, from the lungs, and through the pores of the skin. The quantity of this is said to be five eighths of what we eat.--AUTHOR. Here, then, is one great and general cause of unpleasing dreams. For when the body is uneasy, the mind will be disturbed by it, and disagreeable ideas of various kinds will in sleep be the natural consequences. The remedies, preventive and curative, follow: 1. By eating moderately (as before advised for health's sake), less perspirable matter is produced in a given time; hence the bedclothes receive it longer before they are saturated, and we may therefore sleep longer before we are made uneasy by their refusing to receive any more.