Whether, however, the causes of diseases are more of a mental or corporeal character, is not now the question to decide. For though in the Essay I propose to give on atmospheric influence, I shall endeavour to point out the various causes which may give rise to an irregular display of the spirits of the insane, I am far, however, from denying, that there are alternate states of excitement and depression, of better and worse days, which we may not be able to trace to these causes, but which may depend on principles similar to other physical intermittents, just as we have periodical head-aches, having their accession and intermission most frequently every alternate day, and yet, even in these cases, I have been able to discover the origin of these head-aches, as was my own case, to alternate sleepless and distressing nights. Looking at these periodical exacerbations of insanity, without tracing them up to their first causes, they seem like the operation of some disturbing cause, requiring a given time to arrive at their crisis, or to produce the effect, and when produced, to subside again, and this cause, thus viewed in its less remote operation, seems altogether of a physical nature. But this periodicity, which I also advocate, will be more fully examined in the third section of the Essay on Atmospheric Influence, in which some observations will be ventured on lunar influence. Let the reader in the mean time take it for granted that such an influence exists; we shall then be able to examine the modifying influence which the peculiar state of the mind, in each, among the insane, must have over their manner of exhibiting these alterations in their animal spirits, especially among the old incurable cases, labouring, as it has been said, under the chronic form of insanity.