By way of illustration and with reference to atmospheric influence--the _grand exciting_ cause of disease,--every practitioner is familiar with certain conditions which are favourable to the production of certain disorders, such as common catarrh or cold, and its more aggravated form termed influenza,--hooping-cough, bowel affections, gastric fevers, &c. The effects of external temperature first produce on the functions of the skin (which connects us with the exterior world) an augmentation or diminution of the natural discharges, by suddenly changing or disturbing the balance of the circulating fluids between the external and internal parts or surfaces, irrespective of the perspiratory process, commonly termed sweat, there being innumerable chemical agents more poisonous and of greater importance as to their elimination from the system in the shape of insensible perspiration. I repeat, that the effects of external temperature, by causing a repercussion of the cutaneous exhalations (sensible and insensible), so damage the _vitality_ of the system, as to be the _exciting_ cause of disease; and, although from our imperfect knowledge, it scarcely admits of investigation, setting aside chemical research, so that we cannot explain or define the precise peculiarity of each and every condition of the atmosphere favourable to the production of each variety of disorder, the effects of such external temperature are, however, not the less certain and obvious to our senses. The agencies are all natural, comprised in the excess of state or change acting on individuals whose susceptibility varies greatly from moral as well as from physical _predisposing_ causes; but our inability to define their peculiar conditions or auræ, as noticed by Hippocrates, Sydenham, Bacon, and others, no more militates against our reasoning upon their visible effects, than a knowledge of the gravitating principle or power is essential to the proof of gravity. But to the subject of the causes,--the exciting and the predisposing of disease; it should be premised that when attributing epidemic pestilences or diseases to natural causes, it must not be supposed or inferred that any one of such causes, as heat, cold, moisture, drought, &c., will alone be sufficient to induce disease: to effect that, there must be a combination of these causes in continuance and succession, in occasion and variation, as also in circumstance and virulence.