If, in respect to some diseases, so intimate a connection cannot be observed, the probability is, that it is the obscurity connected with these subjects, the less direct way in which they operate and the remote time at which their effects may be experienced on the body, that are the occasion of the difficulty. The human body, unlike mere inanimate matter, has the power of withstanding, at least for a time, the operation of unwholesome influences, if not very virulent; and it is only natural to allow, that, in respect to a machine so complicated, affected by so many agencies, and standing in so many relations, there will be less complete directness of operation, than with simple or inanimate substances, and more variety in the amount and duration of the effect. On these accounts, the indications of the case are less direct and obvious, and it should cause no great surprise, that being considered, and the fact of the imperfect state of our knowledge on the varieties of the agencies referred to, being kept in mind, that the causes of disease cannot at all times be completely and satisfactorily ascertained. Our knowledge of the derangements of health, from the operation of these agencies, on some of which we are dependent, and with others of which we are constantly brought in contact, is fast increasing, and the relation between the former and the varieties in the latter is becoming clear and precise. There is, therefore, reason to hope that difficulties which now baffle us, will soon be explained away, and that much of that mist that has long overhung the causes of pestilence, will soon be dissipated. Certain circumstances produce certain uniform effects, and in every instance where they are operating, their effects will be produced, provided no agency is acting adequate to neutralize them. Not one hill only, in northern latitudes, has its summit whitened with snow, nor does sterility mark a few spots only in the immense deserts of Africa and Asia. The same features are spread far and wide. They owe their existence to agencies acting in immense spheres corresponding with their own. The sphere of those circumstances connected with the agents so often referred to, that produce disease, is sometimes large, and no astonishment need be felt, if that of disease is also large, since a relation ever holds between the extent of a cause and its effects. It would certainly be ample time to call in the assistance of atmospheric contagion to account for the propagation of disease, when its sphere or circle is found to be positively eccentric of that of those circumstances alluded to. But we are satisfied that such a contingency is of very rare occurrence, and even when it is said to exist, we shall require some undoubted assurance that the non-correspondence is not the result of ignorance of the extent of these hurtful circumstances, rather than the actual absence of relation between them.