Is there any evidence to show that the air of different places, remote from towns, varies in its salubrity in different places, or in the same place at different times? I apprehend that most of the beneficial or evil effects of different air may be ultimately referred to its relations to moisture and dryness. That such changes are considerable and striking, under certain circumstances, have been rendered apparent by the admirable researches of Mr. Da-niell, who, by the invention of a simple and correct hygrometer, has been enabled to throw very considerable light upon this hitherto obscure subject. That the cutaneous discharge is very materially affected by the degree of moisture in the atmosphere is evident; and that the digestive organs may therefore be thus influenced, through its medium, is a corollary which no one will refuse to admit. When the air is very moist, it is a bad conductor of the perspirable matter, which, therefore, instead of being carried off in an insensible form, is condensed upon the surface; hence we appear to perspire greatly upon the slightest exercise, whereas the cuticular discharge is, at that time, absolutely less.

We have all experienced the sensation of heat, and disposition to sweating, during the moist weather which so frequently occurs in this country in April and May, the wind being at the time stationary at south-west or south. On the contrary, during the prevalence of an east wind, the most violent exercise will scarcely prove diaphoretic, and yet the quantity of cutaneous exhalation is far greater than during that state of atmosphere when the slightest exercise deluges us with perspirable matter. Dr. Schmidtmeyer says, that in Chili, notwithstanding the high temperature, which would have been intolerable in Europe, and deluged the inhabitants with perspiration, so rapidly does evaporation proceed, that it might even be doubted whether, after considerable exercise, any perspiration was occasioned by it.* It is scarcely necessary to observe that the atmosphere of Chili is remarkable for its dryness. The functions of the lungs are no less influenced by the state of the atmosphere than those of the skin.

The former organs are constantly giving off water, and if it be not carried off, with equal rapidity, it is reasonable to suppose that some influence will be produced upon them, beneficial or otherwise, according to the peculiar condition of the patient, as I have endeavoured to explain in the last edition of my Pharmaco-logia, under the history of Expectorants. How is the cure of hooping-cough, by change of air, to be explained, unless we adopt the belief which I have endeavoured to enforce?

Experience has taught us that the air most beneficial to the dyspeptic patient is that which is dry and keen; a residence in an elevated spot should therefore be selected: for the atmosphere of a valley, however congenial to the pulmonary invalid, will be generally found injurious to the class of patients whose welfare it is the object of the present work to promote.

* Travels into Chili.

380. The advantages which attend "change of air," in the treatment of various diseases, have been ascribed by many physicians to the exhilarating impressions thus produced upon the mind, and to the simultaneous change of habits which usually takes place upon such occasions. I am willing to admit the extensive and powerful operation of such causes in the treatment of diseases in general, but more particularly in those cases in which the digestive organs constitute the source of the derangement; for such affections are influenced by the state of the mind to an extent to which it would be difficult to assign a limit. It therefore follows that, in the recommendation of a place of resort for invalids, various circumstances are to be taken into consideration: it is no less important to furnish amusement for the mind, than to provide salubrious air and wholesome food for the body. A continual change of residence is, perhaps, better adapted for insuring our object, than a protracted stay in any one place.

The genial excitement, which a succession of novelties produces on the mind, to say nothing of the advantages which necessarily arise from the exercise of the body, is more likely to insure exhilaration and cheerfulness, and to break down the associations which continued disease will always engender, than a monotonous residence in a watering-place, where, after the first few days, the patient becomes familiarized with the objects around him, the spell by which he is to be cured is broken, and his mind is watching every pulsation, in order to discover some indication of that returning health which he so anxiously anticipates. This truth is beautifully illustrated by an anecdote related by Sydenham, and will go further in establishing the importance of the principle I am desirous of enforcing, than any argument which it is in my power to adduce. This great physician, having long attended a gentleman of fortune with little or no advantage, frankly avowed his inability to render him any further service, adding, at the same time, that there was a physician of the name of Robinson, at Inverness, who had distinguished himself by the performance of many remarkable cures of the same complaint as that under which his patient laboured, and expressing a conviction that, if he applied to him, he would come back cured.

This was too encouraging a proposal to be rejected: the gentleman received from Sydenham a statement of his case, with the necessary letter of introduction, and proceeded without delay to the place in question. On arriving at Inverness, and anxiously inquiring for the residence of Dr. Robinson, he found, to his utter dismay and disappointment, that there was no physician of that name in the place, nor ever had been in the memory of any person there. The gentleman returned, vowing eternal hostility against the peace of Sydenham; and on his arrival at home, instantly expressed his indignation, in not very measured terms, at having been sent so many hundred miles for no purpose. "Well," replies Sydenham, "are you better in health?" - "Yes; I am now perfectly well, but no thanks to you." - "No?" says Sydenham, "but you may thank Dr. Robinson for curing you. I wished to send you a journey with some object of interest in view: I knew it would be of service to you: in going, you had Dr. Robinson and his wonderful cures in contemplation, and in returning you were equally engaged in thinking of scolding me." There was more wisdom and address in such a scheme than in that which is said to have been practised by Hippocrates, who sent his patients from Athens with no other object than to touch the walls of Megara, and then to return.

381. I have thus endeavoured to investigate the principles upon which the treatment of indigestion is to be conducted. I might have descended into fuller detail, but the art of selecting remedies, of graduating their strength, and of modifying their powers by combination, constitutes a subject to which I have directed the attention of the practitioner in a distinct work, the extensive sale of which convinces me that it must be already in the hands of every professional reader, and will render any further observations in this place unnecessary. After all, more benefit will arise in dyspeptic diseases, from a judicious regulation of the diet and habits of the patient, than from large quantities of medicines: although I do not intend, by this observation, •to undervalue the importance of such agents, when directed by the skilful hand of the physician. The unhappy invalid, who seeks for relief from the nervous cordials and stomachic mixtures of the empiric, may occasionally derive the feeling of temporary relief, from the operation of stimulants to which the regular practitioner will never resort. Let him remember that such relief, if obtained, must be at the expense of his future welfare.

Such expedients have been aptly enough said to be drafts upon the constitution, payable with compound interest a few months after date. If we examine the history of quackery, we shall generally find that the administration of a boasted panacea has been associated by its crafty author with some reformed system of diet and habits, to which any benefit that may have arisen ought injustice to be assigned; for mystery carries with it a spell that overcomes every prejudice, secures the implicit obedience of the patient, and reconciles the most refractory to privations which reason would in vain endeavour to enforce.

381. I shall now recapitulate some of the more prominent doctrines which have been established in the preceding pages - a plan which will not only have the advantage of placing the subject in a simple and perspicuous point of view, but of affording the reader a convenient reference to the practical parts of the work.