I come now to consider a division of trades which expose to the operalion of some agent, capable, or generally considered so, of acting unfavorably on the human body. Most of these agents we shall find are of a subtle nature, not appearing in a solid tangible form, but eluding the senses, and recognized principally from their effects. They are enemies rather to he fell than seen. They act, for the most part, through the medium of that invisible fluid by which we are surrounded ; some being

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... lved in the air, and thus altering its properties, and others being merely diffused-that is, raised into the atmosphere by their lightness, or the fineness of the particles of which they consist, and thus conveyed to the person on which they act. These agents are heat, cold, humidity or dampness, the gases produced by human respiration and combustion, the fine particles or dust of substances employed, the exhalations from dead animal or vegetable matter, metallic or universal exhalations, and other chemical agents. Of these in their order. Class VII.-Occupations exposing to extremes of température. There are many occupations which require constant exposure lo extreme cold ; but where this circumstance is counteracted by active exercise, it is not only not injurious, but decidedly salutary. Employments of this description have been adverted to and sufficiently considered. Of those which require exposure icilhout exercise, the number is fortunately small. Among them may be mentioned the drivers of public coaches, who in our climate are sometimes obliged to remain for a considerable space of time in the open air, without the means of exercising their limbs, except partially and imperfectly. Under such circumstances, it is not surprising that instances occur of injury to limbs from frost, and occasionally of loss of life. It is, perhaps, more surprising that these accidents are so rare. The question has often attracted attention, whether the use of ardent spirit renders the body more or less sensible to cold. This may be answered in different ways. As respects the nervous sensibility, or the susceptibility to suffering from cold, this is certainly less after the use of stimulus ; and in this .sense it may be said that these substances render the body less sensible to all injurious agents. But it is a very different question whether the body can best bear the continued action of cold without danger to life, with or without alcohol. Those who have heard or read the sensations described which precede freezing, will notice that there is always a disposition to sleep, and that unless this disposition is counteracted by resolution, the sleep proves to be a fatal slumber. Now it is this somnolency, or sleepiness, which the presence of alcohol in the system undoubtedly encourages ; and there can be no doubt that a person exposed to intense cold, will hold out the longest without being actually exhausted, if the system is free from excitement. This is confirmed by what we hear of sailors when wrecked on a coast in winter, and obliged to remain many hours without assistance. Those who remain sober under these circumstances, live longer than those who, to