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as having been near to the bodies of men, are imbued with their effluvia, and in which substances these effluvia are sometimes retained in an active state for a very long time.

The substances thus imbued with an active and infectious matter, may be called fomites; and it appears to me probable, that contagions, as they arise from fomites, are more powerful than as they arise immediately from the human body.

83. Miasmata are next to be considered. These may arise from various sources, and be of different kinds; but we know little of their variety, or of their several effects. We know with certainty only one species of miasma, which can be considered as the cause of fever; and, from the universality of this, it may be doubted if there be any other.

84. The miasma, so universally the cause of fever, is that which arises from marshes or moist ground, acted upon by heat. So many observá ations have now been made with respect to this in so many different regions of the earth, that there is neither any doubt of its being in general a cause of fevers, nor of its being very universally the cause of intermittent fevers, in all their different forms. The similarity of the climate, season, and soil, in the different countries in which intermita tents arise, and the similarity of the diseases, though arising in different regions, concur in proving, that. there is one common cause of these diseases, and that this is the marsh miasma.

What is the particular nature of this miasma, we know not; nor do we certainly know whether or not it differs in kind: but it is probable that it does not; and that it varies only in the degree of its power, or perhaps as to its quantity, in a given space.

85. It has been now rendered probable, that the remote causes of fevers (8), are chiefly contagions or miasmata, and neither of them of great variety. We have supposed that miasmata are the cause of intermittents, and contagions the cause of continued fevers, strictly so named; but we cannot with propriety employ these general terms. For, as the cause of continued fevers may arise from fomites, and may, in such cases, be called a miasma; and as other miasmata also may produce contagious diseases, it will be proper to distinguish the causes of fevers, by using the terms human or marsh effluvia, rather than the general ones of contagion or miasma.

86. To render our doctrine of fever consistent and complete, it is necessary to add here, that those remote causes of fever, human and marsh effluvia, seem to be of a debilitating or sedative quality. They arise from a putrescent matter. Their production is favoured, and their power increased, by circumstances which favour putrefaction, and they often prove putrefactive ferments with respect to the animal fluids. As putrid matter, therefore, is always, with respect to animal bodies, a powerful sedative, so it can hardly be doubted, that human and marsh effluvia are of the same quality; and it is confirmed by this, that the debility which is always induced, seems to be in proportion to the other marks that appear of the power of those causes.

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87. Though we have endeavoured to shew that fevers generally arise from marsh or human effluvia, we cannot, with any certainty, exclude some other remote causes, which are commonly supposed to have at least a share in producing those diseases. And I proceed, therefore, to inquire concerning these causes; the first of which that merits attention, is the power of cold applied to the human body.

88. The operation of cold on a living body, is so different in different circumstances, as to be of difficult explanation ; it is here, therefore, attempted with some diffidence.

The power of cold may be considered as absolute or relative.

The absolute power is that by which it can diminish the temperature of the body to which it is applied. And thus, if the natural temperature of the human body is, as we suppose it to be, that of 98 degrees of Fahrenheit's thermometer* every degree of temperature less than that, may be considered as cold with respect to the human body; and, in proportion to its degree, will have a tendency to diminish the temperature of the body. But as the living human body has in itself a power of generating heat, so it can sustain its own proper heat to the degree above mentioned, though surrounded by air or other bodies of a lower temperature than itself; and it appears from observation, that, in this climate, air, or other bodies applied to the living man, do not diminish the tem. perature of his body, unless the temperature of the bodies applied be below 62 degrees. From hence it appears, that the absolute power of cold in this climate, does not act upon the living human body, unless the cold applied be below the degree just now mentioned. ?

It appears also, that the human body's being surrounded by air of a lower temperature than itself, is necessary to its being retained in its proper temperature, of 98 degrees : for, in this climate, every temperature of the air above 62 degrees, applied to the human body, though still of a lower temperature than itself, is found to increase the

* In every instance of our mentioning degrees of heat or cold, we shall mention' them by the degrees in Fahrenheit's ' scale ; and the expression of higher or lower shall always be according to that scale.

heat of it. And from all this it appears, that the absolute power of cold with respect to the human body, is very different from what it is with respect to inanimate bodies.

- 89. The relative power of cold with respect to the living human body, is that power by which it produces a sensation of cold in it; and with respect to this, it is agreeable to the general principle of sensation, that the sensation produced is not in proportion to the absolute force of impression, but according as the new impression is stronger or weaker than that which had been applied immediately before. Accordingly, with respect to the temperature, the sensation produced by any degree of this, depends upon the temperature to which the body had been immediately before exposed; so that whatever is higher than this feels warm, and whatever is lower than it feels cold; and it will therefore happen, that the opposite sensations of heat and cold, may, on different occasions, arise from the same temperature, as marked by the thermometer.

With respect to this, however, it is to be observed, that though every change of temperature gives a sensation of cold or heat, as it is lower or higher than the temperature applied immediately before, the sensation produced is in different cases of different duration. If the temperature at any time applied is under 62 degrees, every increase

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