of temperature applied will give a sensation of heat; but if the increase of temperature does not arise to 62 degrees, the sensation produced will not continue long, but be soon changed to a sensation of cold. In like manner, any temperature applied to the human body, lower than that of the body itself, gives a sensation of cold; but if the temperature applied does not go below 62 degrees, the sensation of cold will not continue long, but be soon changed to a sensation of heat.

It will appear hereafter, that the effects of the sensation of cold will be very different, according as it is more permanent or transitory.

90. Having thus explained the operation of cold as absolute or relative with respect to the human body, I proceed to mention the general effects of cold upon it.

1. Cold, in certain circumstances, has manifestly a sedative power. It can extinguish the vital principle entirely, either in particular parts, or in 'the whole body; and considering how much the vital principle of animals depends upon heat, it cannot be doubted that the power of cold is always more or less directly sedative.

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This effect may be said to take place from every degree of absolute cold; and, when the heat of the body has upon any occasion been preternaturally increased, every lower temperature may be useful in diminishing the activity of the system;

but it cannot diminish the natural vigour of the vital principle, till the cold applied is under 62 degrees; nor even then will it have this effect, unless the cold applied be of an intense degree, or be applied for some length of time to a large portion of the body.

2. It is equally manifest, that, in certain circumstances, cold proves a stimulus to the living body, and particularly to the sanguiferous system.

It is probable, that this effect takes place in every case in which the temperature applied produces a sensation of cold; and this, therefore, as depending entirely on the relative power of cold, will be in proportion to the change of temperature that takes place.

It appears to me probable, that every change of temperature, from a higher to a lower degree, will prove more or less stimulant; excepting when the cold applied is so intense, as immediately to extinguish the vital principle in the part.

3, Beside the sedative and stimulant powers of cold, it is manifestly also a powerful astringent, causing a contraction of the vessels on the surface of the body, and thereby producing a paleness of the skin, and a suppression of perspiration; and it seems to have similar effects when applied to internal parts. It is likewise probable, that this constriction, as it takes place especially in consequence of the sensibility of the parts to which the cold is applied, will in some measure be communicated to other parts of the body; and that thereby

the application of the cold proves a tonic power with respect to the whole system.

These effects of tonic and astringent power seem to take place both from the absolute and relative power of cold; and therefore every application of it which gives a sensation of cold, is, in its first effect, both astringent and stimulant, though the former may be often prevented from being either considerable or permanent, when the latter immediately takes place.

91. It will be obvious, that these several effects of cold cannot all take place at the same time, but may in succession be variously combined. The stimulant power taking place obviates the effects, at least the permanency of the effects, that might otherwise have arisen from the sedative power. That the same stimulant power prevents these from the astringent, I have said above; but the stimulant and tonic powers of cold are commonly, perhaps always, conjoined.

92. These general effects of cold now pointed out, are sometimes salutary, and frequently morbid; but it is the latter only I am to consider here, and they seem to be chiefly the following.

1. A general inflammatory disposition of the system, which is commonly accompanied with rheumatism or other phlegmasiæ.

2. The same inflammatory disposition accompanied with catarrh.

3. A gangrene of particular parts. 4. A palsy of a single member.

5. A fever, or fever strictly so called, (8), which it often produces by its own power alone; but more commonly it is only an exciting cause of fever, by concurring with the operation of human or marsh effluvia.

93. Cold is often applied to the human body without producing any of these morbid effects, and it is difficult to determine in what circumstances it especially operates in producing them. It appears to me, that the morbid effects of cold depend partly upon certain circumstances of the cold itself, and partly on certain circumstances of the person to whom it is applied.

94. The circumstances of the cold applied, which seem to give it effect, are, 1, The intensity or degree of the cold; 2, the length of time during which it is applied; 3, the degree of moisture at the same time accompanying it; 4, Its being applied by a wind or current of air; 5, Its being a vicissitude, or sudden and considerable change of temperature, from heat to cold.

95. The circumstances of persons rendering them more liable to be affected by cold, seem to be, 1, the weakness of the system, and particularly the lessened vigour of the circulation, occa

sioned by fasting, by evacuations, by fatigue, by a last night's debauch, by excess in venery, by long watching, by much study, by rest immediately after great exercise, by sleep, and by preceding disease; 2, The body, or its parts, being deprived of their accustomed coverings; 3, One part of the body being exposed to cold, while the rest is kept in its usual or a greater warmth.

96. The power of these circumstances (95) is demonstrated by the circumstances enabling persons to resist cold. These are, a certain vigour of constitution, exercise of the body, the presence of active passions, and the use of cordials.

Besides these, there are other circumstances which, by a different operation, enable persons to resist cold acting as a sensation, such as, passions engaging a close attention to one object, the use of narcotics, and that state of the body in which sensibility is greatly diminished, as in maniacs. To all which is to be added, the power of habit with respect to those parts of the body to which cold is more constantly applied, which both diminishes sensibility, and increases the power of the activity generating heat.

97. Beside cold, there are other powers that seem to be remote causes of fever; such as fear, intemperance in drinking, excess in venery, and other circumstances, which evidently weaken the