The range of temperature and the amount of cold and heat the human system is capable of enduring is immense. Thus, we find human beings, as well as various forms of animal and vegetable life, flourishing in the intense cold of the frozen regions, when the thermometer sinks to 40 or 50° below zero, and the night is five or six months long, as well as in more temperate regions and in the torrid zone, where the thermometer remains for weeks from 100 to 110° above zero.

The amount of heat and cold, which the human system is capable of enduring under certain circumstances has been proved by actual experiment to be so enormous, that had not the statements been well attested, we should have been inclined to doubt their truth. In some experiments tried in the years 1760 and 1761 in France, to devise means to destroy an insect which consumed the grain, a girl entered the oven and after remaining two or three minutes marked the point to which the mercury in the thermometer had risen at 260°. Insisting that she felt no inconvenience, she remained ten minutes longer, during which time the mercury reached the 288 degree, or 76° of heat above water when it boils. On coming out, her complexion was heightened, but her respiration was neither quick nor labored.

Another girl remained in the oven the same length of time with equal impunity, and even breathed for five minutes air heated to 325° or 113 above boiling water.

At different times physicians and others have entered rooms heated to 240 and 260° and remained some time, sometimes with, and again without their clothes, and without inconvenience. During this time the animal heat, ascertained by placing the thermometer under the tongue, was scarcely increased at all, but the pulse was much quickened. The heat was so great, that pieces of metal about them could hardly be touched; breathing on the thermometer caused the mercury to fall several degrees, and they also cooled their fingers by breathing on them. In the same air, eggs were roasted hard in twenty minutes, and beefsteak well cooked in thirty.

Taking the other extreme, we find the body capable of enduring under certain circumstances an intense degree of cold without injury. Air being a bad conductor, caloric is exhausted much more rapidly when the air is in motion than when still, and therefore a much greater amount of cold can be endured in the latter than in the former case. Capt. Parry mentions that where the thermometer was 55° below zero and no wind stirring, the hands could remain uncovered without inconvenience for a quarter of an hour, while with a fresh breeze and the thermometer at zero, the pain in the same length of time would be intense.

In the recent expedition to the Arctic regions, under command of Capt. de Haven, in search of Sir John Franklin, the degree of cold to which the men were subject in-a climate where the thermometer for weeks numbered from thirty to fifty degrees below zero, was beyond what we can well realize. Yet the men were generally healthy. There were but very few colds or inflammatory diseases and I believe no deaths.

Thus we see that the body is capable of enduring under certain circumstances, and for a certain time, great extremes of heat and cold. It was formerly supposed, and is to a certain extent now, that sudden changes ol temperature are in all cases hurtful. But this is by no means always the case. The inhabitants of Russia are in the habit, while reeking from their vapor baths, of plunging directly into cold water, or rolling in snow, without injurious effects, and Capt. Parry remarked that during his northern expedition they were constantly in the habit in going from the cabin into the external air, of undergoing a change of from 80 to 120° of temperature in one minute without the slightest unpleasant sensations.

The sudden descent from one point to another in atmospheric temperature, must of course vary in its effect, according to the state of the body at that time. We have heretofore explained the generation of animal heat, and shown that in health, nearly the same degree of inward temperature is maintained in every clime. If the external temperature be lower than that of the body, the heat carried off is instantly replaced in health, by the combustion within, together with exercise and clothing. When it approaches the natural heat of the body, sweat breaks out, and the superfluous heat is thus removed by evaporation. The internal evolution of heat is so constant, that an external temperature of 98, which is about the heat of blood in man, when of course the atmosphere cannot abstract heat from the body, is exceeding oppressive. If the heat is carried off just as fast and no faster than it is developed, no particular sensation of either heat or cold is felt. Therefore the sensations must vary in a great measure with the power which different constitutions possess of evolving heat. Thus one person whose power of evolving heat may be less than another, will be cold in a temperature, which the other would consider warm.

If this power of evolving heat, be entire, active, and continuous, no danger need be apprehended even from various alterations of temperature. Great heat of the body, when the cold is applied, is really a condition of safety instead of danger, provided the heat is steady and permanent. Thus,how refreshing to the heated brow is the application of ice, and how grateful during the burning paroxysm of fever affusions of cold water. This principle also holds good of the application of cold, when the body has been heated by exercise, or from any other cause, provided that cause remains steadily in action, there being no local disease, and the body not fatigued, and fast loosing its heat. But, if a person is exhausted and weakened by exercise, if he is perspiring and rapidly parting with heat, if the exercise is over and he remains at rest during, and immediately after the application of cold, then there is danger of mischief. The danger is not from the application of cold, when the body is hot, but when the body is cooling after having been heated.