328. =Degree of Heat Endurable by Man.=--It was formerly believed that the human body could not endure with impunity, even for a short time, a much higher degree of temperature than that which is met with in hot climates. But in the year 1760 it was accidentally discovered that a much higher temperature than this could be endured. An insect was destroying at that time the grain gathered in some parts of France, and it was found that if the grain was subjected to a certain high degree of temperature the insect was killed, and yet the grain was not injured. In trying some experiments in regard to this matter the experimenters wished to know the point at which the thermometer stood in a large oven. A girl attending on the oven offered to go in and mark the thermometer. She did so, remaining two or three minutes, and the thermometer was at 260°, that is, 48° above the boiling point of water. As she experienced no great inconvenience from the heat she remained ten minutes longer, when the thermometer rose to 76° above that point. These facts were published, and prompted scientific men to try other experiments. In England, Dr. Fordyce, Sir Charles Blagden, and others, went into rooms heated even to 240° and 260°, and remained long enough to cook eggs and steaks, and yet themselves suffered little inconvenience. The pulse was quickened, the perspiration was very profuse, but the heat of the body, as ascertained by putting the thermometer under the tongue the moment they came out, was scarcely raised at all. The air in which they were roasted eggs quite hard in twenty minutes, and when it was applied by a pair of bellows to a steak it cooked it in thirteen minutes. The question arises, how is it that this high degree of heat did not produce more effect upon the body? One reason is that the heat of the air in the immediate neighborhood of the body was continually reduced by the evaporation of the free perspiration, sensible heat being thus converted into latent. Another reason is that the air is not a good conductor, and therefore did not communicate its heat readily to the body. Dr. Fordyce and his friends found that they could not touch with impunity any good conductor, as the metals, and they were obliged to wear upon their feet some non-conducting substance. 329. =Formation of Ice.=--Before dismissing the subject of heat I must notice the grand exception which we have to some of the operations of heat in the formation of ice. Heat generally produces expansion. But in the case of water this law of expansion is set aside, and the reverse is established. This is done, however, only within a small range of temperature, viz., from the freezing point up the scale about seven degrees. In all degrees above that the usual expansion by heat takes place. The exception occurs at this part of the scale for a special purpose, viz., _that water, in distinction from other substances, shall become more bulky, and therefore lighter, as it takes the solid form_.