HEART WOOD. Duramen. The central, old, and colored wood of trees; it is the most durable.
HEAT. .A.griculturally considered, heat is the force through which plants are enabled to grow and perfect their seed. In connection with this, however, there must be moisture and -with moisture substances soluble in -water proper to the sustenance of the plants. Thus under a given heat, or an average heat, extend ing through a certain period, we have the ripen ing point of the various crops cultivated for the use of man. Nevertheless heat and cold are merely relative terms. Thus, if -we immerse one hand in cold and the other hand in warm water, and then successively in water of a medium temperature between the two, this water of medium temperature will feel cool to the hand taken out of hot water, and warm to the hand taken from cold water Again, the temperature .of the hody in health is constant, being about ninety-six degrees. When the temperature is much below this point, or when the body is suddenly cooled from artificial causes, we feel the sensation of cold. If, on the other hand, insensible perspiration is obstructed we are un -easy from the accumulation of heat, the act of perspiring being the natural refrigerating process among animals. In expansion by heat wood is found to do so chiefly in a direction transverse to its fibers, and but little in length. On the -other hand iron expands in the direction of its length to such a degree as to force apart walls, when rigidly placed, and railway bars, unless spaces are left between, will expand to so great a degree as to be sometimes forced up in the shape of a semicircle. Again, the contraction 4)f bodies upon cooling is often not so great as their expansion. Thus we find old fire grates, and iron when it is often strongly heated, much -distorted. Heat is communicated in various -ways, through particles in bodies, generally in two ways; in solids, by passing the heat received from particle to particle. This is called conduction. In fluids heat is com municated by being carried connectively, that is, the heated particles rise by their increasing levity through heat, conveying the heat and giving it out to other parts. Thus water
heated from above, remains comparatively cold at thc bottom, but heated from below the -whole body comes to be of nearly a uniform temperature. In wood the heat conducting power bears no definite relation to its density. "Thus birch one of the lightest of woods, conducts heat better than any other; oak, a very dense wood, conducts nearly as well, but iron-wood, a wood of great density, is a bad conductor of heat. Thus we see why some vegetable sub stances seem colder than others. Again, the conducting power of a medium has much to do -with the sensible hcat experienced. Thus, a metal heated to 120° may burn the hand, while irn mersed in water without motion at 150° it may not scald. So, also, air heated to 300° has been borne by the human body without injury. Feathers, down, wool and cotton, are bad con ductors of heat. Hence their superior warmth as clothing because they prevent the escape of heat from the body. The order of their value in capacity being, furs, -wool, cotton, silk and linen. The rays of the sun striking the earth produce sensible heat; part of this is absorbed by the soil, and part reflected back into space. The relative powers of absorption may be represented by lampblack at 100", common glass will absorb about ninety degrees, and a highly polished metallic surface will absorb but six degrees. Therefore, practically a dark soil will absorb much more heat than a light one The relative power of soils to absorb heat giving the maximum temperatures of variou,s earths exposed to the sun, both moist and dry, the thermometer 77° in the air, was found by Becquerel, to be as follows: the difference in tem peratures between moist and dry being due to the heat spent in the evaporation of a portion of the water of the moist earth, an important point for the farmer to note in periods of intense heat.