HEAT. 688 HEATH. by which the energy used in producing heat- erteets is conveyed from one point to another: couveclion, conduction, radiation. It should be j>articularly noted that in all cases a difference of temperature is essential for a transfer of energ}-, and that it is the body at the lower temperature which gains heat-energy, while the body at the higher temperature loses it. Go«rec- tiui'i has already been described. It consists in applying a llame or 'source of heat' to the lower purtionof a column of some fluid; this portion expands, becoming less dense; then under the iiitluence of gravity it rises to give place to some beavier portion of the Uuid above it. The motion of the wind, of draughts of smoke in chimneys, of water in hot-water systems of heating, etc., are illustrations of this process. Conduction is illus- trated when one end of a long metal rod is raised to a high temperature, while the other is maintained at a lower one. There may be observed a steady increase of temperature from the latter end tip to the former; the heat-energy required to produce this temperature coming from the 'source of heat' which keeps the hot end at its higli temperature by a process of 'con- duction' from particle to particle down the rod. Some bodies conduct better than others, meaning that the effect of the source of heat at the hot end is felt farther down tlie rod. Metals are all good conductors; but silver, copper, and aluminum are the best. Woods and cloths are poor conductors; so are all liquids, with the evception of molten metals, and gases also, rela- tivel,v to their volumes. The principle of the miner's safety lamp depends upon conduction, because the temperature of the gases escaping from the flame through the gauze envelope of the lamp is made so low by the conduction away of heat-energy by the gauze and the metal base that the exterior gas is not ignited. Radiation is the process by which heat-energy is conveyed from one body to another by a wave-process in the luminiferous ether. (See Radiation; Ab- soRPTiox: Ether.) Illustrations are afforded when the hand is held beside a stove or exposed to the sun. All bodies in the universe, so far as is known, are emitting these ether-waves, owing to vibrations inside the molecules. These waves have lengths varying from less than 0.00002 of a centimeter to a few hmidredths of a millimeter. They carry energy; and if they are absorbed by any body upon which they fall, this energy is as a rule distributed throvighout the minute portions of the body, and it manifests lieat-effects. The total amount of energy radiated by a body increases as the temperature is raised; and, further, as the temperature rises, the body emits shorter and shorter waves — finally the bodv may become visible. If two bodies are allowed to radiate to each other, each loses energy by radiation and gains it by absorption until finally there is equilibrium of temperature. It may be shown (see Radi.tiox) that the ra- diating and absorptive powers of any body are the same at any one temperature: and in general a body which is a good absorber is a good radia- tor, e.g. a body painted black; while, if a body is a poor absorber, and therefore a good re- flector, it is a poor radiator, e.g. a piece of polished metal. Some bodies are transparent to ether-radiations of certain wave-lengths and opaque to others: they are called diathermanous if the waves which are transmitted by them carry a large amount of energy, which may be trans- formed into lieat-energy if absorbed. See Dia- thermancy. Consult: Preston, Theory of Heat (London, 189G) ; Tyiidall, Beat as a Mode of Motion (0th ed. London, 1880) ; Maxwell, Theory of Meat (London, 1875). For heat considered as a fac- tor of organic evolution, see Evolution. HEATH (AS. 7i(e1>, Icel. heifr, Goth. kai>i, OHG. hcida, Ger. Heide, heath, OWelsh, Bret. coit, Corn, cuit, forest, Gall, ceto- in Veto-hriija, eto-cetum, Lat. cetum in hu-cetum, cow-pasture, quer-cetum, oak-forest). A term of varied sig- nificance, applied originally to various members of the Ericac;B (see below), and long used to designate areas covered by such plants. It has been still further extended to include most sterile, uncultivated tracts covered with low shrubs. In its more proper ecological sense, however, a heath may be defined as a plant society on sterile and commonly dry soil in which the dominant plant types are evergreen shrubs, chieti.y ericads or ericad-like plants. The heath, defined in this way, is much less common in America than in Europe, although in the Northern United States and Canada small areas of low evergreen shrubs are often found in which the bearberry (Arcto- staphylos) and junipers dominate. Small heath areas are also found in mountain (even alpine) districts. In the far northern regions and some- times locally in the Northern States, habitats like the above are often covered with coarse mosses like Polvtrichum or lichens of the genus C'ladonia (including the reindeer lichen) ; these areas may be called moss or lichen heaths, since the soil conditions are those of the true heath, though the exposure is perhaps too great for the shrubs. The origin of the heath is twofold: it may arise from a moor or undrained swamp (q.v. ), or from a dry sandy area, as a beach or dune. This fact is of great ecological interest, since so far as water content is concerned, a dune is xerophytic, while a swamp is hydrophytic. That dune, heath, and moor are closely related as to their ecological conditions, however, is shown not only by the twofold origin of the heath, but also bythe fact that all typical dune, heath, and moor plants have identical and xerophytic struc- tures (see XEROPnYTE.s) ; not only this, but the dominant plant families, conifers and ericads, Are the same throughout. On the dune transpira- tion is great, perhaps because of the excessive exposure ; on the heath and in the swamp, tran- spiration is great in relation to absorption, prob- ably because of the soil acids and the relative alisenoe of bacteria. Hence in all three cases only those plants can thrive which are able to reduce transpiration by protective structures, and those structures are peculiarly prominent in conifers and ericads. The heaths are small shrubs, distinguished by a calyx of four leaves and a bell - shaped or ovate corolla. The leaves are small, linear, and evergreen. The genus has been separated by some botanists into a number of genera, but the old name. Erica, is still commonl.v re- tained. The name heath, however, is in popular language extended to many plants of genera nearly allied to Erica. The little shrub which chiefly covers the large tracts named moors or heath's (Ger. Hnide) in Great Britain and on the Continent of Europe, is the common ling or
