HEAT HEATH 579 nearly agree with Prof. James Thomson's pre- diction that the fall should be 0'0135 for each additional atmosphere. Mouson has since then succeeded by enormous pressure in reducing the freezing point of water several degrees. The apparatus in which pressure was effected was placed in a certain position and charged with water into which a piece of metal was dropped. The water was then frozen, and cooled to zero, or 32 below the freezing point. A pressure which was estimated to be several thousand atmospheres was then applied, after which the apparatus was inverted and the pressure removed, when on examination the piece of metal was found at the opposite side of the enclosure, thus showing that the ice had been melted. Those bodies which, unlike ice, expand during liquefaction, have their melting points raised instead of lowered by increase of pressure. In this manner Bunsen, Hopkins, and Fairbairn have raised the melting point of spermaceti, which is 120, several degrees ; a pressure of 519 atmospheres raised it to 140, and one of 792 atmospheres to 176. A liquid which, like water, expands on con- gealing, has its particles restrained by pres- sure, and therefore to congeal it the tempera- ture must be lowered; but one which con- tracts in solidifying will have its particles as- sisted by pressure, and hence its melting point will be raised. Many interesting phenomena are exhibited by liquids and gases when sub- jected to great heat and pressure, such as the obliteration of the line of demarkation between the liquid and vaporized portion in which what is called a critical temperature is con- cerned. The subject will be found treated, with that of the tension of vapors, in the arti- cle VAPORIZATION. Chemical action being al- ways accompanied by physical change, as ex- pansion or contraction, liquefaction or solidifi- cation, it is difficult to estimate the effects pro- duced by each. In general it may be held that the heat of chemical combination results from the intense molecular motion imparted by the clashing of combining molecules with each other, and moreover, that whatever heat is evolved by combination will be absorbed, or will disappear in the separation of the constitu- ents of the compound into their original form ; and it is found that generally combination pro- duces heat, and that decomposition produces cold. But the heat which is evolved by the physical changes which accompany chemical action is more easily accounted for. Take for example the condensation which accompanies the union of quicklime with water ; the re- sulting hydrate has less bulk than the sum of the constituents previous to combination. The energy necessary to maintain this excess of volume among particles at insensible distances from each other composing liquids or solids, is enormous; consequently a reduction of the distances, whether accomplished by the influ- ence of chemical affinity or by mechanical pressure, causes a conversion of this energy into another, generally heat. The first change may not, however, be entirely into heat, but, as in the case of the compression of certain crystals, or the combination of a metal with an acid under certain conditions, as in the galvanic battery, there may be a transformation into electric force, but which is supposed finally to become resolved into heat. Sir William Thomson has advanced the opinion that there is a tendency to the conversion of all physical energy into the condition of heat, and to its uniform diffusion throughout all matter; a condition which he regards as involving the cessation of all physical phenomena. The con- clusions of Prof. Thomson are founded upon the law of the French philosopher Carnot, which is that mechanical energy is produced by heat only when it is transferred from a body of a higher to one of a lower tempera- ture. The subject is a difficult one, as there are many possible circumstances connected with the forces and matter of the universe which can never be reduced to an exact basis of cal- culation. The following are the most impor- tant recent works on heat : " Sketch of Ther- mo-dynamics," by P. G. Tait (Edinburgh, 1868) ; " An Elementary Treatise on Heat," by Bal- four Stewart (London, 1872); "Theory of Heat," by J. Clerk Maxwell (1871) ; " Heat as a Mode of Motion," and " Contributions to Mo- lecular Physics in the Domain of Kadiant Heat," by John Tyndall (1873). See also the articles " Heat " and "Radiation " in Watts's " Diction- ary of Chemistry," and various articles in the reports of the Smithsonian institution. HEATH, or Heather, the common name of plants of the genus erica, which contains about 400 species, besides numerous varieties pro- duced by cultivation. The greater number of species of heath are natives of western Africa, some are peculiar to the western portion of Europe and the Mediterranean, and a few extend into northern Europe, one of which is sparingly found in North America. While some of the African species form shrubs 8 or 10 ft. high, those of northern countries are low, much-branched shrubs, seldom exceeding a foot. The small evergreen leaves are entire, usually revolute at the margins and in whorls of three or four, scattered or rarely opposite. The mostly drooping flowers are either axillary or in short terminal clusters ; the calyx of four sepals, sometimes colored ; corolla ovoid, globu- lar, bell-shaped, or sometimes tubular, more or less four-lobed, and drying attached to the cap- sule ; stamens eight, the anthers with two ap- pendages at the back and opening by a chink ; pistil solitary ; capsule four-celled, splitting at maturity into four or eight valves. The genus erica comprises species of great beauty, even the most humble of them being attractive, and is the type of a large order, the ericacece or the heath family, noted for the showy character of many of its genera, about 50 in number, in- cluding rhododendron, azalea, Tcalmia, andro- meda, and others well known for the beauty of
