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of the heart, particularly in the course of acute rheumatism, is liable to inflammation, and consequent to this inflammation the valves become thickened, contracted, or deformed by the deposition of fibrinous concretions on their free edges. The general symptoms of the disease are not well marked; febrile reaction, some local uneasiness about the heart, and the occurrence of murmur at the apex or the base, are those most commonly met with. Rest, regimen, and depletion are the remedies most to be relied on. Endocarditis in itself is very rarely a serious complaint, but it leaves behind it valvular disease, the valves becoming incompetent to the perfect performance of their office, either opposing the free flow of the blood in its proper course, or permitting its regurgitation into the cavity from which it had just been thrown; and this again leads to secondary changes in the structure of the heart itself. When a valvular murmur is once produced, it remains permanent unless it becomes inaudible from an enfeebled action of the heart.-Hypertrophy and Dilatation. The general mass of the heart, as well as its separate parts, are liable to become enlarged, either from an overgrowth of its muscular substance, or from the dilatation of its cavities, or from the combination of both. Both alterations produce an increased area of cardiac dulness, and in both the apex of the heart strikes below and to the left of the normal point. In hypertrophy the impulse of the heart is heaving and forcible, the pulse, if there be no valvular complication, full and strong, the first sound of the heart prolonged; while in dilatation the first sound is short and clear, the impulse feeble, and the pulse weak. Hypertrophy of the heart is almost always caused by some obstruction in the course of the circulation beyond the hypertrophied part; in most instances this obstruction is valvular, or it may be an aneurism or diseased aorta, or some peculiar condition of the blood, as in Bright's disease. Hypertrophy or dilatation with valvular disease, though often compatible with a prolonged and useful existence, sooner or later, if the patient escape death from syncope or apoplexy, gives rise to congestion of the lungs and liver, and finally to general dropsy. The treatment of these diseases consists largely in the avoidance of all physical and moral causes of undue excitement, in the employment of a simple and digestible but nutritious diet, and in the use of passive rather than active exercise. As anæmia greatly increases the violence of the heart's action, the preparations of iron are often useful. When congestions or dropsy supervene; they must be met with suitable treatment.-The heart is subject to changes of consistence, and among these the most important is fatty degeneration. Here the muscular fibre of the heart is affected, becoming in part replaced by fatty and granular matter. The disease is best discriminated by the general signs of fatty atrophy, by the feebleness of the heart's action and sounds, and by liability on any exertion to great irregularity

or frequency of the pulse. It is in cases of such degeneration that the greater number of instances of rupture of the heart itself, which sometimes though rarely occurs, are to be found. These cases, when independent of external injury, occur more frequently in the male than in the female, in advanced than in early life. The immediate cause of the rupture is to be sought in some sudden congestion of the heart, produced by violent effort, sudden passion or emotion, the shock of the cold bath, or otherwise. Death is commonly immediate, or at most is delayed but a few hours. Rupture of one or more of the chorda tendinea, or of one of the valves, though still rare, occurs more frequently than rupture of the heart itself. Faintness, precordial anxiety, palpitation, and irregularity of the pulse come at the moment of the accident, and if a valve be injured are attended with the murmur diagnostic of the injury.The heart is sometimes affected with aneurism, this being confined almost exclusively to the left ventricle. It may consist in a gradual and uniform dilatation of a portion of the wall of the heart, or in a sudden pinching of the wall with a more or less constricted orifice. Its diagnosis is obscure, the disease presenting few or no symptoms unless very extensive, when the symptoms are common with those of dilatation of the heart. The patient either dies suddenly from rupture of the aneurism, or is worn out by the embarrassment of the circulation and its attendants, congestion and effusion. Occasionally both tubercle and cancer attack the heart, but only as part of a general disease whose principal manifestations are shown in other organs. They present no peculiar symptoms, and are not subjects of treatment. The heart is liable to be detruded from its natural position by various intra-thoracic diseases, most commonly and to the greatest extent by pleurisy with effusion. With extensive pleuritic effusion on the left side the heart may beat beneath or even to the right of the right nipple. These cases are readily distinguished by the coincidence of the signs of pleurisy. As, however, the heart is sometimes placed congenitally on the right rather than on the left side, difficulty of diagnosis may occur. This will be obviated by recollecting that when the heart is thus congenitally misplaced, the liver is likewise transposed to the left side, while the spleen is found on the right. Such displacements produce no symptoms. In rare cases calcareous matter is deposited in the pericardium, often in altered and diseased valves, particularly in old persons. Such cases have given rise to the accounts of hearts converted into bone, and the like, which are mere popular exaggerations.

HEAT (Saxon, hat), the name both of a certain primary sensation which can be defined only by its synonymes, warmth, calidity, &c., and also of the unknown agency or cause that produces the sensation, together with a great variety of phenomena in the material world. All bodies with which we are familiar are in

cessantly under the influence of this agent, its presence being an indispensable condition toward fitting the globe on which we are for the habitation of life and intelligence. (See CENTRAL HEAT, and ANIMAL HEAT.) In this article will be considered those fundamental laws of the action of heat upon bodies generally which constitute the science of thermotics, with some reference to applicatious and to the relations of heat to other forces. Our sensations, as well as observations upon bodies, teach us that heat can exist or manifest itself through a wide range of variation. A given point or intensity in this range forms a certain degree of heat, and constitutes the temperature of the body or space affected by it. I. Changes of temperature are accompanied by changes in the volume of bodies. As a rule, all bodies undergo an increase of volume (expansion or dilatation) while heated, and a corresponding diminution (contraction) upon cooling. Supposing, now, a convenient substance found, the expansion of which shall be, through a wide range, exactly proportional to the sensible temperature imparted to it; it is evident that the observed expansion of such substance will indicate the existing temperature, and show its variations. In the common method with us of measuring temperatures, a range equal to of the variation between the freezing and boiling points of water, as shown by the expansion of mercury, is taken as the unit or single degree (1°) of sensible temperature; the succession of degrees of this magnitude constitutes Fahrenheit's scale. To our sensations, a body is hot or cold according to the difference of its temperature from our own; but our sense of heat is inaccurate, and often fallacious. If, having one hand in a vessel of warm, and the other in one of cold water, we at once immerse both in water of a mean between the two temperatures, this will be felt as warm by the hand removed from the colder liquid, and as cold by the other. Heat and cold, as known to us, are relatively, not absolutely different; they are only higher or lower degrees of heat. Increase in length of bodies, due to heat, is termed linear expansion; and increase in volume, cubical expansion. (See EXPANSION.) A few facts may here be added. Solids expand least of all; but their enlargement is easily made sensible, and, by an apparatus in form of the pyrometer, measurable. Under the same augmentation of heat, different solids expand very differently. Kopp finds that certain crystals, as fluor spar, aragonite, &c., expand more than many of the metals, which were formerly ranked first; and the rate of expansion of ice, could it be observed through the same range, is greater than that of any metal, being, between 32° and 212°, one part in 267. Wood expands chiefly in a direction transverse to its fibres, very little in length; and hence wood, as well as lucullite, has been used for pendulum rods. The contraction of bodies upon cooling is sometimes not so great as their previous expansion; perhaps it is never so great. Some

bodies, it is certain, become permanently elongated by repeated heating; hence it is that the bars of old fire grates are often found distorted; and lead pipes conveying hot water have lengthened several inches in a few weeks, being thrown into curves. Glass without lead, and platinum, expand so nearly alike, that they can be soldered or otherwise united in machinery, and exposed to heat or cold without being caused to separate. Most substances expand more rapidly, some very violently, as in ascending they approach the melting or vaporizing point; and in descending, they contract correspondingly just before and after condensation or solidification. In sulphuric acid no such inequality is observed; in water, cast iron, bismuth, and antimony, the result at the melting point is the reverse; but in sulphur, phosphorus, mercury, &c., especially near the freezing point, the disturbance is very marked. Mercury contracts so violently just before and after freezing, at-39°, as to have led some observers into the error that it may freeze indifferently at-38°,

42°, or even -46°. The force with which bodies expand and contract is enormous, and in practical operations must always be allowed for. In middle latitudes, the variation between summer and winter temperatures may be stated at not less than 80°. In iron bars or beams abutting against or immovably fixed in walls, there is in consequence generated an immense pulling or pushing force; this, in a bar no more than 10 feet long, has been calculated at not less than 50 tons to the square inch, acting through the minute distance of the elongation or contraction. Hence, the ends of railway bars cannot be allowed to come into absolute contact; and the parts of buildings or bridges must be fitted to slide or play to a certain extent upon each other. Acting upon the arches of an iron bridge, the sun's heat during the middle of the day has caused an elevation of an inch or more; and a single one of the gigantic tubes of the Britannia tubular bridge has been lengthened from 1 to 3 inches during a hot day, warping toward the exposed side-the tubes, to allow of this play, being on rollers. The Bunker hill monument, a granite obelisk, 221 feet high, is during a bright day so expanded on the side toward the sun, that its top is swayed through an irregular ellipse, returning to perpendicularity only when all its sides are of equal temperature, as on cloudy days or in the night. The snapping of stoves while heating or cooling, of trees in extremely cold weather, and the breaking of thick glass or earthen vessels by very cold or hot liquids, are illustrations in different ways of the principle under consideration. The fracture of glass vessels in heating is avoided by making them very thin, by applying heat gradually, or by thinly coating them without with a conducting body, as copper. (For certain compensations of expansion, see CLOCKS AND WATCHES.) Liquids are more expansible than solids; but they differ widely among themselves. From 32° to 212°, pure water expands in volume about 1

part in 224; fixed oils, 1 in 12. Among solids, those of lowest points of fusion, and among liquids, those most volatile, are in the greatest degree expansible. Expansion in liquids occurs with enormous force, but its effect is usually in part compensated by enlargement of the containing vessel; and because of the latter change, the apparent is usually less than the absolute expansion of a liquid. II. Heat is communicated in various ways, through bodies or spaces. These may be summed up in: 1, conduction, occurring mainly in solids, and consisting in a process by which a substance passes the heat it may receive from particle to particle through its mass; 2, convection, or carrying, occurring in all fluids, in which heated particles rise by their superior levity, conveying their heat with them, to be given out to other parts; and, 3, radiation, occurring through space, and through certain bodies, solid or fluid, termed diathermanous, a phenomenon analogous to the transmission of the rays of light. The conductibility of different solid substances is at once proved and roughly compared by attaching at equal distances along rods of them small weights, as marbles, by wax, and then applying a high heat at one end of the rods. In a homogeneous rod of any metal, the bits of wax will be melted in regular succession; but in some of the rods this travelling of heat will be much more rapid than in others. It is by its rapid conduction of heat that a silver or copper vessel receiving a hot liquid is at once too warm to be held in the hands; while, from want of this property, a glass or earthen vessel can be grasped very near to the portion in contact with boiling water. So, the hand is burned by seizing a metallic rod red-hot at one end; but not by grasping a wooden rod even nearer to the higher heat of a burning part. Thus all bodies are divisible into the classes of good and poor conductors of heat, though among solids this property is possessed in very variable degree. Liquids and gases do not conduct heat in any degree appreciable by ordinary means; a thermometer inserted a little below the surface of water on which ether is burning, or a hot body laid, is scarcely affected; and a mass of air cannot be heated by contact of a hot body above it. But the differing conductibilities of solids, among which silver stands highest, the metals generally best, and all porous and heterogeneous substances, as wood, ashes, the hairy coverings of animals, the plumage of birds, and woven fabrics, owing to constant change of conducting medium from solid to air, very low, afford results of the highest importance in view of the comforts and the arts of mankind. Unfortunately, the tables of conducting power prepared by different observers are somewhat at variance. The latest results are those of Calvert and Johnson: silver, 100; gold, 98. 1 to 84; copper, rolled, 84.5; mercury, 67.7; aluminum, 66.5; zinc, forged, 64.1; iron, forged, 43.6; platinum, 37.9; cast iron, 35.9; lead, 28.7; bismuth, 6.1. They found that .01 of impurity often reduced the VOL. IX. -2

conducting power by to. In metals, moreover, the power of conducting has been found to diminish with rise of their temperature. A very useful table for practical men is that of Mr. Hutchinson, in which the substances are placed in the order of their resistance to the passage of heat, or of their relative warmness for building material, slate being taken as the unit, and lead being last, as the best conductor in the list: thus, plaster and sand, 18.70; plaster of Paris, 20.26; Roman cement, 20.88; lath and plaster, 25.55; fir, 27.61; oak, 33.66; asphalt, 45.19; Napoleon marble, 58.27; brick, 60.14; fire brick, 61.70; Lunelle marble, 75.41; various kinds of stone, 61 to 95; slate, 100; Yorkshire flag, 110.04; lead, 521.34. Bodies perfectly homogeneous, and crystals of the regular system (monometric), conduct heat with equal facility in all directions. Tyndall has found in wood 3 unequal axes of heat conduction, cohesion, and permeability to liquids, which coincide with each other, the greatest with the greatest and the least with the least, and with the axes of elasticity discovered by Savart. Of these, the greatest is that parallel to the fibres, the least that perpendicular to the fibres and parallel or tangential to the rings, and the mean that perpendicular to the fibres and also to the rings. Thus, in cutting staves for casks, it is well known that these must be cut across the rings, the direction tangential to the rings being that of least permeability. The heat-conducting power of wood bears no definite relation to its density. American birch, one of the lightest woods, conducts heat better than any other. Oak, very dense, conducts nearly as well; but iron-wood, density 1.426, is very low in conducting power. Air saturated with watery vapor has its conducting power increased nearly in a triple ratio-an explanation of the fact that damp air most rapidly robs the body of its heat, and hence feels more cold than dry air. As a partial illustration of the relative conducting powers of bodies in different states, it may be mentioned that a metal burns the hand at 120°, while contact with a liquid, without motion, may not scald at 150°; and an atmosphere of 300° has been endured for some minutes without injury. The crust of the earth is a poor conductor, first, because mainly composed of oxides, and secondly, because formed in porous and heterogeneous strata. Hence it is that, in temperate latitudes, freezing can never extend during the cold months to any great depth. Applications of poor conductors for the prevention of the escape of heat from bodies, or its entrance into those designed to be kept cold, are upon the same principle, and very numer ous. We term poor conductors warm, because they retain the heat of the body, not because they have heat to impart. Unrivalled in this respect are the down of the eider duck and the finer white furs of the polar regions. We thus find the philosophy of clothing in relation to temperature, and the order of value in view of warmth, viz.: furs, wool, cotton, silk, linen;

also, in part, of fire-proof safes; and wholly, of the lining of furnaces with fire brick, to keep in heat and intensify combustion; of the wrapping of ice in flannel or burying it in sawdust in summer; of the protecting influence of a coat of snow on vegetation; of the preservation of fire by burying it in ashes; of the construction of ice-houses with double walls, filled between with porous material, as sawdust or straw; and so on. As already intimated, when in liquids and gases heat is applied at a point, the heated parts by rapid expansion become lighter than those about them, rise, and are as constantly replaced by those more cold and dense; so that a circulation of currents of heated fluid upward and colder fluid downward is maintained, until, if that be possible, the whole mass is brought to a common temperature. Hence it is seen why heat should in such cases be applied below; and also why any thing rendering a liquid viscid, as starch, impedes boiling. Oceanic and aerial currents (winds), the warming of buildings by circulation of hot water, the draft of fires and furnaces, and ventilation, are illustrations. Radiation of heat occurs from the surfaces of all masses in a warmer state than those about them, however low their actual temperature. The most valuable observations we possess on this subject are due to Sir John Leslie (1804). He proved that the radiating power of a body, and hence its rate of cooling, are more influenced by the state of its surface than by the nature of its substance. Water which was 156 minutes in cooling through a certain range while in a bright tin globe, cooled to the same extent in 81 minutes when the globe was thinly coated with lampblack. The nature of surface being the same, the intensity of radiant heat is proportional to that of its source; inversely as the square of distance from the point of radiation; and greater as the direction of impingence on the receiving surface approaches the perpendicular. Indeed, it is now considered that all bodies, however cold, must radiate heat upon all sides of them, so that there is a perpetual interchange of heat rays, and the temperature of any body at any given time is that due to the difference between the amount of heat it imparts and the amount it receives within the same time; while, by necessity, the tendency of all bodies and spaces is thus to an equilibrium of temperature, which only fresh sources of heat excitation continually disturb. Thus is explained the apparent radiation of cold; a globe of ice in the focus of a concave mirror causes a fall of the mercury in a thermometer in the focus of an opposite one, because the substance of the thermometer yields more rays to the ice than it receives from it. Hence, too, the peculiar oppressiveness of those days on which the thermometer indicates a temperature nearly or quite that of the blood; the human body then receives heat nearly or quite as fast, so far as radiation is concerned, as it parts with it, and hence cannot so well rid itself of that surplus naturally produced by its own

processes. Thus, the degrees of heat with which we are familiar may be compared to the middle links in an endless chain, of neither extremity of which we can have any knowledge. Yet, reasoning from the relation of the elasticity of gases to their, temperature, it has been conjectured by Joule that an absolute zero of heat (absolute cold) must exist at 491° below the freezing point of water, i. e., -459° F. The greatest cold ever actually produced is -220°; and according to calculations of Fourier, the temperature of the interplanetary spaces is not lower than from -58° to -76°; so that an immense period must elapse before the effect of radiation of the earth's heat could become sensible. The heat rays falling upon any body are disposed of in one of 3 ways: they pass through it as a medium (see DIATHERMANOY); or they enter into its substance and are there arrested, usually producing rise of temperature, an effect known as absorption of the heat; or they are thrown off or reflected from its surface. Almost all surfaces reflect a portion of the heat falling upon them, usually more than 10, never more than 97 per cent. The reflecting power is increased by polish, and in some bodies, as glass, by increase of the angle of incidence. Radiation, on the other hand, is favored by roughness and by darkness of color; and leaving out the amount of rays that in some media are transmitted, this singular relation holds between the 3 processes now treated of, namely, that the power of absorbing incident heat is always and for every substance exactly equal to the radiating power of the same; and that the percentage of incident heat not absorbed by a substance equals that which will be reflected from its surface. The table originally obtained by Leslie has been corrected by the later experiments of Provostaye and Desains, according to which, of a given intensity of heat falling on the bodies to be named, the following percentage will be absorbed: by a smoke-blackened surface, or by carbonate of lead, 100; writing paper, 98; glass, 90; gum lac, 72; silver foil on glass, 27; cast iron, 25; mercury, 23; steel, 17; tin, 14; metallic mirrors, 14; brass, 7; copper, 7; gold plating, 5; silver, 3. All the metallic surfaces were partially or highly polished. The emission or radiation is always in the same proportion as that given; and the proportion reflected is found by subtracting the numbers above from 100. In respect of color, black absorbs and radiates most perfectly; then, in order, violet, indigo, blue, green, red, yellow, white. The applications of these principles are numerous and important. Liquids are kept hot longest in light-colored vessels, as those of silver, and polished; they cool most rapidly in those that are black and roughened. For boiling quickly, in culinary arrangements, the latter surfaces are preferable; and the deposit of soot upon the bottoms of kettles further improves them for this purpose. Stoves and pipes designed to keep their heat, or to convey it to distant rooms, there to be given out, require

a low radiating capacity; in the rooms in which the heat is to be dispensed, a higher one, so that here they are impaired by polishing. For persons of feeble heat-regulating capacity, black clothing is the most unfavorable for all seasons; since it absorbs largely in a warm atmosphere, and radiates rapidly in a cold one. III. The heat indicated by the thermometer, that is, the sensible heat in any mass, is not a true measure of the actual amount of heat which the body may contain, and be capable of restoring. Suppose equal measures of water at 108° and at 32° mixed rapidly; the temperature of the whole will then be an average, or 70°the 38° lost by the one measure being capable of heating the other through exactly the same range. Now suppose equal measures of mercury at 130° and water at 70° mixed as before; the thermometer will now indicate in the mixture only 90°. The 40° which the mercury loses is capable of raising a like weight of water through only about 20°; and when, instead of equal measures, equal weights of the two are taken, it is found that a loss of about 33° in the mercury only suffices to raise the temperature of the water 1°. These results we express by saying that the capacity for heat of different bodies is different; that, volume for volume, water requires about twice as much actual heat to raise it through 1° of sensible temperature as does mercury, while, weight for weight, its capacity is 33 times as great; or that the heat which shows as 33° in mercury, shows as only 1° in water. The relative capacity for heat of any given substance is termed its specific heat; and since there must evidently be some standard for the comparison, water is assumed as such standard, its specific heat being called 1 or 1,000; and the thermal unit, or unit of actual heat, is then so much heat as will raise the sensible temperature of 1 lb. of pure water from 32° to 33°, or through 1o. The methods of ascertaining the specific heat of bodies are various; as, by mixture; by finding the quantities of ice different bodies will melt in cooling through so many degrees; by finding the different rates of cooling under like conditions, &c. (see CALORIMETER); and though the problem is beset by practical difficulties, yet many satisfactory and instructive results have been obtained. The results are generally given for equal weights; and the following are the specific heats of the bodies named: water, 1,000; ice, 513; alcohol, 615; ether, 503; oil of turpentine, 414; charcoal, 241; sulphur, 203; glass, 198; iron, 114; zinc, 95.5; copper, 95.15; silver, 57; tin, 56; gold, 32.44; lead, 31.4; platinum, 32; mercury, 33; bromine, liquid, 107; among gases: air, 266; oxygen, 195; hydrogen, 293; steam, 847; carbonic acid, 221; olefiant gas, 420. That is to say, oil of turpentine boils with less than half the heat actually received that would be required by water, provided their boiling points were the same; or more correctly, to heat the oil through 10° requires only about the actual consumption of heat required by water in heat

ing through the same range; again, the same heat which raises a pound of water 1°, will raise a pound of ice about 2°, a pound of silver about 20°, and so on. Ice and steam have each a less capacity for heat than has water; so that the specific heat changes with change of state. Water has a capacity for heat exceeding that of any other known substance; and as a consequence, the development of a certain sensible temperature in it requires a greater consumption of heat, and hence of fuel, than any other-a conclusion of some moment, when we reflect how vast are the quantities of this liquid that, in cookery and in the arts, must continually be heated or brought to the boiling state. Again, in cooling through a given number of degrees, the same weight of water gives out heat which, entering the air and solids, is equivalent to and produces in them a considerably greater sensible heat than that lost by the water. Thus, in one way, the oceans, lakes, and rivers of the earth become a vast system for equalizing the temperature of the seasons; these bodies of water causing in effect a disappearance of large amounts of the solar heat of summer, and giving this out again to temper the colder air of winter. Thus all latitudes are made more inhabitable; solar heat, drunk in under the equator, is by ocean currents given out all the way to the poles; and the vicinity of large bodies of water, other things being equal, secures a more equal climate, especially a milder winter. Other laws are: 1, that the capacity for heat increases by rarefaction-a result especially manifest in gases, furnishing one reason for the increasing cold of increased elevations above the sea level, as well as for the coldness of the expanding jet of steam escaping under pressure from an orifice in a steam boiler, and a principle now turned to account in the mechanical manufacture of ice; and 2, that this capacity for heat is lessened by compression, a fact long illustrated by the firing of phosphorus by condensation of air in the fire syringe. IV. Heat disappears during changes of bodies from the solid to the liquid, and from the latter to the aeriform state; and it reappears from the occurrence of the reverse changes. If pounded ice be taken below freezing point, a thermometer in it marking the true temperature, say 26°, and if heat be then gradually applied, the mercury rises steadily to 32°; at this point the ice begins to melt, and the mercury remains stationary, until the whole is liquefied. If more heat be added, the mercury again rises until boiling commences, then marking 212°; but it now again remains stationary until the whole is vaporized; when, if the vapor be confined, its temperature may be increased, and the mercury again rises accordingly. It is easily proved that, at both these stationary points of the mercury, heat has disappeared in the substance changing its state. To illustrate in the former case only: if 1 lb. of water at 32° be rapidly mingled with 1 lb. at 174.65°, the resulting temperature will be the average, 103.32°; but if 1 lb. of finely crushed

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