ÆäÀÌÁö À̹ÌÁö
PDF
ePub

ice at exactly 32° be thus mixed with 1 lb. of water at 174.65°, the ice will rapidly and completely melt, and the liquid mass will, if tested at once, show a temperature of only 32°. The 142.65° of heat lost by 1 lb. of water has disappeared in the act of converting into water 1 lb. of ice. The heat thus disappearing is called latent heat of fusion, which, it would thus appear, for water is 143.65°. The heat which becomes latent in evaporating a pound of water is various ly estimated, but is probably about that required to raise, by the re-condensation of the steam thus obtained, 10 lbs. of water through 96.5°, equivalent to a latent heat of 965°. According to the recent views of the relation of heat to other forms of force, the latent heat of fusion is consumed in the work of modifying the cohesion of the particles of a body, so that they lose their character of fixity; that of evaporation, in generating a repulsive energy which results in the gaseous condition. (For many points in connection with changes of state of bodies, see BOILING POINT, EVAPORATION, and FUSIBILITY.) It now appears why changes in the state of bodies must ever be gradual. Time is required, in the case of fusion or evaporation, to furnish the large amounts of heat needful to effect the conversion of form of the body. But when a vapor undergoes condensation, or a liquid solidifies, the same amounts of heat before taken up must reappear in the sensible form, thus for the time raising the temperature of the mass by their liberation; and the vapor cannot liquefy, nor the liquid congeal, until time has been allowed for this extricated heat to be distributed to other bodies by conduction or radiation. The downward changes must therefore also be gradual; and either set must be expedited or retarded by the conditions surrounding the mass undergoing the change. Large bodies of snow and ice are always very long in melting, owing to their reflecting a large share of the radiant heat striking them, and to the imperfect conducting power of the air. Any mass will melt or vaporize rapidly in proportion as it is environed with highly absorbing and conducting materials; and a mass will be allowed to condense into liquid or to congeal rapidly under precisely the same conditions. Again, it is evident that the process of melting or thawing is secondarily, that is, for all bodies in its neighborhood, a process of cooling or freezing; since in order to effect its fluidification the melting body must rob those around it of heat. Hence the peculiar chilling influence of a day when thawing of snow is taking place, with the temperature only a degree or a few degrees above 32°; and the cold experienced in the feet by walking through melting snow. The cooling down of the emitted heat of a stove or fire by the heating of water, due to loss in specific heat, and by the boiling of water, due to latent heat, are effects very sensibly felt and well known. On the other hand, the congelation or condensation from vapor of a mass is secondarily a thawing or warming process; the heat given out by

pro

the condensing or congealing mass warms the bodies around it. It is thus that the air is tempered during the accession of freezing, and that, as an almost invariable rule, the thermometer rises at the beginning of or during a snow storm, at which time heat is escaping from great quantities of moisture undergoing congelation into snow. It is thus also that a vessel of water in a cellar in which the freezing point is reached or barely passed, owing to the more ready freezing of water in mass than when enclosed in the tissues of vegetable or animal substances, serves, so long as its congelation can go on rapidly, to protect the latter; and that steam condensing in an appropriate set of tubes at a distance from a boiler serves to give out a large amount of heat, and a heat that is free from almost all the inconveniences and deleterious effects possible from other warming processes. Further illustrations of cold produced by evaporation are found in the use of alcohol, ether, or other volatile liquids, or even water, over inflamed parts of the body, for the purpose of lowering their temperature, a result which is thus effectually secured; in the cooling effect on the air of evaporation from the surface of the earth after rain; in the ill effects of wet or damp clothing, especially in a wind, which occasions rapid abstraction of the heat of the body; and in the various familiar processes of freezing by evaporation. On this principle, also, the most intense cold known has been duced. Into a powerful cylinder, in Thilorier's apparatus, carbonic acid is condensed under a pressure of 50 or more atmospheres, and its sensible heat abstracted by a surrounding freezing mixture. The gas is thus liquefied; and if a jet be now opened, and the liquid escaping be directed into a strong metallic flask, its immense expansive force causes considerable volumes of it to burst again into the gaseous form, necessarily abstracting enormous quantities of heat from the remainder, and freezing it into a flocculent snow, temperature -94°, which is then quite permanent in the atmosphere, in an ordinarily poorly conducting vessel. With a mixture of this solid and ether, and by aid of their rapid evaporation in the vacuum of an air pump, Faraday obtained a cold of —166°; while, by means of a similar bath of protoxide of nitrogen and bisulphuret of carbon, previously liquefied by cold and pressure, Natterer obtained a cold of-220°. The latent heat of fusion of several familiar substances is as follows: water, 142.65°; nitrate of silver, 113.34°; zinc, 50.63°; silver, 37.92°; tin, 25.65°; sulphur, 16.85°; lead, 9.65°; phosphorus, 9°; mercury, 5.11°; beeswax, yellow, 78°. The latent heat of certain vapors is as follows: vapor of water, 965°; carburetted nitrogen, 108°; absolute alcohol, 374.4°; ether, sulphuric, 163.8°; oil of turpentine, 124°; bisulphuret of carbon, 152°. (See also DISTILLATION.) V. MECHANICAL THEORY OF HEAT; CONVERTIBILITY OF FORCES; CONSERVATION OF FORCE. Certain questions respecting the relations and final expression of

forces have lately grown into great importance. Galileo, Bacon, and others of their time held that heat was a motion of the particles of bodies. This idea, in opposition to the notion of phlogiston, or heat-substance, which was in his day gaining ground, Count Rumford revived in 1778. He found that the friction of a steel borer, used in boring cannon, and turned 32 times per minute under a pressure of 10,000 lbs. to the square inch, evolved heat enough in 2 hours to raise to boiling temperature 183 lbs. of water; and he thence calculated a rise of 1° F. in 1 lb. of water as the result of a mechanical force estimated at 1,034 lbs. Davy in 1799 melted pieces of ice in vacuo by their mutual friction. The production of fire by the rubbing together of dry sticks, Black considered to be an effect of compression, evolving their latent heat; but the fact that, without sensible increase of compression or, wear by friction, indefinite quantities of heat may thus be obtained, is at once fatal to this explanation. Carnot in 1824 took the next step, in establishing the suggestive law that the greatest possible work of a heat engine is related to the amount of change of temperature undergone, during the action of such engine, by the enclosed elastic body. In 1842 J. R. Mayer of Heilbronn, and J. P. Joule of Manchester, England, independently of each other, subjected the question to careful experiment, by observing the heat evolved in agitated liquids, and by the compression of gases and friction of solids. The corrected results attained by Joule give a rise of 1° F. in 1 lb. of water as the equivalent of mechanical force exerted, sufficient to elevate against gravity to the distance of 1 foot a weight of 772 lbs. That is, this quantity of force, expressed as 772 foot pounds, is to be regarded as the mechanical equivalent of 1° of temperature. It has accordingly been termed the thermodynamic unit, and is also now well known as Joule's equivalent. Heat and motive power, then, are mutually convertible; heat requires for its production, and produces by its disappearance, mechanical work to the amount already named. This view, the dynamical or mechanical theory of heat, in the hands of its author, of Prof. W. Thomson, Waterston, Rankine, Regnault, and others, has already led to many important deductions, theoretical and practical. Some of these will here be named; for others, see STEAM ENGINE; also the volumes of the "Philosophical Magazine" from the date of Joule's discovery, the "Philosophical Transactions," 1854, and Comptes rendus, 1853. The development of heat by compression of gases, by chemical union, and by transmission of the electric current along insufficient conductors, was very naturally considered in the next place; and in these conversions, also, an equivalency between the disappearing and the new-appearing forces has been gradually made out. The instances continually extend and multiply. For example, heat is found to appear when light is extinguished in bodies. Galvani and his followers

had before discovered the development of electricity as a consequence of chemical decompositions; Oersted, in 1820, the production of magnetism in the vicinity of electrical currents; and Faraday, in 1831, the converse phenomenon of development of electricity by magnetism. Faraday proved, in particular, the exact equivalency between the quantity of electricity generated in any cell of a battery, and the amount of chemical decomposition in which it had its origin; and that, in an outside or decomposing cell, compounds were electrolyzed in exact chemical equivalents with the change going on in any single one of the generating cells. Almost all the other forms of force may directly originate motion; and the latter may be made to evolve many of them in return. Within a single galvanic circuit a few feet in length, by suitably arranging the conditions at as many breaks in the conducting wire, almost all the known forms of force, beginning with some disappearing affinity and a generated current, can be successively brought into manifestation; even the growth of vegetables, or nutrition, and muscular or nervous actions of animal bodies, it is now ascertained, may form links in this wonderful chain of the relations of forces. The cases of this nature known at that time were in 1845 generalized by Prof. Grove, in relation to physical forces, in the hypothesis that these forces are mutually convertible and equivalent; or, that any one of them is capable, under suitable conditions, i. e., in connection with its proper material substratum, of giving place to the manifestation, in an equivalent amount, of any other. This relation, expressed by Grove as the "correlation of forces," Helmholtz has named the "conservation," and Smee the "monogenesis of force." In truth, it is hard to understand correlation as anything less than convertibility, or convertibility as aught else than unity-at least, a potential unity, giving certain invariable manifestations or forms of power under any given set of material conditions. In 1848 Prof. W. B. Carpenter first distinctly proposed the extension of this theory to embrace a convertibility of electricity and the nerve force in animals; and in 1850, to the idea of a like inter-relation of the vital with the physical forces generally. The supposed law may, in its present form, be thus broadly stated: all forms of force or energy are convertible; and an apparent disappearance of any one is necessarily compensated by the appearance anew of an equivalent amount of one or more other forms. Some recent confirmations of the law may be named. Foucault caused a soft iron wheel between the poles of an electro-magnet to revolve; as soon as, by passing the galvanic current, magnetism was induced, the wheel was seized as if by an invisible hand, and a considerably increased power was required to keep up its velocity, while heat was rapidly developed in the wheel, and by suitable communication was made to boil water. Prof. W. Thomson finds that the freezing or melting point of bodies

22

23

HEAT

is lowered by pressure, that of water by 0.232°
for a pressure equal to 16.8 atmospheres. Fara-
day has shown that the disrupted affinity of a
single drop of water evolves a quantity of elec-
tricity sufficient for a flash of lightning; the heat
and terrific mechanical effects to which the lat-
ter gives rise are but too well known. Bunsen
and Roscoe (1857) have experimentally proved
that the chemical rays of the sunbeam are ex-
tinguished (absorbed) in doing chemical work;
and Niepce de St. Victor claims to have pre-
served, in white card paper saturated with tar-
taric acid or salts of uranium, exposed for some
minutes to the sun and then sealed up, this ac-
tinic energy of solar light, and to have obtained
its effect after intervals of various length, some
as great as 6 months. (See also ANIMAL ELEC-
TRICITY, and FLUORESCENCE.) Of special interest,
as touching this subject, are the recent experi-
ments showing that time is an essential element
in all phenomena involving movement or change;
not less in sensation, perception, volition, and
muscular contraction, than in purely physical
actions. Thus, the average speed of the nerv-
ous change conveying the matériel of sensations
or volitions in man, is by Helmholtz stated at
180 feet per second; slower in some of the low-
er animals; faster in some of the temperaments,
in man, than in others, and also during eleva-
tion of the bodily temperature; but such that,
as a rule, in the human species, the whole in-
terval between the reception of an impression
from without, and the earliest possible conse-
quent muscular movement, is about 0.2 of a
second. Thus the distinctions of quick and
slow thinking seem in a fair way of a physical
explanation; a conclusion further established
by the determinations recently made, for astro-
nomical purposes, of the "personal equation"
of observers, or the organic differences in the
celerity of sight and registration by hand dis-
tinguishing different individuals. According to
the views now stated, the food of any animal
body, that of man included, is merely a vehicle
for the introduction within the grasp of the liv-
ing organism of physical and chemical energies
inwrought into and thus stored up in such pab-
ulum during the process of its formation, and
by it obtained, of course, from the forces of in-
animate nature; and the necessity and value of
food is explained by its capability, at some time
subsequent to its ingestion, of suffering a disap-
pearance of the affinities that had constituted
it an organizable or highly organic substance,
with an equivalent evolution of the forces pe-
culiar to the animal organization. Thus food
serves as the physical link between the mind
and the world of matter on which the former
is enabled to act, and furnishes, not the princi-
ple of intelligence, but that sum of energies
which we term vital forces, and whose mani-
festation is the physical fact of life. Thus man
moves within an ocean of forces, which he daily
draws into his own vitality, and again expends
in manifestations necessitated by the continu-
ance of life or dictated by his intelligence. The

work of generating de novo organic compounds,
including foods, or the vehicles for introducing
physical forces into the living animal body, is
well known to occur in the green leaves of
growing plants; so that at this very point the
And
temporary transfer of energies from the physi-
cal to the vital realm must take place.
accordingly it is not only true that at this point
simply chemical affinities, as those constituting
the compounds carbonic acid, water, and am-
monia, give way, to be replaced by organic
chemical affinities, as those constituting starch,
glucose, and albumen, but there is also a disap-
pearance of certain energies of the sunbeam im-
pinging on the leaves-for instance, most of its
actinic and thermal, and a portion of its lumi-
nous rays. The fact that green paper or a green
wall, of color identical with that of the leaf,
produces the same obvious effect in extinguish-
ing certain portions of the sunbeam, does not
invalidate this view; for it is only necessary to
assume that in the paper or wall, where no or-
ganic compounds are forming, some other equiv-
alent manifestation of the disappearing forces

probably heat, fading (a chemical change), or
electricity-must occur; and the objector must
show, therefore, that between the results in the
plant and the paper there is no such difference.
If not through the conversion here supposed in
the leaves, it is difficult to understand from
what source is derived that vast accumulation
of energies which are as complete in every part
of a tree or of a forest as they were originally
in the single acorn from which the latter may
have sprung. A question naturally growing
out of the view of forces now explained, is that
as to what single form of energy, beneath and
subject to Creative Intelligence, can more than
any other claim to be the fountain or source of
the various forms. Loose, analogical reasoning
has repeatedly named electricity as the parent
force; and pinus, conceiving matter to be
self-repellent, and gravitation an electrical phe-
nomenon-a sort of residual excess of the at-
traction of matter and electricity combined,
over the repulsion due to the former alone-
comes also to this conclusion. But, in view of
the important part played by gravitation as a
cosmical agency, especially if we accept La-
place's nebular hypothesis, it would seem more
reasonable, with Smee and some others, to as-
sign to gravitation this position. Faraday,
indeed, has recently questioned whether the va-
riation of gravitative force in the ratio of the
inverse square of distance be compatible with
the law of conservation of force; whether, if
a body were at a distance 10 from the earth,
and then suddenly brought within a distance 1,
there would not, according to present views of
gravitation, occur an actual creation of force,
in the ratio of 100 to 1, and a corresponding
annihilation on its being as suddenly returned.
To this query, Sir W. J. M. Rankine (April,
1859) replies that the quantity conserved dur-
ing all the mutual actions of a system of bodies,
is always equivalent to a product of two factors;

[ocr errors]

one, a tendency, or impulsion; the other, the distance through which this impulsion is capable of acting, within the time considered. Hence, since in the case of the gravitative attraction of two bodies, which is a mere pressure or phenomenon of statics, the impulsion only is regarded, the quantity of the impulsion is one to which no law of conservation can apply; and consequently the case supposed furnishes no contradiction of the principle at issue. He further remarks that the idea of force, as popularly understood, is mechanically incomplete, in that it may stand for an attraction only; while the conception of energy, properly defined, includes both the factors, and denotes a quantity rigidly and always conserved or replaced by equivalents, in the physical changes thus far studied; so that a better expression for the law would be that of the "conservation of energy." Faraday seems to have overlooked the fact that, to move the supposed body from the distance 10 to the distance 1, would require the expending on its inertia of a power that, thrown into the lighter scale, must neutralize the apparent disparity between the attractions at the two unlike distances; so that his difficulty arises from the supposing of a mechanically impossible case. The most important general consequence of the theory now detailed, is that expressed in the idea of conservation. According to this, every form of energy, upon ceasing to be manifestly active, simply passes into some other form of energy, of which the manifestation may or may not be equally obvious. Hence, no amount or fraction of force can ever be lost or annihilated; and of course, in a universal system, once complete, no quantity of new force can by any means be created. Physicists have long rested in the conclusion that no atom of matter can be added or lost, except through the direct exercise of creative power; and it is strange that the like obvious conclusion in respect to force or energy comes so late in the history of science. The sum total of energies in a universal system is a constant through infinite time; but particular amounts of energy, according to conditions, may pass from one form into another: speedily, as in the conversion of current electricity into heat or mechanical effect; or slowly, and to appearance irrecoverably; or such energy may appear to pass from within the bounds of one partial system to those of another, as in the case of the earth affected by light and heat of the most remote stars. Among practical results of the theory, are such as the calculation of the mechanical energy extricable from a pound of coal; this is, in the different kinds of coal, according to purity, from 5,000,000 to 10,000,000 foot pounds. A horse or a man can turn to account from to of the mechanical equivalent of the entire daily oxidation of food and tissue; the steam engine, by a much less costly fuel, can be made to yield a larger percentage of work. Prof. W. Thomson calculates the mechanical energy of a cubic mile of sun

light; and he also finds that the force of the solar rays falling annually on a square foot of land, in lat. 50°, is equal to 530,000,000 foot pounds, about .01 of which he supposes to be consumed in vegetable growth and secretion. He finds that the heat alone hourly given out by each square yard of the solar surface is equivalent to 63,000 horse power, and would require the hourly combustion of 13,500 lbs. of coal. This estimate, however, which is on the supposition that the evolved energy is obtained from chemical action, requires the consumption during each year of 3,000 times the quantity of matter that, if supposed to act by friction and concussion, as in a meteoric shower, must produce the same result. Under the latter supposition, the angular diameter of the sun, as seen from the earth, would be 100,000 years in increasing by a single second of a degree. To such an action, therefore, the materials for which may in part, perhaps, be found in the zodiacal light, Waterston and Thomson ascribe the origin of the sun's energy. By either theory the sun is losing heat, and the materials for its production are being consumed. Motion and heat seem thus to be gradually passing into other forms of force; and if so, the universe is tending to a state of eternal quiescence. It is doubtful whether these deductions will be found accordant with facts known and yet to be discovered. If true, the progress of science may nevertheless enable man for a time to reconvert, upon his own planet, certain forms of force on a large scale into motion and heat, and thus to delay the threatened catastrophe. "It is impossible," said Prof. Owen before the British association in 1858, "to foresee the extent to which chemistry may ultimately, in the production of things needful, supersede the present vital agencies of nature, and enable us to obtain in a small manufactory, and in a few days, effects which can be realized from present natural agencies only when they are exerted upon vast areas of land, and through considerable periods of time."-The sources of heat may, for convenience, be divided into: 1, mechanical, as in the case of arrested motions, in blows, friction, &c., and the evolution of specific and latent heat by compression; 2, physical, as that obtained by direct and simple conversion of electricity, or other physical forces; 3, chem-, ical, as that due to oxidation and like changes (see COMBUSTION, and FUEL); 4, physiological, or that developed during the vital processes of vegetable and animal bodies-unless, indeed, this division is to be included under the preceding; 5, cosmical, as that due to the radiation of suns, by far the larger part of that by which we are affected coming from the centre of our own system, and also that probably radiating from cooling planetary bodies. Doubtless, these distinctions do not hold in nature; heat from all sources is essentially identical, and it may be regarded as interchangeable with all other forms of force. The term caloric, however, if still retained in science, can no longer, as formerly,

be employed to signify a sort of substance of heat. It may appropriately be used to name that form of energy, considered as a cause, of which warmth and the other known results are the sensible effects. In reference to the question as to whether the process of combustion is retarded by the action of sunlight, it may be remarked that the recent experiments of Prof. Le Conte, and his conclusion that sunlight exercises no retarding influence on the burning of bodies, are probably vitiated by his employment of a glass lens to condense the solar beam. Prof. Stokes found that glass cuts off a large portion of the chemical rays; so that when in experimenting he desired to retain these in the transmitted beam, he was obliged to have recourse to prisms and lenses of quartz. Now, concentration of mere solar heat, as of any heat, must have aided the combustion; it is yet a question whether the luminous rays, per se, have any effect on the burning process; but the probability having been that the chemical rays, and these only, would by their deoxidizing agency interfere with the combustion, Prof. Le Conte should have avoided the use of a medium that, by excluding the greater part of these rays, must leave the question of their influence on burning doubtful as before, or rather incline us still to accept the popular opinion on the subject.

HEATH (erica, Linn.), the common name of one of the most extensive genera of plants, remarkable for the beauty and variety of its flowers. It is unknown to North America, although there are many flowering plants em braced in the natural order (ericea) to which it belongs, which are widely distributed throughout the new world. In Great Britain the heath or heather covers vast tracts of wild land; on the sides of mountains in Scotland and Ireland it forms beds, extending for many miles together, of trailing stems which are 3 or 4 feet in length. In those portions of the country the plant enters into the manufacture of a variety of rude domestic articles. A double-flowered variety of extreme beauty has been known in British gardens. Some species are peculiar to the north of Europe, and a few to the Mediterranean coasts. In Germany and on the mountains of middle Europe generally the flesh-colored heath (E. carnea) is one of the few plants which are the early harbingers of spring. A great many species of the heath are favorite plants for greenhouse culture. These claim the Cape of Good Hope as their native country. In a wild condition there, their external forms and habits are so unprepossessing that they are scarcely noticed among the wild flowering plants; but under cultivation and training they acquire great perfection, one of their principal charms consisting in the production of flowers during the whole year. A very peculiar treatinent, however, alone insures success; and owing to general neglect or to ignorance of this fact, the heaths have often fallen into disrepute. The best practical treatise on their artificial growth is McNab's

"Propagation, Cultivation, and General Treatment of Cape Heaths” (Edinburgh, 1832). The soil which Mr. McNab recommends is a black peat, taken from a dry heath or common which is never overflowed with water. With this a certain proportion of sand is used, when not naturally present. Small fragments of freestone are mingled with the compost, it being found that the smaller and more delicate fibres of the roots seek such substances for moisture. On every new change into larger pots, when it becomes necessary, the original ball of earth is raised a little above the rim of the pot, sufficient space being allowed between it and the pot to allow proper watering. Considerable drainage is recommended, and also plunging the pots in tan or some substance which will keep them cool during the heat of summer, and will secure an equal moisture to those portions of the roots which seek the sides. By such a treatment the heaths can be made very remarkable. Some specimens, according to Loudon, have been known to grow 7 or 8 feet high, and to range from 10 to 26 feet in circumference. All the species do well, because, being natives of the higher regions, they can bear considerable cold. A more uniform climate than ours seems most favorable, however, and in England their cultivation has been carried to a greater degree of perfection. The flowers of the heaths have a 4-leaved calyx, a 4-toothed corolla, and their numerous seeds are contained in a dry, 4 or 8celled capsule, which opens into valves with the partitions projecting from its middle. Their foliage consists generally of very narrow linear leaves arranged in whorls, and are so similar as often to convey the impression that they belong to one species, an idea soon dissipated on the appearance of their wonderfully diversified flowers. It is impossible for words to do justice to the delicacy, elegance, and loveliness of their tissue, colors, and forms. The total number of kinds, according to Don, is from 300 to 400.

HEATH, WILLIAM, a major-general in the American revolution, born in Roxbury, Mass., March 7, 1737, died there, Jan. 24, 1814. When the Massachusetts congress in 1774 voted to enroll 12,000 minute men, volunteers from among the militia, Heath, then a farmer in Roxbury, was commissioned as one of the generals. On June 22, 1775, he received the appointment of brigadier in the continental army, and in Aug. 1776, was created major-general. When the troops moved to New York, Heath was stationed in the highlands near King's Bridge, with orders to throw up fortifications for the defence of that important pass. In 1777 he was transferred to Boston, and the prisoners of Saratoga were intrusted to him. In June, 1779, he was again in New York, at the highlands, with 4 regiments, and he was stationed near the Hudson till the close of the war. He was the last surviving major-general of the war, and his "Memoirs of Maj. Gen. Heath, containing Anecdotes, Details of Skirmishes, Battles, &c., during the American War" (1798), shows him

« ÀÌÀü°è¼Ó »