PERIHELION, in astronomy, that point of the orbit of any planet at which it is nearest to the sun. PERIOSTEUM, the membrane covering the bones. PETRIFACTION, signifying conversion into stone, and applied to animal and vegetable bodies so changed by long exposure to impregnation from earthy substances. PHANTASMAGORIA, an optical exhibition resembling that of the magic lantern. PHARMACEUTICs, a title for the science of pharmacy, which takes cognisance of the preparation of drugs for medical purposes. PNEUMONIA, inflammation of the lungs. POLARITY, the property of pointing to the poles; a word used in reference to mineral bodies when they attract one pole of the magnet and repel another. POLARIZATION OF LIGHT, a changed state of light, in which it exhibits the property of polarity, wnen acted on by certain mediums. POLE, in magnetic science, the two points of a magnet which correspond and point the poles of the world, north and south. POLEMICS, controversial writings on theology. PHARMACOPEIA, a dispensatory, or work which directs varied description. the preparation of drugs. PHARYNX, the muscular cavity at the back of the mouth, leading into the gullet. PHENOMENON, a word signifying" an appearance," and familiarly used in an extended sense, being applied to any remarkable thing discoverable by observation or experiment, whether common or uncommon in occurrence. PHILANTHROPY, literally, "love of mankind." PHILOLOGY, that department of literature which involves the consideration of words and languages, their origin, structure, and significations. PHILOSOPHY, the "love of wisdom or knowledge." PHLEBOTOMY, bleeding or opening a vein. PHONICS, a title for the science of sounds. PHOSPHORESCENCE, a luminousness emitted by certain bodies, animal and vegetable, and unaccompanied by heat. The light of the glow-worm exemplifies the meaning of the term, which is derived from PHOSPHORUS, a simple body, yellowish and solid like wax, and which emits light at common temperatures without much heat. It enters into bones, and forms a variety of compounds in nature. PHOTOGENIC, a term invented for the new mode of drawing by means of light," where, upon a surface rendered peculiarly sensitive by certain preparations, the rays of light impress perfect images of external objects. PHRENOLOGY, a science which holds the intellectual and moral character of men to be determined and recognisable by the magnitude and figure of the skull. PHYLACTERY, a spell or charm of any kind. PHYSICS, a science of vast extent, which explains the doctrine of natural bodies, and all the phenomena connected with them. POLYTHEISM, the belief in many gods. PORPHYRY, a compound rock, granular and crystalline, and susceptible of a fine polish. PORTE (the Sublime or Ottoman), a title for the Turkish sovereignty, derived from the famous gate (porta) of the sultan's palace at Constantinople. POSITIVE, a term used in connection with electricity, to indicate its presence in bodies in a quantity greater than natural. Negative electricity is a phrase expressive of the opposite condition. POSTULATE, a point" demanded" as fundamental in any demonstration. PRENOMEN, a name prefixed to the family name. PRATIQUE, a license to crews to trade after performing quarantine or proving health. PRAIRIE, the grassy plains in North America PRECIPITATE, the chemical term for matters precipitated from solutions by change of affinity. PRIME VIE, the alimentary canal in animals. PRISM, in optics, a triangular glass body used for separating rays of light into their primitive colours. PROGNOSIS, the art of foretelling the issue of maladies from their symptoms. PROLOGUE, an address prefixed to dramatic compositions. PROPAGANDISM, a term commonly given to the system of propagating political doctrines, and originally derived from the court of the propaganda at Rome for disseminating the papal faith. The secret revolutionary societies of France also took the name of propaganda. PROPOLIS, a resin gathered from trees, and used in the architecture of the bee tribe. PROSCENIUM, the front of the stage in the theatres of PHYSIOGNOMY, the study of men's characters, as indi- Greece and Rome. cated by the external features of the face. PHYSIOLOGY, a term confined to that branch of physics which treats of the functions and properties of living bodies, animal and vegetable. PIA-MATER, the inner tunic which dips into and lines all the folds of the brain. PLAGIARY, a literary person who pilfers from the works of others. PLANISPHERE, a sphere laid down on a plain surface, as in the case of maps of the world and the heavens. PLASTIC, a word applied to substances, such as clay, capable of being moulded into any desired shape, as well as to the art of so moulding them. PROSODY, the part of grammar which treats of the quantities and accents of words, and of the rules of versification. PROTOCOL, the first draught of a diplomatic agreement or treaty. PSYCHOLOGY, the doctrine of the nature and properties of the soul. PULMONARY, of or pertaining to the lungs. PUNDIT, a learned Brahmin, or one versed in Hindoo learning. PUPA, the chrysalis state of the insect, or that inter. mediate between the worm and the insect. PYLORUS, the orifice by which the stomach communicates with the intestines. PYROLIGNEOUS, an epithet for acetic acid. or vinegar PLEONASM, the use of a redundancy of words to express an idea, as "I heard it with my own ears." PLETHORA, a condition of the body in which the ves- produced from wood. sels are surcharged with blood. PLECRISY, inflammation of the pleura or membranous covering of the lungs. PNEUMATICS, the science which treats of the mechanical properties of air and other compressible aëriform fluids. PYROTECHNY, the art of arranging fire-works. QUADRATURE (of the CIRCLE), a problem in geometry long undetermined, of which the object is to find a right-lined figure equal to the area of any given circle. QUARANTINE, denoting the period of restraint, formerly forty (quaranta) days, to which ships' crews are subjected when infection is presumed to be among them. QUARTAN, a fever or ague, of which the paroxysm recurs every fourth day. QUARTO, a term for that size of books formed by dividing a sheet into four leaves. QUARTZ, a species of stone, often found in pure white crystalline masses, and sometimes endowed with various tints by other minerals, as in the case of amethyst, which is a purple quartz. QUININE, a bitter alkaline body, extracted from Peruvian bark, and much used as a tonic in the form of sulphate. QUOTIDIAN, an intermittent fever, of which the fit occurs once every day. RADIAL and RADIATED, adjectives applied to bodies of a figure resembling in whole or in part a cart-wheel, of which each spoke is a radius or ray, such being the name given to lines passing from the centre to the circumference of every circle. RADIX, a root. RAMADAN, a solemn fast kept by the Mohammedans during the ninth month of the Arabic year, and lasting, each day, from dawn till sunset. RATIO, the proportion of one thing in regard to any other thing. REBUS, properly a species of riddle, in which some name or word is represented by figures or pictures. RECIPE, a receipt or prescription. RECITATIVE, a kind of musical composition, in which the accentuations of common speech are imitated. RECTANGLE, a right angle or angle formed by two sides which are perpendicular to one another. RECTUM, the terminating section of the intestines. REGATTA, a word adapted from the Italian, and applied to boat or yacht races. REGIMEN, a regulated course of diet in medical language. RELIEVO (or RELIEF), a word applied to that mode of sculpture or carving in which figures are raised more or less from the surface. RETINA, an expansion of the optic nerve, on which external images are cast, and through which ocular perception is effected, the other parts of the eye being strictly mechanical. RHETORIC, the art of speaking with propriety, elegance, and force. RHOмBOID, a four-sided figure of which the opposite sides and angles are equal, but which is neither equalsided nor right-angled. RHOMBUS Oг( RHOм B), a four-sided figure with equal sides, but not right-angled; for example, a card-diamond. RHYTHм, the measured division of time in music, or in verses of poetry. RIFACCIMENTO, a word from the Italian, signifying something dressed up anew. RONDEAU, a short species of poem with few rhymes, in which the sense of the opening lines is repeated, or nearly so, at the close. ROSICRUSIANS, a sect of mysticists of the middle ages, who called themselves Brethren of the Rosy Cross, and prosecuted in secret the search for the philosopher's stone and the elixir of life. ROUND-ROBIN, a mode of addressing or petitioning, in which, to prevent any subscriber from seeming more forward than another, the names are arranged in a circle. RUNIC, the title of the characters forming the language of the ancient Scandinavians or northern Goths. SABBATICAL, an epithet given to every seventh year in the Jewish economy, because they then allowed their fields to rest without tillage, and gave external nature a long Sabbath SACCHARINE, of or pertaining to sugar. SACRUM, the small terminating bone of the back-vona SALIQUE, the title of the old law of France, which excluded females from the throne. SALIVATION, an increased flow of saliva from the glands of the mouth, caused by medicines. SANHEDRIM, a Word signifying the great public council, civil and religious, of the Jews. SANSCRIT, the old or dead tongue of Hindostan, in which much valuable learning is contained. SAPPHIC, a species of verse among the Greeks and Romans, consisting of four lines, and named from Sappho. SARCOPHAGUS, a word applied to tombs, and derived from the body-consuming effects of a species of limestone which was anciently used for making coffins. SCAPULA, the shoulder-blade. SCARABEUS, the beetle. SCROFULA, a disease consisting in hard tumours of the glands, chiefly of the reck. SENSORIUM, the brain or centre of nervous energy, including sensation and volition. SEPTIC, any thing that promotes putrefaction. Antiseptic signifies any thing that checks it. SEPTUAGINT, a Greek version of the Old Testament, named from its being executed by seventy (septuaginta) Jews, or perhaps seventy-two. SERRATE (OF SERRATED),something notched like a saw. SERUM, a very thin and transparent fluid, which lubri cates those surfaces in the interior of the body which do not communicate with the external air. SETON, an issue on the body formed by the insertion of a cord. SIDEREAL, of or pertaining to the stars. SIENITE, a compound, granular, grayish-tinted rock, named from Siene in Upper Egypt. SIERRA, a Spanish name for an eminence or chain of hills. SILICA (or SILEX), a primitive earth, the main constituent in all stones and rocks whatsoever. SINAPISM, a mustard poultice. SIPHON, in hydraulics, a tube from which the air is extracted for the purpose of raising fluids, by the atmospherical pressure behind, above the level of the reservoir. SMELTING, the art of fusing ores for the extraction of the metallic parts from the refuse. SOLECISM, in grammar, a violation in one way or another of the rules of the art. SOLSTICE, the term given to the two periods at which the sun enters the tropics of Cancer and Capricorn, which is respectively on June 21st, and December 21st. SOMNAMBULISM, a word denoting the act or phenomenon of sleep-walking. SOPHISM, a false species of reasoning, not supported by the premises. Sophistry is the art or practice of using sophisms. SOPORIFIC, an epithet for any thing inducing sleep. SOPRANO, in music, a term applied to a species of treble, suited to the female voice. SPECIFIC, in medical language, a remedy which cures any special disorder with more than common certainty. SPECTRUM, a bright spot formed by admitted light on any surface, or the image of an object seen after the eye is withdrawn from it. SPECULUM, in optics, a polished body impervious to light, or which reflects it. SPHEROID, a body nearly approaching a sphere in shape. SPONDEE, a poetic foot, or division, of a line, consisting of two long syllables. SPORADIC, an epithet opposed in sense to epidemic, and meaning diseases which are neither general nor contagious STALACTITE, a name for the concretions formed of carbonate of lime, which accumulate in consequence of the drippings from the roofs of caverns. STAMEN, the male organ of fructification in plants STANZA, a word now used to designate every portion of a poem united by rhymes. STATICS, that department of mathematics which has reference to bodies at rest. STATISTICS, a science of a comprehensive order, em-` bracing every thing connected with the population of a country, their condition and employments. STEARINE, the solid constituent of oils and tallow. STENOGRAPHY, the art of writing in short-hand. STEREOTYPE, a solid plate of metal, cast from a proparation of stucco, and receiving from it an impression of drawings or letter-press, previously communicated by common gravings or types. The art of stereotyping is now of great use in giving to publishers permanent impressions of their works. TARIFF, a table of the customs or duties chargeable upon goods. TARSUS, the bones of the foot immediately adjoining the heel. TAUTOLOGY, the needless repetition of the same words or ideas in speech or writing. TECHNOLOGY, a treatise on the arts; a word derived from techne (art), and logos (a discourse). The epithet technical, denoting something belonging to art, is from the same source. TELEGRAPH, a Word signifying "writing to or for a distant point, and applied to the various inventions by which news is communicated between distant spots by flags or other means." TELESCOPE, a term signifying and applied to an in STERTOR, a noisy kind of breathing, following affec-strument through which distant objects are viewed. tions of the brain. SETHOSCOPE, a tubular instrument, by applying the ear to which internal diseases of the chest or abdomen are discovered. TENTACLES (TENTACULA), the organs of feeling, prehension, and motion, in various insects and other animals, and sometimes viewed also as organs of hearing. TERMINOLOGY, that branch of philological science STIGMA, in botany, the top of the pistil into which the which explains the sense of terms of art. pollen is received. STRABISMUS, in technical language, a squint. STRATIFICATIOx, the process by which any portions of the earth have been arranged in layers or beds, called strata. Generally speaking, all stratified rocks have been subjected to the influence of water, while unstratified rocks are more or less volcanic in their origin. STRIATED, streaked or marked with lines. TERRA-COTTA, an Italian word signifying baked clay, and applied to a class of relics of art, such as vases and the like, formed from that substance, and found in considerable quantities in Tuscany. TERTIAN, an ague of which there are two paroxysms every three days. TERTIARY, a term used to denote the later formations in the earth's crust, comprehending the superficial alluSTROPHE, the first division of a Greek ode, succeeded vial deposits, and such as are composed chiefly of sand by the antistrophe. STRUMOUS, an epithet applied to glandular tumours. STYPTICS, medicines which check bleeding. SUBLIMATION, the process of volatilizing or distilling a dry substance by heat. and clay. TESTACEOUS, a word given to animals which have a strong thick shell, such as oysters, and are included in an order called by the general name of testacea. TESTUDO, the tortoise tribe of animals. THEISM, the doctrine of the existence of a God, opposed directly in sense to atheism. THEOLOGY, literally, a discourse on divinity, and commonly denoting the study or science of religion. THEOREM, a speculative proposition deduced from several definitions compared together. THERAPEUTICS, a term applied to the study of the symptoms of disease and its remedies, and denoting, in short, the healing art generally. THERMAL, an epithet equivalent to worm or 'tepid, and SYLLOGISM, an argument consisting of three parts, called the major, minor, and the conclusion, and which Dear a relative sense, such as is exemplified in the follow-usually assigned to mineral waters so characterized. ing case-Every madman should be confined: A. B. is a madman, therefore A. B. should be confined." All accurate reasoning is syllogistic. SYMPOSION, a social entertainment among the ancients. SYNCHRONISM, a word expressing the simultaneous occurrence of two events. SYNCOPE, a faint or swoon. SYNONYM, a word having the same meaning with another. TABLEAUX (VIVANTS), a term applied to groups of persons, arranged scenically, and so dressed as to represent the actors in some famous historical incident, or the scene of some noted painting. TALC, an earthy stone, usually of greenish tint, and found in lustrous layers or plates of a soft or unctuous feel. TALMUD, the book of the oral law of the Jews, of great antiquity, and containing many of their traditions. TANGENT, in geometry, a line which touches a curve, but does not cut it. THERMOMETER, an instrument for measuring heat by means of a graduated scale of degrees. THESIS, a theme or proposition advanced and maintained by illustration and argument. THORACIC, of or pertaining to the thorax or chest. TONICS, medicines which increase or restore the healthy tone of the coats of the stomach and muscles generally. TOPOGRAPHY, a description of places, or minute branch of geographical science. TORNADO, a whirlwind. Tonso, the trunk of a statue deprived of head and limbs. TOURNIQUET, a surgical instrument for repressing the flow of blood. TOXICOLOGY, a treatise on poisons, or the science which takes cognisance of them. TRANSCENDENTAL, the philosophy of pure or speculative reason, which occupies itself not so much with objects as with the way of knowing them. TRANSITION, the term applied to those parts of the earth's crust supposed to have been arranged when the earth was passing from une uninhabitable to the habitable TANNIN, the principle in galls, oak-bark, and other sub-state. stances, upon which their astringent qualities depends. TARANTULA, a name given to a large spider, the bite of which, long held to be venemous, was deemed curable by music alone. VOL. II.-10 TRANSUBSTANTIATION, the conversion of the sacra mental bread and wine into the body and blood of the Saviour, held by Roman Catholics to take place in reality. G TRAP, a dark semi-volcanic rock, usually found in a columnar form, or arranged in successive layers like stepping-stones. TRAPEZIUM, a geometrical figure having four unequal sides. TRAVESTIE, a burlesque imitation of grave writing. TREPANNING, in surgery, an operation by which the skull is perforated in order to raise a depressed portion. TRIGONOMETRY, the art of measuring the sides and angles of triangles. TROCHEE, a poetical foot of two syllables, one long and the other short. TUBERCLES, in anatomy, small round suppurative tumours, such as those affecting the lungs in consumptive disease; the adjectives tubercular, tuberose, and tuberous, are applied, in medical and botanical language, to denote the presence of knobs or growths so shaped. TUBULAN, having the form of a tube. TUMULUS, a barrow or mound of earth or stones formerly erected over the dead. TUNICATED, Covered with one or more tunics or coverings. TURBINATED, in conchology, a term applied to any shell wreathed serpentinely from a broad base to a narrowed apex. TURBINITE, a fossil turbinated shell. TUSCAN (ORDER), an ancient, massive, and simple style of architecture. TYMPANY, flatulent distension of the abdominal region. TYMPANUM, the drum of the ear, or partition dividing the outer from the inner parts of the organ of hearing. TYPHUS, a dangerous species of continued fever of a contagious nature, and marked by a tendency in the system to putrefaction. TYPOGRAPHY, literally, "writing with types," or the art of printing. ULTRAMONTANE, signifying « beyond the mountains." UNGUICULATE, provided with claws or nails. URANOLOGY, a discourse on the heavens. themselves. Velocipedes have as yet been objects of cu riosity merely, not of utility. VENTILATION, free introduction of air into any place. VENTRICLES, a name given to cavities in the heart and brain. VENTRILOQUISM, a word signifying "speech from the stomach," and erroneously used to denote the art by which the voice is made to appear as if it came from different places-an art dependent on skilful management of the voice in the windpipe, and other accessary means of illusion. Ventriloquism, in short, seems to consist simply in a vocal mimicry of a very perfect kind. VERMICULAR, of or belonging to worms, called the vermes in the Linnæan classification. VERMIFORM, shaped like worms. VERTEBRE, the twenty-four strong and united bones which form the spine or vertebral column, and sustain the trunk and head. VERTEX, the top or summit of any thing; whence the adjective vertical, applied commonly to any thing placed or rising directly upwards in the air or heavens. VIADUCT, a carriage-way, raised or arched over any hollow or low-lying spot. VILLOUS, Covered with down or soft hairs. VITREOUS, a term signifying glassy, and applied to the soft pellucid humour filling the fore-parts of the eye. VITRIFACTION, conversion into glass. VIVIPAROUS, a term applied to animals which bring forth living young, as opposed to egg-bearing creatures. VOLCANO, in geology, a burning mountain or eminence from which ignited and melted matters are cast forth. Volcanoes have evidently been instrumental in moulding a great proportion of the existing crust of the globe. Traces of them, in an extinct state, are noticed almost everywhere. VOLTAIC PILE, the upright series of alternate zinc and silver plates, which the chemist Volta formed, as a mode of charac-developing the galvanic power, after its discovery by Galvani. The pile is now disused, the galvanic trough being substituted for it, as more useful and convenient. VORTEX, the centre of a whirlwind or whirlpool, or of any body or bodies in rapid circular commotion. URANUS, the planet Herschel or Georgium Sidus. UTILITARIAN, an epithet first applied to the followers of Jeremy Bentham, or those who estimate all things by their degree of usefulness in promoting 66 the greatest happiness of the greatest number." UVULA, a small dependent body at the back of the mouth, familiarly called the pap of the throat, and useful as a sort of defence to the tops of the windpipe and gullet. VACCINATION, the operation of introducing cow-pox matter into the human body, in order, by producing a greatly mitigated disease, to preserve the system against natural small-pox, which rarely occurs twice in one person. From noticing that cow-milkers were strangely free from liability to small-pox, Dr. Jenner discovered the invaluable secret that certain pustules on the udders of cows possessed the property described. VACUUM, a space named as being void or vacant, but always containing, in reality, some amount of highly rarefied air even under the most powerful air-pump. VARICOSE, an epithet for veins distended in an uneven or knotted manner. VASCULAR, provided with or pertaining to vessels. VATICAN, the title of a palace built on a hill of the same name at Rome, and containing the magnificent library collected by successive popes. VELOCIPEDE, a wheeled machine so constructed that a man, while seated on a sort of saddle, can propel the whole by pressing on the ground, or acting on the wheels VULCANIC, the title sometimes given to the theory of Dr. Hutton, which ascribes almost all geological phenomena to subterranean fire. WERNERIAN, a name for the aqueous theory of the earth, or that which regards water as the chief geological agent, derived from the German philosopher Werner. WRANGLER, a term applied to the successful competitors for degrees in the English universities, particularly in mathematical contests. ZENITH, that point in the heavens which is directly above or vertical to the spectator, or to any given spot of the earth. ZERO, the point of the thermometer from which it is graduated, or the numbers are begun. Fahrenheit's zero is thirty degrees below the freezing point of water. ZODIAC, an imaginary belt or broad circular space in the heavens, within which the whole of the planets make their revolutions. It is divided into twelve parts of 30 degrees each, called the Signs of the Zodiac, and named respectively from the constellations which were observed to pass them. ZOOLOGY, the science which treats of the structure, character, and varieties of animals or living creatures. ZOOPHITES, a class of remarkable animals, of which sponges and corallines are specimens, and which resemble plants, having stems more or less calcareous, and in which many of the animals are congregated to gether. THE STEAM-ENGINE. TEE apparatus which, after numerous improvements by WATT and others, has assumed its complete form of a Steam-Engine, and has been universally adopted as a convenient and economical means of impelling machinery, is dependent on the properties of water and heat for its source of power, and an account of these seems an indispensable preliminary to any description of its character. We shall, therefore, in the present sheet of popular information, begin by giving, first, a history of the properties of water, with the general nature of aerial bodies; embracing an account of the different kinds of fuel which are employed to convert the water into steam: and then proceed to a detailed account of the various parts which enter into the formation or construction of the engine, taken in the widest sense of the word, and the mode in which these are arranged so as to obtain the maximum of power, including the different kinds of engines adapted to navigation, railroads, &c. The whole to conclude with an historical sketch of the invention, from the earliest period, until its almost final perfection in the hands of Watt. WATER. matters, or substances, are oxygen and hydrogen; combined together in the proportion of one by bulk of the former, with two by bulk of the latter, they constitute the compound water, which had been, until nearly the close of the last century, considered as an elementary body. But water, as it is found in nature, though it is essentially composed of the matters now mentioned, does not consist solely of these, inasmuch as whether it be taken from springs, from lakes, from the sea, from rivers, from melted snow or ice, or from rain, or from any other source, it contains many other substances held in solution, and which affect its character very much-rendering it, indeed, often totally unfit for those purposes to which it is usually applied. When devoid of these substances, the water is considered by chemists as pure. Watt. The matter of which the external world is composed presents itself to our notice in three palpable forms or conditions, namely, the solid, the liquid, the aëriform. Stones or pieces of metal belong to the first kind; water and quicksilver are instances of the second; in the air, and in gases, such as carbonic acid gas, we find examples of the third. But all the different kinds of matter, whether simple or compound, are not specially found only under one or other of these forms; for it may be truly said, that the solid, the fluid, or the aëriform condition, is merely contingent, that is, depending on certain circumstances besides the inherent nature of the particles of matter of which each body individually is composed. Hence it is that the same kind of matter may, on changing those conditions referred to, assume first the solid form, then the fluid, and finally the aerial state; or, conversely, being in the aërial state, it may be rendered fluid, and lastly become solid. The law applies to bodies whether they are simple or compound, and is beautifully seen in the different forms which water assume when exposed to a varied range of temperature. For, below 32 degrees of Fahrenheit, it is solid (ice); between 33 and 212 degrees, it is fluid (water); and above 212 degrees, it is in the form of vapour (steam); changes in its physical form, immediately related to and connected with changes in the amount of heat with which it has been supplied. Water, which forms the grand agent in the steamengine, is not a simple or elementary body, but consists of two distinct kinds of matter, the natural condition of which, when free under the ordinary circumstances of our globe, is that of an aerial substance. These two distinct James Watt, whose discoveries entitle him to be called the inventor of the steam-engine, was born at Greenock, in Scotland, January 19, 1736. At fourteen years of age he removed to Glasgow, and there. while afterwards acting as mathematical instrument-maker, he began those great improvements on the steam-engine. which were completed in future years. Watt died on the 25th of August, 1919. The substances which are dissolved by the water, and which render it impure, are of two distinct kinds-solid matters, such as lime, magnesia, and iron; gaseous matters, such as the elements of the air, oxygen and nitrogen, and carbonic acid. The proportion of solid matter varies considerably. In the waters of the sea which surround our shores, the amount of solid matter is estimated at nearly about 3 per cent. Again, in river water, the proportion of solid matter is considerably less than that found in the sea. The quantity of gaseous matter varies, but not so much as that of the solid matter. In rain water, there is usually noticed 24 per cent. of atmospheric air, but in which the usual proportion between the oxygen and nitrogen is not preserved, as there is 32 of oxygen out of the 100, the remainder being nitrogen, whereas 21 is the proportion of oxygen in atmospheric air. Carbonic acid gas also is found in water. On boiling the water, these gaseous bodies are set at liberty, and pass off as gases. Also the solid substances, such as the compounds of lime, are deposited, and form thick incrustations on the boiler, which requires to be removed, otherwise it would be rendered totally useless. It is only the pure matter of oxygen and hydrogen-the actual water, as it may be termed—which is required in the working of the steam-engine; the other substances, whether aëriform matter or the solid particles held in solution, being not only useless, but even injurious. It will be seen that there are particular contrivances devised in the structure of the steam-engine to remove these. Water is a fluid at ordinary temperatures, but may become solid on the one hand, or aëriform on the other, by changes in the amount of caloric (heat) with which it is supplied. These two remarkable changes in the condition of water occur at specific temperatures; it becomes solid when the degree of temperature indicated by the thermometer of Fahrenheit is 32 degrees, and passes off in the state of vapour or steam when the same thermometer indicates the temperature to be 212 degrees. On the fluid being cooled down to 32 degrees, it becomes ice, the temperature 32 degrees being named the freezing point of water. When the temperature is increased, so that the thermometer indicates 212 degrees, the water becomes steam |