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extremely fond of attention, and seems to repay kindness with affection.

Cheever, GEORGE BARRELL, American clergyman, born in Maine in 1807, was educated at Bowdoin College and Andover Theological Seminary, and from 1832 to 1870 was pastor of Congregational and Presbyterian churches in Salem (Massachusetts) and New York. He delivered numerous lectures on religious, literary, and social questions, and published a number of works, the most popular of which is his Lectures on the Pil grim's Progress (1844), which, with his Wanderings of a Pilgrim in the Shadow of Mont Blanc (1845-46), has had a wide circulation in England as well as in America. In his prime Dr Cheever was an active and uncompromising opponent of intemperance and slavery. Died October 1, 1890. Che-foo (properly the name of the European colony of the Chinese town of Yen-Tai), a treaty port on the north side of the peninsula of Shan-tung, at the entrance to the Gulf of Pechili, in which it is the only port that remains open throughout the winter. The foreign quarter, with about 120 Europeans and Americans, is in some sense a colony of Shanghai, and, having the wholesomest climate of all the treaty ports, it is much resorted to by convalescents. The Chinese town, built on the sandy shore, with exceedingly dirty streets, has a fort, a signal-station, and about 32,000 inhabitants. As a market for foreign manufactured goods, particularly English cotton yarn and American sheetings, Chefoo is of great and increasing importance. In 1887 the value of imports was £1,760,967, of which £1,145,749 were foreign goods; of exports, £1,347,427. The principal articles of import besides those mentioned are sugar, paper, iron, edible seaweed, matches, and opium. The chief exports are silk, straw-braid, bean-cake, and vermicelli. The total number of vessels entered and cleared during 1887 was 2081, of 1,656,075 tons, of which 1130, of 958,821 tons, were British. The Che foo Convention, which settled several disputed points between China and Great Britain, and extended certain commercial advantages to the latter country, besides throwing open four new treaty ports, was signed 13th September 1876.

Cheiranthus. See WALLFLOWER, Cheiro'lepis, a genus of fossil ganoid fishes, characteristic of the Devonian strata. The generic name, meaning scaly-hand,' was given in allusion to the large pectoral scaly fins.

Cheiromys. See AYE-AYE.

Cheironectes, an aberrant genus of Opossums (q.v.), with webbed hind-feet and of unique aquatic habit. There is but one species, C. variegatus, palmatus, or Yapock (q.v.).

Cheiroptera (“wing-handed'), the technical name for the order of Bats (q.v.).

Cheirothe'rium (Gr., 'hand-beast'), the name originally given to the Labyrinthodont (q.v.), from the peculiar hand-like impressions left by it in the Triassic rocks.

Cheke, SIR JOHN, one of the revivers of Greek learning in England, was born in 1514 at Cambridge, and in 1529 obtained a fellowship of St John's College, where he embraced the Reformed doctrines. He laboured earnestly to advance the study of the Greek language and literature; and when a regius professorship of Greek was founded at Cambridge in 1540, Cheke was appointed its first occupant. A new mode of pronouncing Greek which he introduced was assailed by Bishop Gardiner, the chancellor of the university; but notwithstanding, Cheke's system prevailed. It resembled that still in vogue in England, as opposed to the continental system. In 1544 Cheke

CHELONIA

became tutor to the Prince, afterwards Edward VI., whose elevation to the throne secured him rank, wealth, and honour-a seat in parliament (1547), the provostship of King's College (1548), and knighthood (1552). He was stripped of everything at Mary's accession, and went abroad, but in 1556 was treacherously seized in Belgium, and brought to the Tower. Fear of the stake induced him to abjure Protestantism, and fresh lands were given to him in the place of those he had forfeited, but his recantation preyed on his mind, and he died in the course of the following year, 13th September 1557. Of more than thirty Latin and English books by him, one is a translation of St Matthew's Gospel (edited by Goodwin, 1843), exemplifying a plan for reforming the language by eradicating all words save those of English origin. See his Life by Strype (best ed. Oxford, 1821).

Che-keang, an eastern and maritime province of China (q.v.). Capital, Hang-chow.

Cheli'ceræ, a technical term usually restricted to the biting organs which form the first pair of appendages in spiders, scorpions, and other Arachnida.

Chelifer. See BOOK-SCORPION.

Chelmsford, the county town of Essex, at the confluence of the Chelmer and the Cam, 29 miles NE. of London. It has a corn exchange (1857), a shire hall (1792), a grammar-school, founded by Edward VI. in 1551, and a parish church, which, all but the tower and spire, was siderable trade in agricultural produce. On a small island in the Chelmer there has long been a ludicrous mock-election during the county elec tions. Pop. (1851) 6033; (1881) 9885.

rebuilt between 1803 and 1878. There is a con

Chelmsford, FREDERIC THESIGER, BARON, born in 1794, was a midshipman in the navy, but exchanged the sea for law, and was called to the bar in 1818. He was knighted and made solicitorgeneral in 1844, attorney-general in 1845 and 1852, and lord chancellor in 1858 and 1866. He died October 5, 1878.-His son, FREDERIC AUGUSTUS THESIGER, second BARON, was born in 1827, entered the Rifle Brigade in 1844, became major in the Grenadier Guards in 1855, and served through the Crimea, the Indian Mutiny, and the Abyssinian campaign of 1868. He was adjutant-general in Bengal (1869-74), and commanded the forces in the

Kaffir war of 1878 and in the unfortunate Zulu war of 1879, having resigned the governorship of Cape Colony. Appointed lieutenant-general in 1882, he was made lieutenant of the Tower of London in 1884.

Chelonia, an order of reptiles including the various forms of. tortoise and turtle. Their most

distinctive character is the more or less complete inclosure of the body by a dorsal and a ventral shield, of which the former is in part due to a modification of the vertebral spines and of the ribs. Within these shields the head, limbs, and tail can be more or less retracted. The absence of teeth is also very characteristic. The Chelonia include marine, fresh-water, and terrestrial forms; the known living species number about 260, the majority occurring in warm countries; they are represented by numerous fossil forms from the Upper Jurassie onwards. The large family Textudinidæ, with oval, horny-plated dorsal shield, includes both terrestrial and marsh forms, such as Testudo e.g. the Greek tortoise, Terrapin, Emys, Chelys; a second small family, Trionychida, with oval, softer shield, includes river forms, such as the well-known snapping-turtle (Trionyx ferox); a third family, Chelonidae, with heart-shaped shield, includes five species of marine turtles-e.g.

CHELSEA

CHEMICAL AFFINITY

145

Chelone viridis. The order will be chiefly discussed again, and there are always very many men seeking under the titles TORTOISE and TURTLE.

Chelsea, a suburb of London, on the north bank of the Thames, here crossed by bridges to Battersea (q.v.). In the 16th century the village of Chelsea was the residence of Sir Thomas More, Queen Catharine Parr, the Princess Elizabeth, and Anne of Cleves. Afterwards Walpole, Swift, Steele, and Sir Hans Sloane, and, in later years, Leigh Hunt, Carlyle, Rossetti, and George Eliot lived here. In the 18th century Ranelagh was much resorted to, and Cremorne (closed 1877) was at one time a popular attraction. Besides its Hospital, Chelsea has à Royal Military Asylum for soldiers' children, large barracks for the Foot Guards, a botanic garden, water-works (1722) to supply London, a river-pier, and an enbankment (1873) extending to Battersea Bridge on the west. The famous 18th-century porcelain is noticed under POTTERY. The borough has returned one member to parliament since 1885, when its limits were greatly reduced. Pop. of parish (1887) 112,316. CHELSEA HOSPITAL is an asylum for old and disabled soldiers of the British army. The gradual decay of the feudal system rendered it necessary to make some new provision for sick and maimed stiers, consequently various statutes were passed daring the reigns of Elizabeth, James I., and Carles I, throwing their maintenance on their respective parishes, under directions from the erty justices. This system was abrogated during the Commonwealth as a matter of policy, and the expense met out of moneys arising from sequestrations of the estates of the vanquished royalists. After the Restoration a new act was passed (1662) agan throwing their maintenance on the parishes,

but this was so burdensome that it was never reenacted. Sir Stephen Fox, the first paymaster general of the forces, who had long been an exile in France, and was no doubt well acquainted with the erection in 1671 of the Hôpital des Invalides at Paris, first suggested the building of Chelsea Hospital. The foundation stone was laid by Charles II. in 1682, and the building, designed by Wren, was opened in 1692.

The funds for its lands and buildings, and for many years the maintenance of its inmates, were derived chiefly by deductions from the pay of the trips themselves-viz. 1s. in the £1, as well as

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the position so vacated. Chelsea Hospital is locally known as Chelsea College'-the Hospital having been erected nearly on the site of James I.'s shortlived College for Religious Controversy (1610). See the official Early History of the Royal Hospital at Chelsea (1872), and Martin's Old Chelsea (1888). Chelsea in Massachusetts, U.S., is a northeastern suburb of Boston, from which it is separated by the estuary of Mystic River (see BOSTON). Pop. (1870) 18,547; (1880) 21,782; (1890) 27,909. Cheltenham, a fashionable watering-place of Gloucestershire, on the Chelt, a little affluent of the Severn, 44 miles NNE. of Bristol, 47 SSW. of Birmingham, and 121 WNW. of London (by road only 95). It lies in a picturesque and fertile valley, on the east and south-east half encircled by the Coteswolds. A saline spring was discovered here in 1716, and from a mere village the place gradually increased till 1788, when the benefit derived by George III. from its waters suddenly made it a resort of fashion. The four spas-Royal Old Well, Montpellier, Pittville, and Cambray-are all saline but the last, which is chalybeate; they are deemed efficacious for liver complaints and dyspepsia. With its squares, crescents, and terraces, its gardens and promenades, its clubs and pump-rooms, its August cricket week,' its healthy climate, the cheapness of living, and the happy absence of manufactures, the town offers many attractions both to visitors and residents, the former largely foxhunters in winter, the latter retired Anglo-Indians. It is, besides, a great educational centre, the seat of the Proprietary College, for 700 boys, founded in 1840, and occupying a splendid Tudor pile of 1843; a grammar-school (1586; reconstituted 1883); a large ladies' college (1854); a Church of England training college for schoolmasters (1847); and private schools beyond number. Noticeable buildings are the 14th-century parish church; the Roman Catholic Church (1857), with a spire 205 feet high; the Corn Exchange (1863); and the handsome Free Library. Cheltenham has memories of Lord Tennyson, Frederick Robertson, Sydney Dobell, and Dean Close, under whom (1824-56) it became a stronghold of Evangelicalism. It was incorporated as a municipal borough in 1876, and has returned one member to parliament since 1832, the parliamentary boundary having been extended in 1885. Pop. (1804) 3076; (1841) 31,411; (1881)

50,842.

Chelyuskin, CAPE (formerly North-east Cape, and sometimes called Cape Secero), the most northerly point of Asia, on a peninsula of the same name, which forms the western arm of the eastern half of the Taimyr peninsula. It is named after a Russian officer who led an expedition thus far in 1742, and here succumbed, with his wife, to the fatigues of the journey; it was not revisited till 1878, when Nordenskjold, in the Vega, spent the 19 and 20th of August here. He found it a low promontory, divided into two parts by a small bay; the lat, of the western is 77 36′ 37′′ N., that of the eastern 77 41 ́ N.

he days pay in each year. Since 1783 it has, however, been almost entirely supported by annal parliamentary grants. All Pensions (q.v.) "anted to soldiers are awarded by the CommisGers of Chelsea Hospital, who are appointed by the crown. Originally it was contemplated that all pensioners would become inmates of Chelsea Hospital, but this was soon found imple, and thus those who could not gain de.ttance were granted allowances termed outThe out-pensioners in 1888 numbered *7.73 men, including negroes in the West Indies ani West Africa, Maltese, Singhalese, and Lascars, Ard were estimated to cost £1,770,000. The in ensoners numbered about 550. They are selected fro a such out-pensioners as desire to become in fulles, according to merit, age, and sufferings from Chemical Affinity is the name given to the Wilds or other disabilities, and are provided tendency to combine with one another which is with board, lodging, clothing, including the well- exhibited by many substances; or to the force by known red coat and cocked hat, nursing and which the substances constituting a compound are etical attendance, together with a small weekly held together. The tendency of any given element wance in money according to rank. The to unite with a number of other elements varies jesson vote for the year 1888 89 amounted greatly. Chlorine, for instance, unites with great ding, however, charges relating to the grant, readiness with most metals and with many non4 out pensions, but excluding cost of repairs to metallic elements, much heat being produced dur Mlags &e) to £27,083, against which there is, ing the union; but it has little or no affinity to set off the amount of the out-pensions, which for, or tendency to combine with, oxygen, so that st by statute be surrendered on admission. compounds of chlorine with oxygen can only be Time within can at any time become out-pensioners obtained by roundabout methods, and are very

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liable to sudden and explosive decomposition into chlorine and oxygen. Where the affinity of elements for each other is great, the compounds produced by their union are decomposed with difficulty, and where the affinity is feeble, decomposition is easily effected. See also CHEMISTRY.

Chemistry. Although chemistry has only taken its place as an exact science based upon accurate experimental investigation within a comparatively recent period, yet its origin dates back to the earliest times of philosophical study. It will be convenient to give in the first place a short sketch of the history of chemistry, and then to state some of the principles of the science, illustrating these from the simplest facts. When possible, such illustrations will be chosen as are likely to be not altogether unfamiliar to non-scientific readers.

Historical Sketch. The word chemistry has come to us from the Greek through the Arabic, as shown in our article ALCHEMY. With regard to the chemistry of the ancients, we know that the ancient Egyptians, Phoenicians, Greeks, and Romans were acquainted with a very considerable number of useful substances, and that their processes for preparing some of these did not differ in any essential particular from those now in use. It does not appear, however, that they have left any chemical records behind them, or that they knew anything of the science of chemistry. Several metals were known to, and employed by, these ancient peoples, who were acquainted with processes for reducing them from their ores. Amongst these metals were gold, silver, mercury, copper, tin, lead, and iron; whilst they also knew and worked with brass, although they were not aware that it was an alloy of copper and zinc. Various alloys were employed for bronzes for statues, and these usually contained copper, lead, and tin. The processes for manufacturing soap, starch, glass, leather, various mineral and vegetable pigments, stoneware, and other useful substances, were all known and carried on in very early times; and wine and beer appear likewise to have been prepared and used as beverages long before the process of distillation, which was unknown to the ancients, had been introduced. Vinegar, sulphur, and carbonate of

soda were also known.

We find the application in medicine of many chemical products at a comparatively early period, and the Arabians appear to have been the first who tried to prepare new medicines by chemical methods. Geber, who lived in the 8th century A.D., is the most noted of the Arabian chemists, and he has left some writings which show us what was the state of chemistry at that early date. Geber knew, for instance, how to make and distil vinegar and nitric acid, and even sulphuric acid was made and used as a solvent by him. He knew, amongst other substances, white arsenic, borax, common salt, alum, sal-ammoniac (ammonium chloride), cop. peras (ferrous sulphate), nitre (potassium nitrate), and corrosive sublimate (mercuric chloride), and was acquainted with a number of their properties. He used almost all the kinds of apparatus that were commonly in use down till the 18th century, and understood the processes of distillation, filtration, sublimation, and crystallisation. In one of his works he describes the construction of furnaces for chemical purposes.

From the 8th till the 17th century but little real progress was made in chemistry as a science. The new knowledge that was gained during this period was mainly due to the assiduity of the alchemists, who, in their vain search for the philosopher's stone, necessarily made useful discoveries from time to time. Many of the alchemists so called were mere tricksters who deceived their

dupes by more or less clumsy experiments, which appeared to demonstrate the production of gold from baser metal. Others, however, were really earnest and untiring in their labours, and held the fullest belief in the prospects of the ultimate success of some fortunate worker. The new substances obtained by the alchemists were frequently used in medicine, and it is to these infatuated workers, therefore, that we owe our first knowledge of many potent medicines. The writings of many of the alchemists are preserved, but numbers of them are entirely worthless from a scientific point of view, as the descriptions of processes are mixed up with so much of mystery and extravagance that they present a wholly unintelligible jargon. For more detail, however, regarding this remarkable period in the history of chemistry, see the article ALCHEMY.

As Geber has been called the patriarch of chemistry, so Robert Boyle (1627-91) has been called the father of modern chemistry, since it was Boyle who first tried to free chemistry from the trammels of alchemy and to place it upon a true scientific basis. Boyle in his Sceptical Chemist tried to discredit the salt, sulphur, and mercury of the alchemists (as well as the Aristotelian earth, air, fire, and water) as elements or ultimate constituents of substances, and he gave a scientific definition of an element. Boyle was an experimental investigator of considerable skill, and to him we owe the introduction of the air-pump and the thermometer into this country. His experiments upon the physical properties of gases led to the formulation of the law concerning the relation of the volume of a gas to the pressure, which is commonly known as Boyle's Law.

Theory in modern chemistry begins with Becher (1635-82) and Stahl (1660-1734). The latter adopted, with some modifications, a theory propounded by the former concerning elements and compounds, and formulated the phlogiston theory of combustion. The views of Becher and Stall regarding elements were not so enlightened as those of Boyle, and must be considered as retrograde. Stalil's phlogiston theory (1697) was at once adopted almost universally by chemists, and for fifty years it was held to give the full explanation of the phenomena of combustion. According to this theory phlogiston was a constituent of all combustible substances. When a substance burned, the phlogiston made its escape, and the product of combustion was regarded as the other substance with which the phlogiston had been previously united. When a metal such as lead was heated in the air, it lost its phlogiston, and the oxide formed was looked upon as the other constituent of lead besides phlogiston. The process of reduction of lead from its oxide by means of charcoal was the transfer of phlogiston from the charcoal to the lead. It did not present itself to the adherents of the theory as an absurdity that a metal, in losing its phlogiston on oxidation, gained weight, although some of them at least were aware of the fact. The

idea of gain of matter being a necessary accompaniment of gain of weight is so familiar to us that we can scarcely realise that it was not always so regarded. To this may fairly be attributed the persistence with which the phlogiston theory held its ground for so long a period.

The Dutch chemist Boerhaave (1668-1738), who did not accept Stahl's theory, published in 1732 his system of chemistry, which was a compilation of practically all that was known up till that date, collected with great labour from a large variety of alchemical and other writings.

The interval between the introduction of the phlogiston theory and its overthrow by Lavoisier in 1772-85 was one of great advance in chemical

CHEMISTRY

knowledge, and a number of very eminent chemists 1 preceded and were contemporaries of Lavoisier.

In Germany, Marggraf (1709–82) studied the properties of the almost unknown alumina and mnesia, and made considerable advances in the quitative analysis of substances in solution.

Amongst British chemists of note may be mentioned Hales (1677-1761), who was amongst the first to experiment on gases; Black (1728-99), who in 1756 published his research on Magnesia Alba, showing the nature of fixed air or carbonic acid gas, and of the difference between caustic and mild For carbonated) alkalies; Priestley (1733 1804), who, in addition to his discovery of oxygen in 1774, investigated nitric oxide, nitrous oxide, sulphurous acid, carbonic oxide, hydrochloric acid, and aumonia gases, being specially attracted to the study of gaseous substances and their properties; and Cavendish (1731-1810), who investigated the nature and properties of hydrogen, analysed atmospheric air, and discovered the compound nature and composition of water and of nitric acid.

Lavoisier (1743-94) was one of the ablest chemists of his time, and his labours include a vast variety of subjects. His attack upon, and eventual demolition of the phlogiston theory, and his experiments in connection with his new theory of combustion, cupied him for a considerable number of years. He taught that combustion was the union of the combustible substance with atmospheric oxygen; he was the first to introduce system into chemistry and chemical research; he determined the constituents of a large number of substances, including 1.pluric, phosphoric, and carbonic acids, numerous metal'te oxides, and many animal and vegetable #tances; and he, along with Berthollet, Fourtroy, and Morveau (1737-1816), introduced a new and consistent system of chemical nomenclature. Iwo contemporary Swedish chemists, Bergman 1735 84 and Scheele (1742 86), must be mentioned before leaving the phlogiston age. Bergman investigated, amongst other things, carbonic and gas, studied the phenomena of aflinity, and ¦ Blade advances in the processes and reagents used qualitative analysis. Scheele was one of the most laborious chemists of his time. He discovered eitrie, malie, tartaric, oxalic, lactic, hydrocyanic, arserie and other acids, and chlorine, besides investigating the nature of a large number of other bies and independently discovering oxygen.

It was towards the end of the 18th century that the value of quantitative analysis of substances egan to be generally recognised. The question as to whether the quantitative composition of a given *stance was always the same gave rise to a discassion which lasted for several years, and was at eth decided in favour of constant composition. I in researches of Richter (1762-1807) on the 114ntities of various acids neutralised by a given v saistīts of a base, and of various bases neutralised aven quantity of an acid, led him to the neral conclusion that the quantities of two acids, sanila, which form neutral salts, a b, and a' b', with the quantities of two bases, b and b', are just nantities required to form two other neutral ab and ab. This fundamental discovery Was erroneously attributed to Wenzel by Berzelius a 19. and the error has been carefully per wtuated in a considerable number of text-books *** that time (Kopp, Entwickelung der Chemie; sader memeren Zeit, p. 251).

Berthoilet 1748-1822), who was one of the most **ive opponents of the theory of the constant com*tion of chemical substances, contributed valuresearches into the laws of chemical affinity, aged chlorine to processes of bleaching. The *ses of chemical analysis were improved, and 2. nambers of analyses, especially of minerals,

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were carried out by Klaproth (1743 1817), Vauquelin (1763–1829), Foureroy (1755-1809), and others; and many quantitative observations of all kinds were made about the end of the 18th century, all preparing the way for Dalton's statement of the Atomic Theory (q.v.) in 1803-4.

The progress of chemistry during the present century has been immense, and it is not possible to do much more than mention the names of some of the most prominent workers. A stimulus was given to research by the publication of Dalton's atomic theory; and the labours of Gay-Lussac (1778-1850), who experimented with gases, of Dulong (17851838), and Petit (1791-1820), who pointed out the relation between specific heats and atomic weights of elements, and of others, supported and amplified Dalton's views.

Wollaston (1767-1829) discovered palladium in 1803, and rhodium in 1804. The first alkaloid (morphine) was obtained pure by Serturner in 1816, and this led to the discovery of a number of others in a short time.

The decomposition by electricity of the bases potash and soda by Davy (1778-1829) in 1807, and the separation from these of the metals potassium and sodium, threw an entirely new light on the nature of these substances. The metals were more fully investigated by Gay-Lussac and Thenard (1777 1857). Davy is noted also as the inventor of the miners' safety-famp, and for experiments on the respiration of nitrous oxide and other gases.

Amongst the foremost chemists of the earlier part of the 19th century was the Swede Berzelius (1779– 1848), whose careful and exact analyses of mineral substances contributed a good deal to the confirmation of the law of constant proportions and to the fixing of the atomic weights (see ATOMIC THEORY) of the elements. Berzelius was very conservative with regard to new theories, which he declined to accept without putting them to the strictest experi· mental tests. He formulated the electro-chemical theory of the constitution of salts, introduced great improvements into the methods of quantitative analysis, increased the value of the blowpipe as an aid in mineral analysis, discovered many new substances, and further examined and elucidated points concerning many already known, both inorganic and organic.

The artificial production of urea in 1828 by Wohler (1800-82) marks the beginning of a new era in the branch of organic chemistry, and enormous strides have been made in this department since that time by Dumas (1800 84), Liebig (180373), Laurent (1807-53), Gerhardt (1816-56), Wurtz (1817 84), Kolbe (1818 84), Baeyer, Cannizzaro, Frankland, Hofmann, Kekule, Williamson, and many others. Advances in general inorganic chemistry and analysis have been made by Leopold Gmelin (1788 1853), H. Rose (1795 1864), SainteClaire Deville (1818-81), and Bunsen; whilst in connection with advances in chemical physics may be mentioned Faraday (1791 1867), Mitscherlich (1794-1863), Graham ( Ì805-69), Regnault (1810-78), Andrews (1813-85), and Berthelot. These lists do not include all of even the most prominent names that might be mentioned in connection with each department.

The most striking feature of modern chemistry is the extraordinary development of organic chemistry, the account of one branch of it-the chemistry of the coal-tar products- constituting of itself quite a literature which receives additions every day.

Amongst the most recent triumphs of chemical research may be mentioned the artificial production of indigo and grape-sugar, and the isolation, in sufficient quantities to study its properties, of the hitherto all but unknown element fluorine.

Of the greatest possible interest from a theoretical

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point of view is the fact that since 1870 three new elements have been discovered-gallium, scandium, and germanium-the existence of all of which had been predicted, and the properties of which had to a certain extent been described beforehand by Mendeleeff. (See periodic law in article ATOMIC THEORY.)

Of late much attention has been given to measurements of the quantity of heat produced in various chemical changes, notably by Berthelot and Thomsen.

Elementary Principles of Chemistry.-The science of chemistry deals with a certain class of changes which matter undergoes when subjected to par ticular conditions. Similar treatment may produce very different effects upon different substances, as, for instance, the effect of strong heat upon a piece of quartz, a piece of limestone, and a piece of sugar. The quartz does not suffer any permanent change, that is, it has the same properties after it is cold again as it had before the action of heat. The limestone, although not necessarily much altered in appearance, has its properties entirely changed, and what remains is a new kind of matter-quicklime. The sugar melts, darkens, and chars, and becomes quite manifestly changed into more than one new kind of matter, for gaseous products, having the smell characteristic of burnt sugar,' go off, whilst a black coaly mass remains.

The first of the above changes is merely a physical change, from cold to hot; the other two are chemical changes, which result in the production of new kinds of matter having properties entirely different from those of the kinds of matter from which they were obtained. The existence of chemistry depends upon the existence of different kinds of matter, and it is with such different kinds of matter and the change from one kind to another that chemistry has to do.

first the saltpetre and then the sulphur, and thus recover all three ingredients separately. The expiosion of gunpowder when heated to a sufficienti high temperature is due to the occurrence of a series of changes of the kind we call chemical, for these changes result in the production of new kinds of matter, gaseous and solid, which possess properties in no way resembling those of sulphur, charcoal, or salt petre, and from which these substances cannot now be dissolved out.

A mixture possesses to a greater or less extent the properties of its respective ingredients; a compound, on the other hand, has not as a rule any properties resembling those of its constituents. À piece of magnesium wire heated in the air to a sufficiently high temperature takes fire and burns. This is a chemical change in which the metal mag nesium combines with the oxygen of the air to form a white, brittle, solid compound called magnesia or magnesium oxide. This magnesia does not in the least resemble either magnesium or oxygen in its properties, and the most powerful microscope fals to reveal particles of either of these substances to our vision.

The Atomic Theory (q.v.) is based upon the assumption that matter of every kind is made up of extremely minute indivisible particles called atoms. The atoms which exist in a substance may be all of the same kind, as in elements, or of different kinds, as in compounds. Chemists believe that the element hydrogen consists of molecules of aggregates of atoms-each molecule consisting of two atoms; further, that the compound substance water consists of molecules, each composed of two atoms of hydrogen and an atom of oxygen united to each other by that force which is called Chenacal Affinity (q.v.); and that similarly every other compound substance is composed of molecules, each molecule consisting of two or more different kinds of atoms united by chemical affinity. The weight of a new compound formed by the union of two or more substances is in every case equal to the sum of the weights of its constituents. In chemical actions it is only the kind of matter which is changed, whilst, as in every physical change, the quantity of matter concerned remains constant and unalterable.

When the properties of matter are studied, it is found that for chemical purposes all kinds of matter may be divided into two great classes, which are called respectively elements and compounds. The name element is applied to any kind of matter that has not been proved to be composed of more than one simpler kind of matter. This conception of an elementary substance we owe to Boyle, and it will be noted that some of It has already been seen that one of the charthose substances which are now looked upon as acteristics of the chemical combination of two sub elements (see article ATOMIC THEORY for a list of stances is that the properties of both disappear and the 68 known elements) may hereafter be proved are not observable in the compound. Another and to be compounds, or kinds of matter composed of a most important characteristic is the evolution of more than one simpler kind, just as some sub-heat, which is a very frequent although not an instances which were at one time rightly classed as variable accompaniment of chemical action. The elements (according to Boyle's definition) are now best examples of this may be seen in the ordinary known to be compounds of two or more elements. phenomena of Combustion (q.v.). All combustion, whether it be of magnesium wire, coal, phosphorus, paraffin oil, or a candle, is nothing more than a chemical action accompanied by the evolution of heat and light, oxygen gas of the atmosphere being almost invariably one of the substances taking part in such action.

The compound nature of a specimen of matter may be proved in one or other (or both) of two ways. One of these methods is called Synthesis (q.v.), and consists in building up the compound from the component simpler kinds; the other is called Analysis (q.v.), and consists in separating more than one simpler kind from the compound kind.

The distinction between chemical compounds and mere mechanical mixtures is a fundamental; one, and must be fully understood. The substance gunpowder, for instance, is an intimate mixture of finely powdered sulphur, charcoal, and saltpetre (potassium nitrate), certain precautions being observed during the mixing in order to avoid explosion. These substances are not combined together chemically in gunpowder, but are only mixed, a fact as to which we can easily satisfy ourselves in various ways. We may examine the gunpowder under the microscope and identify the separate particles of the ingredients; or, by the use of appropriate solvents, we may dissolve out

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The conditions under which substances act chemi cally upon each other are very various for different substances. In the first place, certain substances cannot be got to act upon each other at all. Such substances may have little affinity for each other as chlorine and oxygen, or no affinity, as fluorine and oxygen. Other substances, again, only act up-t each other with difficulty. The main conditions upon which action of one substance upon another depends are the state of physical aggregation and the temperature. Certain chemical actions take place at ordinary temperatures, as, for instance, the combination of chlorine with metallic antimony or copper, or the spontaneous ignition of one of the compounds of phosphorus and hydrogen when brought into contact with oxygen. Other actions

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