ACCLIMATISATION

climates. Among animals, we find great powers of adaptation to various climates in the horse, dog, cat, and rat; and among plants, in the various cereals, in potatoes, and in several weeds common to almost all climates; but there seems to be a limit to the power, at least as seen in the individual. Acclimatisation beyond a certain point is the work of some generations. Almost all the domestic animals now commonly spread over Europe, and even in high northern latitudes, were originally natives of warm climates. The change produced by the acclimatising of animals may be either an improvement or a deterioration; of the latter, we have an instance in the Shetland pony; of the former, we see an example in the merino sheep of Spain. The reindeer may serve as an instance of the want of the faculty of becoming acclimatised; removed from the cold north to the fertile valleys of a temperate clime, it degenerates and dies. On the other hand, the horse, which is native in the East, arrives at its highest development in England; and the Syrian sheep, brought northward as far as Spain, becomes remarkable for its fine fleece. Spain has a climate much warmer than that of Silesia and Pomerania; and yet the merino sheep bred in these countries have become superior to their ancestors imported from Spain. This is a proof that art may do very much in modifying the influences of climate. Silkworms, brought from China first into Italy, have been acclimatised not only in the south of France, but even on the coast of the Baltic. Recently, attempts have been made to acclimatise in France the llama, the vicuña, and the alpaca of Peru, and with some success in the last instance, as alpacas have been found to thrive pretty well in the Pyrenees.

In America, some interesting experiments in naturalisation have been made. Many European birds have been set at liberty by local societies, and a few species promise to become Americanised. The camel breeds well in a half wild state in Nevada and Arizona; while alpacas, though repeatedly tried, have nowhere thriven. Ostrich-farming promises well in the Argentine Republic, but the Californian experiments with African ostriches have been reported to be failures. Various Australasian trees, notably the Eucalyptus, thrive wonderfully in California, and successful experiments have also been made with them in the cottongrowing states; the tea-plant also grows well in various parts of the United States. The camel does well in Australia, and has been found highly useful in the desert tracts. Several species of trout, salmon, and other fishes have been successfully naturalised in Australasia, notably in Tasmania and New Zealand. America and Australia alike have become the abodes of many noxious weeds from Europe. The English sparrow is a great nuisance in North America; the English rabbit is extremely destructive in Australia and New Zealand. In like manner, the Anacharis canadensis, a harmless water-plant in America, has proved an annoying obstruction in many British canals. On the other hand, in the island of Arran and elsewhere interesting and successful experiments in acclimatisation, especially of Australian plants, have been made. In the case of one species, Eucalyptus or gum-tree, the rate of growth has been even greater than in Australia. The introduction of coffee into the West Indies and of cinchona into India offer further examples of successful acclimatisation. It has been very generally believed that plants may become gradually inured to a climate so different from that to which they have been accustomed, that if they had been at once transferred to it they would have perished. On the other hand, it is maintained that each species of plant has certain limits of

temperature within which it will succeed, and that alleged instances of acclimatising have been merely instances of plants formerly supposed to be more delicate than they really were. But as it is certain that different varieties of the same species are often more and less hardy, it would seem that in the production of new varieties by seed, there is still a prospect of the acclimatising, to a certain extent, of species of which the existing varieties are too delicate to grow well in the open air. Of Acclimatisation Societies, the best known is the Paris Société d'Acclimatation. The progress which has been made during the past few years in the science of medicine and in sanitation, renders it more probable that attempts made by Europeans to become acclimatised in tropical countries will be successful.

Biologically considered, acclimatisation is part of the general process of modification of organism by environment. When the conditions in the new home are approximately similar, no fresh changes will be imprinted on the organism, and the survival of the imported form is obviously natural. Such cases are instances simply of dispersion, generally by human selection, and hardly of acclimatisation in the strict sense. At the other extreme, the sum of the external forces, or natural selection,' may be predominantly adverse, the consequent changes pathological, the result non-survival. The term acclimatisation should thus be restricted to cases between these two extremes, where the plastic organism becomes actively and passively adapted to the new environment. That modifications do take place in consequence of a change of climate and other external conditions, has been recognised from the time of Hippocrates, but how soon these may become really hereditary is still a matter of much dispute. See CLIMATE, DISTRIBUTION, DOMESTICATION, ENVIRONMENT, HEREDITY, PISCICULTURE; Darwin's Animals and Plants under Domestication; works on anthropology by Tylor and Waitz; H. Weber's Climatic Therapeutics.

Accolade, an essential part of the ceremony by which knighthood was and is conferred. Originally, the grand-master of the order, in receiving the neophyte, embraced him by folding the arms round the neck (ad collum). Later, a blow was given with the fist or the flat of a sword (perhaps as the last the knight should suffer to pass unavenged). Now, the sovereign gives the accolade by laying a drawn sword on the shoulders of the kneeling knightelect, and bids him rise, addressing him with 'Sir' prefixed to his Christian name. See KNIGHT.

Accommodation, a name given in theological phraseology to a method in Scripture interpretation which explains the form as not necessarily more than the vehicle by which divine and spiritual truth is conveyed to the human understanding. Without such an adaptation, the divine revelation would not be intelligible to man, and thus much of the symbolic method, especially of the Old Testament, is merely a compromise with human weak

ness.

The method of Jesus, in his teaching, is also claimed as an example of accommodation, in his selection of familiar natural phenomena and ordinary human experiences, as the means of conveying to the mind abstract spiritual truths. The secondary fulfilments of prophecy, and the New Testament explanations of the manner in which these were seen in the life of Jesus, are supposed by some to be accommodations, to which Jesus and the writers of the Gospels assented for the sake of their didactic value.

Accommodation, in Commerce, is either a loan of money directly, or the service rendered when one becomes security for a sum advanced to

another by a third party, as by a banker. For Accommodation Bill, see BILL.

Accompaniment, in Music, is the assisting or aiding of a solo part by other parts, which may consist of a whole orchestra, or a single instrument, or even subservient vocal parts. It is either ad libitum, when it may be omitted at pleasure, or obbligato, when it forms an integral part of the composition. It serves to support and beautify the solo part, and therefore should not predominate, but merely assist to place the solo part in its brightest light. In this point of view, modern composers have often erred by making the accompaniment too full, and causing it to stand out so independent, that the solo part is often, as it were, entirely lost. This abuse takes place mostly in vocal music; and not only is the effect destroyed, but the vocal organ of the singer is frequently ruined. This is a result, though not a necessary one, of an increasing aim towards a polyphonic style, in which the parts are all of nearly equal value and importance; and it is undoubtedly in this style that the greatest music has been written. The works of Richard Wagner are notable as containing many passages in which the voice is overwhelmed by the orchestral accompaniment, and but few singers have ever been able to cope with this difficulty. The operas of Mozart may probably be regarded as striking the balance most happily on this point. In accompaniment, the composer should keep three principal points in view-harmony, rhythmical figure, and suitable choice of instrumentation, in respect to volume and character of tone; but all must be subservient to the ruling character of the solo part. The accompaniment should, above all things, by its certainty and firmness, prevent wavering. Good accompaniment is as creditable as solo playing; and all qualified orchestras view it as of great importance. In the scores of the older masters, especially Bach and Handel, frequently very faint indications are given of the parts of the accompaniment beyond a figured bass-i.e. the bass part with certain recognised figures written above it-indicating the harmony to be played to each note. At that time, the art of playing from this was in general practice among musicians; but it is now necessary to have these parts written out. The work of supplying additional accompaniments to these scores, adapted for the modern orchestra, has been performed by numerous eminent musicians, Mozart, Mendelssohn, and others, the most successful of all being probably Robert Franz.

Accordion, a musical instrument invented in 1829 by Damian, of Vienna. It consists of a small hand-bellows, with a keyboard on one side, containing from five to fifty keys, acting on free metal reeds, so arranged that each sounds two notes, the one in expanding, and the other in contracting the bellows. Its capabilities are extremely limited, and it is in fact little more than a toy.

Accosted, in Heraldry, a term often applied to a bend, chevron, or fess, placed between two cotises. See COTISE.

Accoucheur. See MIDWIFERY. Account, in Law, means a statement of money transactions showing a balance due by one party to another, or it may show only goods supplied or services rendered by one party, in respect of which money is due to the other. In England, where the parties have agreed to the balance, this is called an account stated,' or, if the parties are trustee and beneficiary, it is called a settled account' (in a sense of course quite different from that in which settled' is used of money accounts settled or paid). In Scotland, a fitted account' is entitled to the privileges of a document in re mer

ACCRINGTON

catoria-i.e. it does not require to be signed before witnesses. It is often stipulated, as in the case of a cash-credit, or of a bank and its agent, that an account stated by the official of the bank shall be conclusive, but this affords only a prima facie case. Even where accounts have been docqueted, errors of calculation can be corrected, and if the settlement has been obtained by misrepresentation or There is in concealment, it may be set aside. Scotland a triennial prescription of tradesmen's accounts (see PRESCRIPTIONS). The action by which in Scotland a party liable is made to account, is called 'count reckoning and payment.' In England, such actions are generally brought in the Chancery Division. Under the Legacy and Succession Duty Acts, executors and others have to file statutory accounts showing the position of the estate. The Bankruptcy Law makes the failure of a tradesman to keep proper accounts of his business a criminal offence. Corporations are generally required by public law to make a certain publication of accounts; in the case of life-assurance companies, a very elaborate form of return or account is required by Mr Cave's Act of 1870. Officers appointed by the court, such as judicial factors, are bound to lodge annual accounts in court. The Accountant-general was in 1726 made (see INTEREST) responsible for the accounts of money paid into the Court of Chancery, but in 1872 this duty was transferred to the Paymastergeneral.

Account, Account Current, Account Sales. See BOOK-KEEPING.

Accountant. Most public companies, such as banks and railway companies, have an officer called the books and accounts of the concern, and to make an accountant, whose duty it is to take charge of up periodical statements and balance-sheets. It is only in recent years that the profession of an branch of business, its functions having been accountant has come to be recognised as a special usually performed, as they still sometimes are, by lawyers and agents. The business of an accountant cannot well be very strictly defined, but it may be stated generally as falling under two divisions : (1) The management or realisation of estates, whether of bankrupts or others; and (2) all matters involving the investigation of business books, as auditing the books of private firms or public companies, and making up balance-sheets, statements ties of accountants incorporated by royal charter, of all kinds, and reports. There are several socieand a member or fellow of one of these is termed a chartered accountant (C.A. or F.C.A. ). principal society in England is The Institute of Chartered Accountants in England and Wales,' incorporated in 1880; and in Scotland there are chartered institutes in Edinburgh (1854), Glasgow (1855), and Aberdeen (1867).

The

Accra, since 1875 capital of the (British) Gold Coast Colony, and after Cape Coast Castle, the most important town on the coast, lies slightly to the W. of the long. of Greenwich. It is a healthy place, much attention being paid to the drainage and water-supply, and has considerable export trade in palm-oil, ivory, gold dust, indiarubber, monkey skins, gum copal, and camwood. The town has telegraphic communication with England, the Niger, and the French and Portuguese settlements to the south. Pop. about 10,000.

Accrington, a manufacturing town of Lancashire, incorporated as a municipal borough in 1878. It lies in a deep valley, surrounded by hills, 22 miles N. of Manchester, and 5 miles E. of Blackburn. The oldest church dates from 1554, and was rebuilt in 1763. The town-hall (1857) is a handsome building, and there is a neat market-hall.

ACCUM

The industries are mainly calico-printing, Turkeyred dyeing, iron-founding, with coal-mining in the neighbourhood, and chemical works. Pop. (1841) 8719; (1881) 31,435; of whom 6000 are employed in the cotton manufactures.

Accum, FRIEDRICH, born in Westphalia in 1769, came to London in 1803. He is known in this country chiefly on account of his Practical Treatise on Gas-light, and other chemical works. He greatly promoted the introduction of gas-lighting. In 1822 he became a professor in a technical institute in Berlin, where he died in 1838.

Accumulation is a legal term applied to the putting by of interest, or rents, and converting them into capital by investment, of which the income is again capitalised. By the Thellusson Act (q.v.) of 1799, it is forbidden to accumulate income for a longer period than the life of the truster or settlor, and twenty-one years thereafter. This act applies to movable estate in England and Scotland, and by the Rutherfurd Act, 1848, the prohibition was extended to the rents of land in Scotland. If the

direction is given to accumulate for a longer period, the settlement is not void, but is given effect to so far as permitted by law. It is usual in settlements to direct the accumulation of income during the minority of the children after providing for their maintenance. Accumulation is also applied in Scotland to the charging of compound interest. Such accumulation is permitted on bankers' accounts periodically settled; on India accounts by custom; on the cash-accounts of judicial factors and law-agents; and in certain cases of fixed commercial usage. See INTEREST.

Accumulator. In such pieces of hydraulic apparatus as hydraulic cranes or hoists, unless the height of the available column of water, or headpressure, as it is called, is considerable, the necessary amount and constancy of pressure is obtained by means of an accumulator. This usually consists of a dead weight acting by means of a plunger on the water column. Sometimes, however, steam is used to put on the required pressure, in which case the arrangement is called a steam accumulator.-In Electricity, the accumulator is an arrangement by which electrical energy can be stored for a considerable time in some potential form, so as to be used at will for the production of electric currents. See

ELECTRICITY.

Acel'dama (Chaldee, field of blood'), the name given to the potter's field bought by the priests, as a burial-place for strangers, with the money which Judas had received for betraying Jesus, and which, in the horror of his repentance, he flung at their feet before hanging himself.

Aceph'ala (Gr., 'headless'), a term used from the time of Aristotle in reference to the class of bivalve molluscs or Lamellibranchs (q.v.). It has in the main retained its application, though some forms which it once included, such as the sea squirts in Cuvier's classification, have been removed to other divisions.

Acer and Aceraceæ. See MAPLE.

Acerra, an ancient city of Southern Italy, 9 miles NE. of Naples by railway. It has a cathedral and seminary. The country around is fertile, but malarious. Pop. (1881) 14,121.

Acetal, C2H(OC,H), is a colourless liquid, of an agreeable odour, and a flavour said to resemble that of the hazel-nut. It is one of the products of the slow oxidation of alcohol under the influence of finely divided platinum, or of chlorine, or of dilute sulphuric acid and peroxide of manganese. Its specific gravity is 821, and it boils at 221° F. (105° C.). It yields various reactions and products of interest in organic chemistry.

ACETONES

0

33

Acetic Acid, the sour principle in vinegar, is the most common of the vegetable acids. If alcohol, diluted with water, be mixed with a ferment such as yeast, and exposed to the air at or a little above its ordinary temperature, it is rapidly converted into vinegar or acetic acid. The views held by Liebig regarding the part that wood-shavings, sand, ash, &c. play in condensing oxygen, and transmitting it to the alcohol, are now sup planted by those of Pasteur, who maintains that the true acetifying matter is a very minute mycoIt derma-a special vegetable organised being. vegetation, consisting of extremely minute spores is impossible to conceive a more simple form of arranged in chains; each spore having a mean diameter not exceeding th of an inch, and the length being about twice as great. The rapidity of circumstances, is almost inconceivable; and the the development of the spores, under favourable power which they possess in fixing the oxygen of the air, and of transmitting it to the alcohol, and of establishing an incomplete combustion of the latter, is no less wonderful. A surface of a square yard covered with this plant, is able, in the course of 24 hours, to fix the oxygen of more than 1000 quarts of air. The temperature of the surface of the fluid at which this slow combustion is proceeding is considerably raised, and often remains for several days at 21° or 25° F. (12° or 14° C.) above that of the surrounding air. The process which has just been described bears a very close analogy to the respiratory process, the oxygen of the air being in one case fixed by minute vegetable cells, and in the other by the blood corpuscles. The change is accompanied by the absorption of oxygen, one atom of which combines with two of hydrogen to form water, aldehyde being left. Further oxidation then takes place, acetic acid being formed thus:

Alcohol. Aldehyde. Water. Aldehyde. Acetic Acid. C2HO+O = C2H ̧O+H2O C2H ̧0 + 0 = C2HO, From the mode in which acetic acid combines with bases to form salts, it is evident that one atom of the hydrogen differs from the other atoms in being replaceable by a metal or an alcohol radical (as ethyl, C,H,), and on this account acetic acid is called a monatomic acid, and its formula is usually represented as HC,H3O2; that of acetate of potash being KC,HO, and of acetate of ethyl, CH,CH,O. A striking experiment may be made illustrating the mode in which alcohol is converted into acetic acid. If slightly diluted alcohol be dropped upon platinum-black, the oxygen condensed in that substance acts with great energy on the spirit, and acetic acid is evolved in vapour. Here the whole office of the platinum is to determine the oxygen of the air and the hydrogen of the alcohol to unite. In the commercial processes for manufacturing vinegar, some vegetable substance containing nitrogen (one of the albuminous principles) takes the place of the platinum-black, and determines the same change. Pure acetic acid is a crystalline solid at ordinary temperatures. It is obtained by distilling dry acetate of potassium and sulphuric acid:

2KC,H3O+H2SO, = 2HC2H2O22 + K2SO4. The anhydride of acetic acid (see ANHYDRIDES) is formed by the action of chloride of acetyl on acetate of potassium. It has the composition (CHO),0, and unites with water to form acetic acid. The salts of acetic acid, called ACETATES, are numerous and important in the arts. The most important is acetate or sugar of lead (see LEAD). For the commercial processes of manufacturing acetic acid, see VINEGAR.

Acetones, or KETONES, are the aldehydes of secondary alcohols (see ALCOHOL). Thus secondary

known as acetone.

Secondary Propyl Alcohol.

CH, CHOH CH3

=

Acetone.

H2 CH3 CO - CH3. A series of such acetones is known, of which acetone is typical. It may be prepared by distilling acetate of calcium. It is a limpid liquid, having a taste like that of peppermint, and is readily soluble in alcohol, ether, and water. Its specific gravity is about 79, its boiling-point being 130° F. (56° C.). It has recently been used in America for the manufacture of chloroform, which is obtained from it by distillation with bleaching-powder. It is a solvent for gums and resins, as well as for gun-cotton. A'cetyl is an organic radical not yet isolated, but which is supposed to exist in acetic acid and its derivatives; the rational formula for acetic acid being on this hypothesis (CHO)OH. See CHEMICAL TYPES. The reason for assuming the existence of this radical in the acetic compounds is, that the formula to which it leads affords the simplest explanation of the most important reactions of acetic acid. Thus, when acetic acid is treated with a metallic oxide or hydrate, the basic atom of hydrogen is replaced by a metal, and an acetate of the metal, (C,H,O)OM, is produced. The term acetyl was formerly applied to the radical C2H ̧. Achæmen'ides, a dynasty in ancient Persia, from which sprang Cyrus, the founder of one of the great monarchies of the world. The ancestor of the family was Achæmenes, whose name is a Greek form of the Persian name Hakhamanis. His descendants gradually united the Persian tribes into a kingdom under the over-lordship of the Medes. Cyrus the Great finally extended his rule over the whole of Iran, and the dynasty reigned in Persia until the overthrow of the empire by Alexander the Great, 330 B.C.

Achaia, a small Greek district lying along the northern coast of the Peloponnesus. In the modern kingdom of Greece, Achaia forms, along with Elis, a department in the extreme north-west of the Morea, and its chief town is Patras (q.v.). Excepting the west coast, the land is fertile, and produces corn, wine, and oil. As the Achaians (Achæans) were the ruling people of the Peloponnesus in heroic times, Homer speaks of the Greeks generally as Achaioi. Their twelve little towns formed a confederacy which was dissolved in the Macedonian times; but was renewed in 281 B.C., and subsequently extended itself, under the name of the Achæan League, throughout Greece, until 146 B.C., when Grecian liberty fell under the power of Rome.

Achalganj, a small town of British India, in the south part of Oude, near the Ganges. Pop. 5000, almost all Hindus.

Achard, FRANZ KARL, born in 1754 at Berlin, distinguished himself by his improvements in the manufacture of beetroot sugar; the king of Prussia having given him a farm whereon to found a model factory. Achard was called to Berlin as director of the physical class in the Academy of Sciences, and died in 1821.

Acha'tes, the constant companion of Eneas in his wanderings after the capture of Troy. He is always styled by Virgil 'fidus Achates,' hence the name has become a synonym for a trusty defendant and companion.

Acheen. See ATCHEEN.

Achelo'us, now called Aspropot'amo, the largest river in Northern Greece, rises in Mount Pindus, flows south and south-west, dividing in its lower course Etolia from Acarnania, and falls into the Ionian Sea opposite Cephalonia. The alluvial

ACHILLES

In

deposits at its mouth are very extensive. Greek mythology, the god of this river was the oldest of the river gods, and was the son of Oceanus and Tethys.

Achene, a dry, indehiscent, single-seeded fruit. The term is often restricted to fruits like those of the common dock, resulting from a superior ovary, the wall of which does not adhere to the seed. But it is perhaps better to extend the term achene to all dry, indehiscent fruits, including forms like grains of wheat, 'seeds' of dandelions, nuts of hazel, and so on. See FRUIT.

Ach'eron, the name given to several rivers by the ancients, always with reference to some peculiarity, such as black or bitter waters, or mephitic gases. The Acheron, in Thesprotia in Epirus, flowing through the lake Acherusia into the Ionian Sea; another river of the same name in Elis, now called Sacuto; and several streams in Egypt were all supposed to have some communication with the infernal world. According to Pausanias, Homer borrowed from the river in Thesprotia the name of his infernal Acheron, which the later poets surrounded with many imaginary horrors. Other lakes or swamps of the same name occur near Hermione in Argolis, between Cuma and Cape Misenum in Campania, and in Egypt, near Memphis.

À-cheval Position. When troops are arranged so that a river or highway passes through the centre and forms a perpendicular to the front, they are said to be drawn up in a-cheval position.

Achievement, in Heraldry, originally arms granted for the performance of an honourable action; hence a complete representation of one's armorial ensigns. See HATCHMENT.

Achill, or Eagle' Isle, off the west coast of Ireland, is reckoned within the county of Mayo. It is 15 miles long by 12 miles broad, and has a very irregular coast-line. It has a wild and desolate appearance; most of the surface is boggy; of the 51,500 acres which the island contains, not 500 are cultivated. There are three villages in Achill, and a number of hovels or huts scattered over its barren moors, sometimes in small clusters, forming hamlets, but so wretched as hardly to be fit for beasts. Achill rises towards the north and west coast, where the mountains attain an elevation of 2000 feet. One of them, Achill Head, composed, like the rest of the island, wholly of mica slate, presents towards the sea a sheer precipice from its peak to its base, a height of 2192 feet. There is a mission-station in the island. The population, gradually decreasing, amounted in 1881 to 6700.

Achillæa. See MILFOIL.

Achilles, the hero of Homer's Iliad, was the son of the nereid Thetis and Peleus, who was son of Eacus, and king of the Myrmidons at Phthia in Thessaly. He was taught eloquence and the arts of war by Phoenix, and the healing art by the centaur Chiron. He led his Myrmidons in fifty ships In the war he was the bulwark of the Greeks, being to Troy, although he knew that he would not return. at once the swiftest and bravest hero in the army. He destroyed many towns in the Troad before his quarrel with Agamemnon, with which the Iliad opens. A pestilence in the Greek camp being ascribed to the anger of Apollo, whose priest had been robbed of his daughter Chryseïs by Agamemnon, that chief was compelled by the army to send the girl back to her father. On this he carried away Briseïs, the fair captive of Achilles. The latter now retired to his tent, and neither the splendid offers made by Agamemnon nor the disasters of the Greeks could afterwards move him to take any

ACHILLES TENDON

part in the contest, until his dear friend Patroclus was slain by Hector. The hero then buckled on his armour, which had been made for him by Hephaes tus, and of which the shield is described at great length by Homer. The fortunes of the field were now suddenly changed in favour of the Greeks; and the vengeance of Achilles was not satiated until he had slain a great number of the Trojan heroes, and lastly Hector himself, whose body he fastened to his chariot and dragged into the Grecian camp. He then buried his friend Patroclus with great funeral honours. King Priam, the father of Hector, came by night to the conqueror's tent, and prayed that the body of his son might be given to him. Achilles consented; and with the burial of Hector the Iliad closes. The hero himself fell in battle at the Scaan gate before the city was taken. His death is not expressly mentioned in the Iliad, but in the Odyssey his remains are buried, together with those of Patroclus, in a golden urn on the coast of Hellespont, where a mound was raised over them. Such is the Homeric account of Achilles, the swift-footed, fair-haired hero of the Iliad. He is at once the handsomest and bravest of the Greeks, terrible to his foes, tender and gentle with his friends, magnanimous and proud, defiant to the unjust prince, but reverent and obedient to the gods. He loves music, is the most devoted of friends, has a passionate hunger for glory, and dies in the full splendour of his youth. There are many later traditions which fill up the bare outlines of his history. His mother dipped him when an infant into the Styx, and hence he became invulnerable except in the heel by which she held him. To escape the fatal expedition to Troy, she hid him in the disguise of a girl at the court of Lycomedes at Scyros, but here his sex soon made itself known, for one of the king's daughters became by him the mother of a son, Pyrrhus or Neoptolemus. Ulysses discovered him by an artful stratagem. Disguised as a pedlar, he came to offer his wares for sale: the girls at once showed a natural interest in the articles of dress and ornaments, but the eager interest which he could not hide in the weapons of war at once revealed the youthful hero. Among his achievements at Troy are his conquest of the Amazon Penthesilea and of Memnon. Nor does he fall by human hands alone. Some say that he was killed by Apollo himself; others, that the god merely guided the weapon of Paris. Another story tells how the hero fell in love with Polyxena, daughter of Priam, and how he came unarmed to meet her in a temple of Apollo, where he was shot in the vulnerable heel by the treacherous Paris. His body was rescued by Ulysses and the Telamonian Ajax, and these heroes had a fierce contest for his famous armour. The hero was carried to the islands of the blessed, where he was united to Medea or Iphigenia.

Achilles Tendon (Tendo Achillis) attaches the soleus and gastrocnemius muscles of the calf of the leg to the heel-bone. It is capable of resisting a tension strain equal to 1000 lb. weight; and yet it is occasionally ruptured by the contraction of these muscles in sudden extension of the foot, such as may occur in the case of old gentlemen dancing, or in the attempt to recover equilibrium after a stumble. For the name, see ACHILLES. Ancient surgeons regarded wounds or serious bruises of the Achilles tendon as fatal.

Achi'menes, a genus of plants of the natural order Gesneracea, cultivated in stoves and greenhouses for the beauty of their flowers. The species are numerous-natives of tropical America.

that in which the pustules are very small, but have large inflamed bases. They are most common on the faces of children. They seem to arise from inflamed hair sacs or sebaceous follicles.

certain combinations of lenses, &c. refract a beam Achromatism, the property in virtue of which of light without producing coloured fringes. Any arrangement of lenses or prisms which refract light without dispersion (see under LIGHT) is achromatic. Newton, misled by certain imperfect experiments, concluded that dispersion could not be annulled without annulling refraction. Hall, in 1733, and later, Dollond (independently), found that certain media give large refraction with small dispersion, while others give small refraction with large dispersion; so that the dispersion produced by one medium can be made to annul that due to another, while its refraction is not entirely annulled. For example, by properly combining a convex lens of crown-glass with a concave one of flint-glass, a compound achromatic lens can be produced. The achromatism in the above arrangement, and in every other arrangement yet tried, is media do not give exactly similar spectra () not absolutely perfect. The reason is that such (see SPECTRUM)-i.e. the ratio of the distances between any two pairs of rays is not quite the same for the different media. A combination of three lenses, or prisms, gives a better approximation to absolute achromatism than a combination of two. Blair, in 1791, constructed an achromatic telescope giving far better definition for high magnifying power than has since been obtained. He used a compound lens consisting of two glass lenses inclosing a liquid.

Acidimetry is the determination of the percentage of real acid contained in a sample of a hydrated acid, as sulphuric or nitric acid. In most cases, if we know that no foreign body is present, it is possible to determine the percentage by means of the specific gravity, as indicated by the Hydrometer (q.v.). Usually, however, other substances, which alter the specific gravity, may be present, and recourse is then had to one of the following methods:

(1) By volumetric analysis, in the manner described under Alkalimetry (q.v.).

(2) By the gravimetric process. This may be conducted in two ways, which will be best understood by an example of each. Sulphuric acid forms several insoluble salts, the sulphate of barium refusing to dissolve, not only in ordinary fluids, but even in strong acids. When chloride of barium is added to a liquid containing sulphuric acid, the sulphate of barium is precipitated, and after due precautions have been taken to insure its purity and to avoid erroneous conclusions, it may be weighed and the amount of sulphuric acid calculated therefrom.

sample some carbonate of soda, and noting the A more rapid method consists in adding to the amount of carbonic acid disengaged. This is readily accomplished by performing the operation in a weighed flask, and determining the loss of weight after the carbonic acid gas has been liberated. Of course many precautions are essential. See also ANALYSIS.

Achlamydeous. See APETALOUS.