corps of laborers, and its bulky and valuable publications, promises to become in the future one of the great scientific forces of the world.

tional Academy was incorporated at Washington by Congress in the year 1863. Its main object was to examine and report upon any scientific questions submitted to it by the Government departments, and thus to bring The institutions above described are the more importthe knowledge of specialists to the aid of the Govern- ant of the American scientific societies, but there are ment when necessary. In such investigations the Gov- many others of minor importance, several of them active ernment pays the actual expenses, but no compensation. and useful. Of these may be named the Society of The membership was originally limited to fifty, selected Natural History, of Portland, Maine, incorporated in from the scientists of the country. This restriction no 1850; the Essex County Natural History Society, of longer exists, and the number of members is now nearly Salem, Mass., incorporated in 1833, and now merged a hundred. The society is divided into two classes in the Essex Institute; the Albany Institute, organized one devoted to mathematics and physics, and one to in 1828; the Poughkeepsie Society of Natural History, natural history. It holds two stated meetings yearly-organized in 1874; the Buffalo Society of Natural Hisone in January at Washington, and one in August at tory, organized in 1861; the Elliot Society of Natural some other city. At these meetings general scientific History, of Charleston, S. C., organized in 1853; the subjects are discussed. The publications of the National New Orleans Academy of Science, organized in 1853; Academy consist of Annuals and Reports dating from the Ann Arbor Scientific Association, of Ann Arbor, 1863, and of Memoirs from 1866. Mich., organized in 1875; the Minnesota Academy of Natural Sciences, of Minneapolis, Minn., organized in 1873; the Wisconsin Academy of Science, Arts, and Letters, of Madison, Wis., organized in 1870; the Kansas Academy of Science, organized in 1867, and holding migratory annual meetings; and the Rochester Academy of Natural Sciences, established in 1881. In addition to these are many local institutions; various State cabinets of natural history, some of which are of considerable extent; and societies devoted to some single branch of science. Of these latter may be named the Massachusetts Horticultural Society, incorporated at Boston in 1829, and the American Chemical Society, of New York, organized in 1876. All the institutions here named issue publications, frequently of considerable value.

The Smithsonian Institution, at Washington, is now the most important of American institutions. James Smithson, an English scientist, who died at Genoa in 1829, left by will a bequest to the United States of America "to found at Washington, under the name of the Smithsonian Institution, an establishment for the increase and diffusion of knowledge among men." The amount of this bequest, when received at Washington in 1838, was $515,169. The institution was founded, in accordance with the terms of the will, by act of Congress in 1846, and the interest of the bequest, amounting to $242,129, was applied to the erection of a suitable building. By judicious management, aided by small subsidiary bequests, the endowment had increased on Jan. 1, 1881, to $719,434.53. It is the intention to limit its increase to $1,000,000. Much of the work of the institution has been in connection with the National Museum, instituted in 1842 to receive the scientific material collected by the Wilkes Exploring Expedition. This museum was placed under the care of the Smithsonian Institution in 1858. Its collection has been increased by the numerous exploring expeditions of the Government, by the material gathered in the geological survey, and by the highly valuable contributions made by foreign Governments to the United States at the close of the Centennial Exhibition of 1876, in addition to the stores gathered by the institution itself. The museum is now closely affiliated with the Smithsonian Institution, though partly supported by annual appropriations from the Government. Its collection is of great value, and is very rich in many departments. It may, as a whole, be ranked with the largest European collections, and in such fields as the ethnology, zoology, and mineralogy of the United States it has no rival among the museums of the world. The large library of the institution, which yearly receives extensive additions, has been transferred to, and incorporated with, the National Library.

Of late years the American colleges and universities have been active in collecting scientific material, and much good work in this direction has been done within their walls. The Johns Hopkins University, of Baltimore, Md., has a fully organized corps of scientific professors, and does much original work, whose results are embodied in annual publications. Harvard and Yale are also active in scientific labor. Yale has a Peabody Museum and the valuable Marsh collection of vertebrate fossils. Harvard possesses the important Museum of Comparative Zoology and a magnificent herbarium which ranks with the best in the world. Columbia College has an herbarium, probably ranking second in the United States; Princeton College, the fine E. M. Natural History Museum, etc. The noble collections of the Cornell University, at Ithaca, N. Y., and the much older, and in some respects more complete, museum of Amherst College, are worthy of mention.

The scientific institutions herein described, and the course of science which is now becoming an important feature of college study, are what America has to show for a century of scientific progress. They form a broad foundation for its future development. Work of the greatest importance in embryology, histology, comparative anatomy, paleontology, etc., is now being actively prosecuted by specialists throughout the country, the Government is giving abundant aid to special investigations, and American science promises ere long to rival that of the much older and better-endowed institutions of Europe. (C. M.)

The special and most important labors of the Smithsonian Institution are, however, in another direction. It yields financial aid to important scientific researches. It acts as a medium of international exchanges of books and specimens between learned societies and individuals of the Eastern and Western hemispheres. It supplies great numbers of duplicate specimens to learned institutions, it aids the investigations of specialists by the ACCENTOR (Lat. accentor, a chanter). 1. A genus loan of valuable specimens, and it publishes the results of oscine passerine birds of uncertain position; by some of such researches and other papers of scientific import-placed among the thrushes, chats, and warblers of the ance in the highly valuable series of works which it issues. These works, which have given the Smithsonian Institution a high standing in the scientific world, consist of Annual Reports from 1846 to date; of Miscel laneous Collections, begun in 1862, and now numbering 21 volumes; and of Contributions to Knowledge, begun in 1848, and numbering 23 volumes. In addition, it issues the publications of the National Museum, consisting of Proceedings and Bulletins, the Publications of the Bureau of Ethnology, and the Bulletin of the Philo sophical Society of Washington. This institution, with its ample endowment, its special advantages, its skilled

ed. (p. 82 Edin. ed.).

in

equity in favor of certain persons. Relief will also be afforded where boundaries have become by accident confused, where there has been an accidental omission to indorse a promissory note, and in certain other cases of less frequent occurrence.

Old World; by others made the type of a sub-family, Accentoring, next to the chats (Saxicolina). 2. Taken as the English name of the hedge-sparrow, hedgechanter, or hedge-accentor, A. modularis of Europe. 3. In the U. S. sometimes used as the English name of species of the sylvicoline genus Siurus, the usual The peculiar jurisdiction of equity with regard to pensong of which is chanted in monotone. (E. C.) alties and forfeitures is also supposed to be traceable in ACCEPTANCE, ACCEPTOR (from the Latin ac- its origin to the head of accident. The failure to make See Vol. I. cipere, to receive). In the law, generally, the payment or perform the act in default of which the P. 79 Am. acceptance means the receipt of anything penalty or forfeiture has been provided was presumed pursuance of a contract on the part of to have been occasioned by accident, and the interpoone person to deliver and of another to re-sition of a court of equity for the relief of the person in ceive it. Thus delivery and acceptance are, as a rule, default was thus justified. (L. L., JR.) necessary to a complete contract for the sale of chattels. ACCIPITRES (Lat. accipiter, a hawk; ad and From its more general meaning it has acquired in mer- capio, I take, seize), an order or sub-order of birds, cantile law the more particular signification of the act birds of prey. 1. In its larger (ordinal) acceptation of one who promises to pay in money a bill of exchange the term is equivalent to the Raptores of Illiger and or draft when it shall fall due, for he thereby becomes Cuvier or Rapaces of Temminck, and covers all the the principal debtor on the bill, and, like the maker of rapacious birds, as the owls and American vultures, as a promissory note, undertakes to pay the amount of it well as the hawks, etc. In this sense it corresponds at maturity. A bill of exchange being a request by with the AETOMORPHE of Huxley (which see). 2. In the drawer of it to the drawee, or person on whom it is its lesser signification, as a sub-order of Raptores, it indrawn, to pay a specified sum at a certain time for his cludes only the diurnal birds of prey, excluding the (the drawer's) account, the acceptance of it by the owls (Striges) and also the American vultures (Cathardrawee creates an obligation on his part to pay it at tides) and African secretary-bird (Gypogeranides). maturity, and the drawer becomes his surety for its payment. The usual form of acceptance consists in writing the word "accepted across the bill and signing the acceptor's name. But the acceptance need not be written on the bill, but may be in writing on another paper, and need not necessarily be in writing at all. In England, however, by statute, an acceptance must now be in writing, and similar statutes have been enacted in this country in the States of Alabama, California, Delaware, Kansas, Maine, Michigan, Minnesota, Missouri, Oregon, New York, Pennsylvania, and Wisconsin.

Taken in the (usual) ordinal signification, Accipitres furnish the following characters: The bill epignathous (hooked), and furnished with a soft cere, in or at the edge of which the nostrils open, and feet not zygodactyle. This expression is diagnostic, for the parrots, the only other birds with hooked and cered bill, are zygodactyle. There are two carotid arteries; the syrinx, when developed (there is none in Cathartides), has but one pair of intrinsic muscles. The sternum is ample and deep-keeled, its posterior margin being doubly or singly notched or fenestrate on each side, or entire, with central emargination; the furculum ankylosed or not. The acceptance may be absolute, which is a positive Angle of mandible not recurved; maxillo-palatines agreement to pay the bill according to its tenor; or it united to an ossified septum; basipterygoid processes may be made conditional upon some contingency, such present or absent. Hallux always present, usually as the receipt of a particular shipment of goods against valid and insistent; outer toe reversible in some cases, which the bill is drawn. But such condition or quali- never permanently reversed. Ambiens muscle present fication must receive the assent of all antecedent parties (except in Striges). Coca coli and tufted oil-gland to the bill in order to hold them liable. (s. w.) ACCIDENT. An accident, in law, may be defined ed or not. Nature altricial, yet ptilopædic, the present (except in Cathartides). Plumage after-shaftyoung See Vol. I. as an unforeseen and injurious occurrence being downy, yet long reared in the nest. The aliP. 79 Am. not attributable to mistake, neglect, or mis-mentary canal varies with the families, but differs from ed. (p. 83 conduct. The principles of the common law Edin. ed.). make ample provision for relief from the consequences of accident in certain cases. Where, for example, written instruments are lost, parol evidence to prove their contents will generally be admitted in a court of law. Resort, however, is usually had in cases of accident to courts of chancery, where relief may often be obtained in accordance with the well-settled principles of equity jurisprudence. A court of equity will not administer relief in cases of accident (1) where there is an adequate remedy at law; (2) where the person seeking relief has by his own gross neglect or misconduct contributed to or caused the accident; (3) where the equity of the party against whom the relief is sought is equal or superior to that of the party invoking the aid of the court; (4) where the relief sought is an entire release from an obligation to perform an express covenant entered into by the suitor, the performance of which, owing to some intervening events, has become unexpectedly harsh or burdensome. Subject to the qualifications above stated, equity will relieve in many instances of accident where otherwise damage might ensue.

Where title-deeds have been lost, equity will in certain cases decree a re-execution of them. Where a bond or other instrument for the payment of money has been mislaid, the creditor may assert his rights in equity and recover the amount of his debt, indemnity being usually required to protect the debtor against any contingent liability growing out of the subsequent discovery of the instrument.

The defective execution of powers will be aided in

that of vegetarian birds in adaptation to an exclusively animal diet. In the higher accipitrine types the whole structure betokens strength, activity, and ferocity, carnivorous propensities and predaceous nature. Most of the smaller or weaker species feed much upon insects, others more particularly upon reptiles and fish, others upon carrion; but the majority prey upon other birds and upon mammals. To this end the claws, no less than the beak, are specially adapted by their development into great talons. These weapons of offence and defence are as a rule of large size, strength, curvature, and acuteness, and also peculiar in being convex on the sides, gradually narrowing to the point, and little or not excavated underneath. The inner claw is larger than the outer, and the hinder one is not smaller than the middle front one; all are very flexibly jointed, so that they may be strongly bent underneath their digits, carrying to an extreme the prehensile power of the feet. The legs are muscular and largely free from the body, feathered to the suffrago or beyond; when unfeathered the tarsal envelope is variously scutellate, reticulate, granular-rugose, etc. The wings are ample, and, as usual in birds below Passeres, the secondary coverts are long and numerous, covering three-fourths or more of the folded wing. The tail, though very variable in shape, has twelve rectrices (with rare exceptions).

Representatives of this order are found in every part of the world. They include four types of structure, of more classificatory value than that attaching to families. One of these, Gypogeranides, consists of the single

species (genus and family) Gypogeranus serpentarius, the serpent-eater or secretary-bird of Africa; it shows a curious grallatorial analogy, being mounted on long legs like a crane and having several important structural modifications. The other three are the Striges, or owls; the Accipitres proper, embracing all the hawks, eagles, etc., and the Old-World vultures; and the Cathartides, or American vultures. The last named are more different from the others collectively than these are from one another. The three groups being well represented in North America, we offer the following scheme of classification:

See Vol. I.

ed. (p. 91

Edin. ed.).

counts.

ACCOMPLICE, in criminal law, one who is in some way concerned in the commission of a crime, though not as a principal. It was formerly doubted whether a prisoner might be convicted on the unsupported evidence of an accomplice. This doubt has been resolved in the affirmative. It has, however, become the practice for the judge strenuously to advise the jury to acquit unless the evidence of the accomplice be corroborated either by other testimony or by the attendant circumstances. (L. L., JR.) ACCOUNT, in law, a statement of the mutual demands in the nature of debt and credit Feet scarcely raptorial, with weak, blunt, lengthened, p. 86 Am, arising out of contracts or some fiduciary relation. and little curved or contractile claws. Hind toe eleIn mercantile law, when an account vated, not more than half as long as outer toe, with is made up to a certain time and a balance small claw; middle toe lengthened; outer toe not struck, and this has been examined and accepted by the versatile; front toes all webbed at base; basal joint of parties expressly or by acquiescence, it is an account middle toe longer than either of the succeeding ones. stated. Books of the original entries of the charges for Nostrils large, perforate. Bill little raptorial, length-goods sold and delivered in mercantile transactions are ened, and somewhat contracted in its continuity; tomia never lobed or toothed; tip blunt, little hooked.' Head valuable as evidence of the sale and delivery, being the largely naked. Index digit with a large claw. No memoranda made at the time of the transaction, and syrinx, cœca, after-shafts, or tufted oil-gland. Ambiens are generally, in this country, admissible as evidence in muscle present; femoro-caudal present or absent; semi- courts of law, with the testimony of the person who tendinosus and its accessory present CATHARTIDES. made the entries, to prove prima facie the contract unDiurnal; gressorial; feed upon carrion Cathartida. der which the plaintiff seeks to recover. So an account Feet highly raptorial, with large, strong, sharp, curved contractile claws. Hind toe not elevated, lengthened, stated may form the basis of a suit to recover the balmore than one-half as long as outer toe, with large claw; ance shown by it to be due to the plaintiff. Account outer toe often versatile; front toes with slight basal forms a distinct branch of equity jurisdiction concurwebbing between outer and middle, or none. Nostrils rently with courts of law, but extending also (1) to small, imperforate. Bill short, stout, seldom contract- dealings so complicated that they cannot be adjusted in ed in its continuity, tomia often once or twice lobed or toothed, tip sharp, much hooked. Head feathered, tion between the parties; (3) to discovery of facts in the a court of law; (2) to the existence of a fiduciary relacompletely or in greatest part. Syrinx developed, with one pair of intrinsic muscles. Coca present. Plumage knowledge of the parties or the production of papers with or without after-shafts. Ambiens present or ab- relating to their rights; and (4) to the appropriation sent. Femoro-caudal present. Semi-tendinosus and of payments. Both in the courts of law and equity the its accessory absent. As a rule saltatorial, and kill disposition is to refuse to interfere with stated actheir prey. (s. w.) ACETIC ACID (formula, C,H,O.OH). Acetic acid See Vol. I. was the only acid with which the ancients p. 88 Am. were acquainted, and frequent references to ed. (P. 93 vinegar, its impure form, with more especial reference to its solvent power, are found in Pliny, Livy, and Plutarch. This vinegar of the ancients was an impure wine-vinegar, and the alchemists were the first who prepared pure acetic acid by distillation. Wood-vinegar was also known in the time of Glauber, who refers to it in 1648. Lavoisier first showed that acetic acid is a product of the oxidation of alcohol. Acetic acid at the present time is made from one of two sources-either as a product of the oxidation of alcohol or by the destructive distillation of wood. Under the first head we have the production of vinegar from wines as carried out in wine-growing countries, from malt liquors as carried out in England, or from cider and fruit-juices as carried out in this country. In these several cases the process of its preparation, known as the "acetous fermentation," is largely but not solely one of atmospheric oxidation, as a vegetable organism, the Mycoderma aceti (see FERMENTATION, Vol. IX.), plays an important part. der the first head also we have the "quick-vinegar process, whereby weak spirit containing 5 to 7 per this case casks are used loosely filled with beech-wood cent. of alcohol is rapidly changed into vinegar. In or deal shavings, and the spirit, slightly warmed, is made to slowly trickle through the cask. The large surface exposed allows of an abundant action of atmoSpheric oxygen, and this, at the temperature of 97°104° F. which is kept up, rapidly changes the alcohol into acetic acid. Under the second head we have the production of what is known as "pyroligneous acid," or wood-vinegar. This is produced chiefly from oakand beech-wood, and is in its crude state a very complex mixture, containing, besides acetic acid, propionic acid, acetone, wood-spirit or methyl alcohol, and other impurities. The acetic acid is obtained by neutralizing the crude pyroligneous acid with lime, removing the brown acetate of lime which forms, and after heating

Physiognomy not peculiar; no great lateral expansion of the cranium or thickening of its walls with diploë; eyes looking sideways; no facial disc, or only an imperfect one; base of bill not hidden by antrorse appressed feathers. Nostrils pierced wholly within the cere. No external ear-conch. Tomia

nal

shafted

shafts

of bill usually toothed or lobed. Outer toe not shorter than inner one, and rarely versatile. Basal joint of middle toe longer than the next. Feet (with rare exceptions) in greatest part or entirely bare of feathers, scutellate or reticulate, or both; toes always bare and scaly. Sternum commonly single-notched or single-fenestrate on each side behind; sometimes entire. Oil-gland tufted. Plumage compact, usually after-shafted; flight audible. Ambiens muscle present. Habits diurACCIPITRES (proper). Outer toe not reversible; plumage usually afterFalconidae. Outer toe reversible; plumage without afterPandionidae. Physiognomy peculiar by reason of great lateral expansion, lengthwise contraction and diploic thickening of the cranium, which is often unsymmetrical. Eyes looking forward, surrounded by a radiating disc of modified feathers, in front antrorse, appressed, hiding base of bill. Nostrils usually at edge of the cere. A large external ear-conch often developed. Tomia of bill never lobed or toothed. Outer toe completely versatile, shorter than inner toe. Basal joint of middle toe not longer than the next, much shorter than the penultimate one. Feet usually feathery or bristly to or on the toes. Oil-gland nude. Plumage without after-shafts, soft and lax; flight noiseless. Ambiens absent. Habits nocturSTRIGES. Sternum entire behind, with central emargination; furculum ankylosed. Middle claw pectinate. Facial disc complete, triangular (embracing only the "barn-owls," which are related to Steatornithidae and Caprimulgidæ). Aluconida. Sternum double-notched or fenestrate behind; furculum free. Middle claw not pectinate. Facial disc circular when complete (ordinary owls), Strigidæ. (E. C.)

nal

Edin. ed.).

Un

it sufficiently to carbonize the tarry impurities, thereby lime, or litharge it forms a silicate. The precipitated changing it to gray acetate, distilling with sulphuric or HSiO is then called hydrated silicic acid, to distinhydrochloric acid. The acetic acid is thus liberated guish it from the former.

66

We now define an acid as the compound of an electronegative element or group of elements with hydrogen; and this definition is sufficiently comprehensive to include all true acids, either inorganic or organic. At the same time, we recognize the fact that oxides of electronegative elements largely constitute those groups which by combination with hydrogen form acids.

How is this combination with hydrogen brought about? The case of the several halogen acids is simple. They are direct binary_compounds of the halogen element with hydrogen. In other cases it is by the union, either directly or indirectly, of the negative oxide with water or hydrogen oxide, so that the hydrated acid of the old nomenclature is the true acid of the new.

in a pure state, although in a dilute form. Concen- This undoubtedly arose from the prevalence of Gmetrated acid is gotten by distilling the fused sodium saltlin's views as perpetuated for many years in what is now with strong sulphuric acid. Among the more import- called the "old notation," whereby the oxide of an ant acetates, or salts of acetic acid, are-sodium and electro-negative element was called the acid, instead ammonium acetates, both of which are used in medi- of the acid-forming oxide or acid anhydride, as it cine; calcium, aluminum, and iron acetates, which are really is. That HCI and the hydrogen compounds used in calico-printing as mordants, the solution of the of the halogens were acids, even as gases, has never second being known as "red liquor," and of the last as been disputed. With the development of organic chemblack liquor;" lead acetate, both the normal and the istry, when organic acids had to be formulated which basic salt, the former as “sugar of lead" and the latter contained both replaceable or basic hydrogen and hydroin solution as Goulard's extract, quite extensively used gen unreplaceable or belonging to the radical, the old in medicine; and cupric acetate, which as basic salt is methods of nomenclature broke down, and a new, truer, extensively manufactured under the name of "verdi- and more comprehensive system had to be adopted. gris." The ethers or organic salts of acetic acid are This is found by going back to the earlier electroalso technically important, as several of them, including chemical views of Berzelius, and looking strictly at the ethyl and amyl acetate, are used in the manufacture of electro-positive or negative character of the elements artificial fruit-essences. for a solution of the question as to whether they are In the year 1880, according to the census, there were acid- or base-forming. And as these characters are not manufactured in the United States 7,233,009 lbs. of fixed and absolute for all conditions of combination, we acetate of lime, valued at $166,092. (S. P. S.) I need look only at the question as to which part an eleACHENBACH, ANDREAS, a German landscape- ment or group of elements plays in the particular compainter, was born at Hesse Cassel, in 1815. He studied pound. From this standpoint it is possible to establish at the Düsseldorf Academy under Schirmer, and a pic-a definition that shall include all true acids and explain ture entitled The Academy of Düsseldorf first brought all cases, whether real or seeming, in which we appear him into notice as an artist of promise and good gifts. to have the reaction known as acid. He has been a great traveller, and his landscapes are reminiscences of the most striking scenery of Europe from Norway to Italy. Of his important pictures may be mentioned A Storm at Vlissingen, Hildesheim, Fish at Ostend, Storm and Flood on the Lower Rhine in 1876, A Norway Torrent, Fishing-boat at Sunset, A Storm Clearing Off, Coast of Sicily, and The Return of the Fishermen at Evening. Achenbach is a diligent student of Nature, and has, in common with most German landscape-painters, a liking for strong contrast and bold effect. His pictures are uneven in quality, but the best of them are very admirable performances. Many of his important works are in the Pinakothek at Munich, and good examples of his style are to be found in nearly all the best collections of Germany and the United The simplest class of acids are the so-called "halogen States. In addition to his oil-paintings, Achenbach has acids," where hydrogen is directly combined with the made many pictures in water-color, and he has prac-electro-negative element, as in HCl, HBr, HI, and HF. tised etching and lithography. He is a knight of the In all the reactions in which these acids enter they simply order of Leopold, a chevalier of the Legion of Honor, exchange hydrogen for metal, forming thereby binary and a member of the academies of Berlin, Amsterdam, haloid salts. Thus, and Antwerp. He has received a number of medals at important exhibitions. (W. J. C., JR.) ACHENBACH, OSWALD, a German artist, was born at Düsseldorf in 1827. He is a brother of Andreas Achenbach, and received his instruction from him. This artist has devoted himself chiefly to Italian subjects, such as A Fête at Genazzano, Villa Torlonia near Frascati, Vesuvius at Twilight, Market-place at Amalfi, The Festival of St. Anna at Ischia, A Storm Effect at Naples, and The Environment of Naples. Oswald Achenbach, although his works are strongly marked by the characteristics of the Düsseldorf school, is a more refined painter than his brother, and his best pictures suggest Calame and his school of landscape art rather than the performances of the Düsseldorf artists. He is particularly skilful in the management of groups of figures, while his atmospheric effects are frequently very charming. (W. J. C., JR.) ACID, a chemical term which may be applied in a general but loose way, or in a strictly scientific way which is at the same time thoroughly comprehensive as well as correct. The Edin. ed.). general application of the word is to any chemical compound, whether binary or ternary, whether containing oxygen or not, whether containing hydrogen or not, that neutralizes the substances called bases. Thus, the term acid is applied in this loose way to CO2. SO2, and SO,, as well as to H,CO, H2SO,, and H2SO4, and similarly to HCl, HBr, HI, and HF. SiO is often called silicic acid, because when fused with soda,

See Vol. I. p. 91 Am. ed. (p. 97

HCI + KOH = KCl + HOH

is an example where the hydrogen displaced from the acid goes with the hydroxyl of the base to form water. Again,

(HCl)2 + CaO = CaCl2 + H2O

is an example where the hydrogen displaced from the acid unites directly with the oxygen of the oxide to form water. Or the halogen acid may be made to decompose a salt of a weaker acid, as

(HCl)2+ CaCO3 = CaCl2 + H2CO3 or H2O + CO2, in which the displaced hydrogen would unite with the electro-negative group CO, to form the free carbonic acid H2CO3, which, being unstable at ordinary temperature, breaks up into H2O and CO2, an electronegative oxide or anhydride.

The next class of acids are those in which hydrogen is combined with the electro-negative element only indirectly or by the aid of oxygen, making an electronegative group, as it is termed. In this category belongs the greater number of acids. Here great care must be taken to note the valence of the electro-negative group, for upon this depends the number of hydrogen atoms that it is capable of uniting with, or, as it is conveniently expressed, the basicity of the acid. The electronegative element which constitutes the foundation of the molecular structure may vary in its valence, giving us a series of acids with perhaps the same number of basic hydrogen atoms, but with a different number of oxygen