260. Hydrates. On the foregoing view acids and bases belong to the same class of compounds, and are called hydrates-acid hydrates and basic hydrates. Caustic potash and nitric acid are opposite extremes of the same series which are connected by bodies of intermediate gradation. Hydrogen is an essential constituent of all acids and bases. Salts contain no hydrogen, and possess neither acid nor basic properties.

261. Quantivalence of Hydrates.-The hydrogen, which is directly linked to the atomic group by oxygen in acids, is termed basic hydrogen; that in bases, acid hydrogen. It is readily replaceable in the former case by other positive elements; in the latter case by other negative elements or radicals. Hydrates are univalent, bivalent, trivalent, etc., according to the number of these replaceable hydrogenatoms, or hydroxyl groups; and acids are said to be mono-, di-, or tri-basic; and bases, mon-, di-, or tri-acid, in the same conditions.

262. Kinds of Acids and Bases.-Acids or bases in which all the oxygen (or analogous element) performs a linking function, are called ortho-, those which also contain oxygen that is not linking, are termed meta-, acids or bases. The atoms of ternary molecules may be connected by the negative dyads, sulphur and selenium, as well as by oxygen, giving rise to sulphur and selenium acids, bases, and salts.

263. Classes of Salts.--Salts containing neither acid nor basic hydrogen are said to be normal. Acid salts are those which contain basic hydrogen, and manifest acid reactions; basic salts contain acid hydrogen, and produce basic effects. Double salts are such as contain two or more positive atoms.

264. Anomalous Bodies.-As common salt, the substance which, above all others, was long considered as the type of saline character, contains no oxygen whatever, it cannot properly be included in this class of bodies. It consists of one atom of each of the monad elements, so

dium and chlorine, directly united. Substances of analogous composition have been called haloids-bodies resembling salt. As the compounds of hydrogen with chlorine, and other elements analogous to it, likewise contain no oxygen, though often termed acids, they must also be excluded from this group as above defined. They are sometimes distinguished as hydracids.

265. The Ammonia Type.-In acids, bases, and salts, the radicals are linked by dyads, but triadic elements, as nitrogen, phosphorus, or arsenic, may perform a similar linking function, and then we have a corresponding series of bodies on a new type. Nitrogen is the nucleus of the most important group, and its molecule in ammonia, H

is the type of a large class of compounds. If

H-N-H the hydrogen is replaced by negative radicals, an amide is produced; if by positive atoms, an amine results; if by one positive and one negative radical, an alkalamide is the product. By thus substituting simple or compound radicals for the hydrogen of the ammonia-molecule, we get the H

derived ammonias. If we write ammonia as HN, and H

then substitute for its hydrogen the negative radicals, cyanogen, iodine, and chlorine, the formation of the amides will be made clear:

By replacing the hydrogen with positive radicals, we get

§ 6. Theory of Isomerism and Allotropism.

266. Inorganic Chemistry in Relation to Theory.— Chemical science has been long divided into two great branches-inorganic chemistry, which treats of non-living or mineral substances, and organic chemistry, which treats of matter that composes the parts of organized things. In the former department the chemist deals with all the elements of Nature in their simple physical conditions; in the latter he is occupied with only a very few elements in circumstances of great obscurity and complexity. In fact, organic chemistry was long regarded as an impossibility, under the belief that the vital force dominates in the organic sphere, and suspends the ordinary laws of chemical action. It was therefore natural, and indeed inevitable, that inorganic chemistry should be cultivated first, and that the earlier theories of the science should be framed upon the knowledge obtained by studying the simpler and more general phenomena. Yet the domain was partial, and the knowledge limited; organic chemistry was a legitimate and most important division of the science, and its numerous and remarkable facts being left out, the prevailing theories were necessarily defective.

267. Organic Chemistry in Relation to Theory.-But it was impossible to confine the chemists within these early and arbitrary limits; they pressed into the organic field, and were rewarded by the discovery of multitudes of new substances, many of them of great importance. They also made an unexpected conquest by forming, artificially, in the laboratory, various compounds which had hitherto been regarded as producible only under the influence of life. Much ingenuity was, however, at first expended in the. effort to bring the new facts into harmony with preëxisting theoretical views. The efforts, however, proved futile. A new chemistry sprang up in the new province, which, instead of being subordinated to old theories, powerfully re

acted upon them. And thus, from a neglected region, long supposed to lie beyond the bounds of the science, there came an influence that has changed its whole theoretical character. The modern ideas that are distinctive of the new system-changes by substitution, types, unitary groups, atomicity, and the controlling importance of molecular structure-have all arisen through the modern investigation of organic substances.

268. The Organic Elements.-Four substances make up the main bulk of organized bodies throughout the entire vegetable and animal kingdoms, viz., hydrogen, oxygen, nitrogen, and carbon. The properties of these bodies are remarkable. The three gases have never been condensed to the liquid or solid state by any application of mechanical force, although they are constantly reduced to these conditions by chemical action. Carbon, on the other hand, is an equally invincible solid, never having been liquefied or vaporized in its separate state. Hydrogen is the most attenuated of bodies, the unit of the chemical system, and the most widely-diffused element in Nature. Oxygen is the most abundant element on the globe we inhabit, has an extreme range of attractions, and forms compounds of all grades of stability. Nitrogen performs peculiar offices of the highest importance in the world of life, giving quality to the most complex and changeable organized compounds. Carbon is the common solidifying element in all organized products, and by its peculiar chemical relations stamps the character of this division of chemistry. Hydrogen is monadic, oxygen dyadic, nitrogen triadic, and carbon tetradic. The latter element, by its high multivalence, combines with itself in interminable series of radical groups, which become the skeletons or nuclei of a countless host of compounds by linking with atoms and groups of other elements. Organic chemistry, under this title, has in fact now disappeared, and so important is the part played by earbon that this division of the subject is known as the

"Chemistry of the Carbon-compounds." Prof. Cooke, indeed, does not hesitate to say that "the number of known compounds of this one element is far greater than those of all the other elements besides."

269. Isomerism.-The old conditions of analytic inquiry here obviously failed. It was not enough merely to analyze organic substances, and state the elements and the proportions of the elements that they contained. Analysis, in fact, now broke down so completely as to leave no alternative to chemists but to seek the explanation of the properties of bodies in their atomic arrangements; for compounds of the most diverse properties were found to consist of exactly the same elements in exactly the same proportions. Butyric acid, an oily liquid, not easily inflammable, which has the disgusting smell of rancid butter, and gives the acid reaction, has the formula C,H,O,; while acetic ether, a limpid liquid, non-acid, easily inflammable, and having the pleasant, fruity smell of apples, has also the formula C,H,O,. These substances are therefore said to be isomeric, a term meaning equal measure. There is no way of explaining this difference of properties, except on the theory that the constituent atoms are differently grouped in the two cases. And this is proved by acting on the two molecules with chemical agents, when they break up in very different ways, and give rise to different products. Isomeric compounds are often convertible into each other without loss or addition; their different properties must therefore be ascribed, not to the presence or proportions of certain elements, but to the influence of molecular clustering and structure.

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270. Kinds of Isomerism.-Isomeric phenomena are so important that they have been discriminated as of different kinds. If bodies have the same absolute composition, as in the example just quoted, they are said to be metameric compounds. But substances sometimes have not the same atomic composition, although represented as identical on a