Isomerism (Gr. equal, and part), a term used in chemistry to express the relation existing between those substances which, while they possess the same ultimate composition, exhibit essentially different chemical and physical properties. The term isomeric is often applied indiscriminately to several classes of bodies which will here be separately considered. Strictly speaking, it ought perhaps now to be confined to those cases in which the elements composing the dissimilar substances are both quantitatively and qualitatively the same. Difference of properties can here be readily explained by admitting that the elements of the different bodies are arranged or grouped in different ways. Thus both formiate of ethyle and acetate of methyle - very different substances - have the ultimate composition expressed by the formula C3H6O2 sometimes written C6H12O4. They are however proximately composed of

That such compounds really contain different proximate constituents is proved by the fact that they afford different products when decomposed under similar conditions. Hence they are regarded as distinct chemical substances, and not as modifications of one and the same body. The different grouping of the elements of these compounds has been compared to that of letters in words like ate, eat, tea, etc, on the arrangement of which the meaning of the word is entirely dependent. All bodies which are thus isomeric with each other, all that have absolutely the same ultimate composition, must of course possess identical equivalent weights. Such substances are often called metameric (Gr. indicating change or alteration), in contradistinction to polymeric substances (Gr. many), which are composed of similar elements united in the same relative proportion in each case, but in different absolute quantities; the equivalent weights in which these substances combine with other bodies being unlike. This distinguishes them from members of the preceding class, in which both the relative and absolute number of equivalents are the same. Very many polymeric substances are known, whole series of organic compounds being formed of them. As an example, olefiant gas and cetene both contain 85.7 per cent, of carbon and 14.3 per cent, of hydrogen. If nothing were known of their chemical comportment, the empirical formula CH2 would be applicable to both; but by studying their properties it has been found that 2C and 4H have united to form olefiant gas, the rational formula of which is therefore C2H4, and its combining equivalent 28; while 16C and 32H have united to form cetene, which has consequently the rational formula C16H32, and the equivalent number 224. Between these two bodies there are 14 others polymeric with them and with each other.

Differences like this have been compared to those between words like ma, mamma, tar, tartar, etc, which contain the same letters arranged in the same way, but in different quantities. The arrangement of the elements in polymeric substances is not however of necessity the same; thus, the ether of wood spirit is polymeric with common alcohol; yet the rational formula of the former is CH4O; of the latter, C2H6O.-Until a comparatively recent period it was the prevalent opinion among chemists that bodies of similar composition must of necessity possess similar properties. Any observations tending to throw doubt upon the correctness of this belief were considered erroneous. Even the discovery, by "WOhler and Liebig, that cyanic and fulminic acids are of like percentage composition although they possess very different properties, was attributed to errors of observation, and generally discredited. Faraday's investigation of several isomeric hydrocarbons in 1825 first proved the fallacy of this supposed law. Its exceptions, being now more carefully observed, were found to be very numerous. In 1830 Berzelius proposed that they should be classified as isomeric substances.

It was however soon perceived that the doctrine of isomerism could not with propriety be employed to explain the cause of all the differences which had been observed; least of all, to explain those which occur among the elements themselves - bodies which, from his inability to decompose them, the chemist is forced to regard as simple. In 1840 Berzelius suggested that these peculiarities might depend upon some absolute difference of quality in the different varieties of a substance, and not upon any dissimilarity in the arrangement or number of its molecules. He proposed the term allo-tropism (Gr. of a different nature) to express this idea, which has ever since steadily gained favor, although directly opposed to the doctrine of the immutability of matter, one of the principal tenets on which the chemistry of the first half of the present century was based. Allotropism is of special interest from the fact that several of the most common and best known elements may occur in two or more allotropic states. Thus, pure charcoal (lampblack), graphite, and the diamond are essentially identical chemical substances. The element phosphorus, as it commonly occurs, is as oft, waxy, yellowish white, exceedingly inflammable, and very poisonous substance, with a strong odor and taste, luminous in the dark, and readily soluble in bisulphide of carbon. It may easily be transformed, however, into another allotropic state, in which it is of a dark red, nearly black, color; is hard, brittle, and devoid of taste or smell, and, so far as is known, of poisonous properties; is not luminous, and is completely insoluble in bisulphide of carbon. It differs moreover from ordinary phosphorus in specific gravity, and entirely in its affinity for other substances.

Indeed, it is not known that it is itself combustible; for it may be heated without undergoing change to about 500° F., at which temperature it is reconverted into ordinary phosphorus. These two conditions of phosphorus are so utterly unlike in all their properties, excepting the weight of their equivalent, that were it not in the power of chemists to prove their identity by converting them one into the other, they would without hesitation be considered distinct elements. Similar instances occur among gases. For example, ordinary oxygen gas may be converted into an allotropic modification called ozone, which possesses properties entirely different from those of the original oxygen. Chlorine gas also, according to Prof. J. W. Draper of New York, after exposure to strong sunlight, possesses the power of combining with hydrogen even in the dark, and exhibits other properties unlike those of chlorine which has been kept from the light. Several other elements are known to be capable of existing in two or more allotropic states; and a considerable number of compound bodies occur under different modifications, which, it is not unlikely, may yet be found to depend upon the allotropism of one or more of their elements.

Indeed, these instances are so common that some chemists have been led to believe that most if not all of the elements may exist in distinct allotropic states. It has not as yet, however, been well ascertained to how great an extent the peculiar state of an element can influence the properties of the compounds it may form by uniting with other bodies. Schonbein, the discoverer of ozone, was confident that it exists, as such, chemically combined in several oxides. Other chemists have referred the dissimilar varieties of certain compounds of phosphorus, arsenic, etc, to the allotropism of their elements. Berzelius long ago pointed out that the different states of sulphide of mercury, iodide of mercury, etc, were probably to be attributed to a similar cause. Berthe-lot has advanced the opinion that the allotropic modifications of sulphur are intimately connected with, if not directly dependent upon, the electrical relation which this substance bears to the elements with which it is or has been united. When separated, by agents which are without action upon it, from those compounds in which it acts as an electro-positive body, as in sulphurous acid, it is amorphous and insoluble in bisulphide of carbon and other neutral solvents.

On the contrary, when obtained from compounds in which it plays the part of an electro-negative element, as in sulphuretted hydrogen, it is susceptible of crystallization, and is soluble in bisulphide of carbon, etc. Berthelot also states that the modifications of selenium exhibit a similar comportment, and has suggested that the different states of phosphorus may in like manner represent respectively electro-negative (ordinary phosphorus) and electro-positive (red phosphorus) conditions. It is worthy of remark that these views, which are of prime importance in their bearing upon the theory of substitutions, are almost identically the same with those concerning chlorine published some years since by. Prof. Draper. Although the correctness of the observations of both these chemists has been called in question by other observers, it cannot as yet be admitted that their views have been disproved; they still deserve the most careful consideration. The apparent relation between some of the phenomena of allotropism and those exhibited by substances when in the so-called nascent state (a phrase used in reference to the well established fact that many bodies can be made to combine with other substances with much greater facility at the instant when they escape from some of their combinations than at any other time) has been remarked by several chemists.

Intimately connected with this view is the theory of chemical polarity advanced by Brodie ("Philosophical Transactions," 1850, p. 759), who assumes that under certain conditions, as at the moment when a body enters into combination, a chemical difference exists between the particles of which the body is composed; so that these particles are to one another in a peculiar relation which is expressed by the terms positive and negative (+ and - ). Several of the phenomena of allotropism may be explained by this theory. Thus, ozone may be regarded as polarized (active) oxygen, while ordinary oxygen is that in which the positive and negative particles are combined, and in the quiescent state. In like manner ordinary white and red phosphorus represent respectively polarized and indifferent conditions. It is customary to speak of the different allotropic states of a substance as if each were something absolute, and not liable to any variation. But there are numerous facts which go to prove that this is not always the case, and that the peculiar characteristics of the allotropic conditions of several bodies are themselves subject to certain variations.

In support of this view may be instanced the great diversity of properties exhibited by different specimens of graphite and the various kinds of coke allied to it, or by the different sorts of sulphur. - In addition to the several classes of phenomena already alluded to, the peculiarities of which are strongly marked, there is another class of analogous facts which deserves mention. Many well known substances exhibit differences in hardness, color, specific gravity, solubility, etc, according to the circumstances in which they have been produced. Thus, carbonate of lime, when precipitated from a cold solution of a salt of lime, is readily soluble in an aqueous solution of chloride of ammonium; on the other hand, when in the form of marble it is scarcely at all soluble in this menstruum. Red oxide of mercury, which has been prepared by precipitation in the wet way, is decomposed with much greater facility when heated than that obtained by exposing nitrate of mercury to a high temperature. These differences, though subject to considerable variations, are rarely strongly marked. Since they do not affect to any great extent the chemical behavior of the substance, they are not classed as allotropic conditions, but are supposed to depend upon different states of aggregation of the substance.

Some of these variations are probably more intimately connected with allotropism than has heretofore been admitted; thus, the dissimilar properties exhibited by different specimens of silicic acid would now be attributed by most chemists to the known allotropism of its components. But most differences of this sort are so slight that they cannot be regarded as being dependent upon allotropism; they seem rather to be allied to those variations to which, as already stated, even the allotropic conditions of substances are themselves liable. It would appear indeed as if every substance, in each of its allotropic conditions, must have a point of maximum activity, at which point its properties are normal, subject however, like everything else in nature, to perturbations by which its peculiar properties may be somewhat changed. In compound bodies it is not always easy to distinguish between allotropism and isomerism properly so called; indeed, both may occur at once, i, e., both the arrangement and quality of the elements of two or more substances of the same ultimate composition may be unlike. There is also a large class of bodies to which the general term isomeric is still applied, some of which may be allotropic, while many are probably polymeric.

As examples may be mentioned the numerous metallic oxides which undergo changes when heated. The very remarkable circumstance noticed in this connection, that these bodies while undergoing change give off a quantity of heat which they must have previously possessed in a combined or latent form, has led some chemists to seek for an explanation of all the phenomena of allotropism by assuming that heat is a material constituent of substances, capable of modifying their properties according as it is combined with them in greater or less quantity. This is however entirely matter of conjecture, and, in view of our limited knowledge respecting the true nature of heat, can hardly be admitted. Nor has the direct influence of heat been proved in all the cases of allotropism which have been studied. That it is nevertheless intimately connected in some way with these phenomena is evident. This is of special interest in view of the changes which heat is known to effect in the ordinary conditions of matter; the solid, liquid, and gaseous forms, which all substances are supposed to be capable of assuming, being unquestionably dependent upon the temperature to which they are exposed. These conditions must not however become confounded with those dependent on allotropism, which are essentially different.

Other chemists have regarded allotropic modifications as dependent upon different states of aggregation of the hypothetical atoms of which, as they suppose, all bodies are formed. In their eyes, the chemical peculiarities of charcoal depend upon its amorphous state; those of the diamond are different because it is crystalline, and those of graphite unlike those of the diamond because its crystals belong to another system. They would call the ordinary state of phosphorus crystalline, the other condition amorphous, and refer all difference of properties to this difference of form. Diversity of crystalline structure, or its entire absence, is however evidently only one of the many differences of properties incidental to allotropism; in many cases it must be regarded as a consequence of the latter, by no means as its cause. At all events, the cases of allotropism which occur among gases cannot be explained by this theory. Others, without paying special attention to crystalline form, have supposed that all cases of isomerism, taken in its widest meaning, depend upon variations in the grouping of the molecules of bodies.

They even refer the instances which have here been classed under allotropism to differences in the arrangement of the particles of matter of which the elements themselves are composed. But few, however, now hold this opinion, the doctrine of allotropism being generally admitted. Although the mere term allotropism conveys no definite idea of the different conditions of matter which it indicates, and is, strictly speaking, nothing more than a convenient name for a class of phenomena as yet inexplicable, the fact which it denotes, that an element can exhibit the properties of two different substances, is of preeminent importance. Important contributions to our knowledge of isomerism have been made in modern times by But-lerow, Kekule, Erlenmeyer, and Gibbs. (See Allotropism.)