Page:The American Cyclopædia (1879) Volume IX.djvu/448

 434 ISOMEEISM ISPAHAN its allotropic conditions, must have a point of maximum activity, at which point its proper- tics are normal, subject however, like every- thing 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 sub- stances of the same ultimate composition may be unlike. There is also a large class of bodies to which the general term isomeric is still ap- plied, some of which may be allotropic, while many are probably polymeric. As examples may be mentioned the numerous metallic ox- ides 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 al- lotropism 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 con- nected 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 tempera- ture to which they are exposed. These condi- tions 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 hy- pothetical atoms of which, as they suppose, all bodies are formed. In their eyes, the chemical peculiarities of charcoal depend upon its amor- phous state ; those of the diamond are differ- ent because it is crystalline, and those of graph- ite nnlike those of the diamond because its crystals belong to another system. They would elill the ordinary state of phosphorus crystal- line, 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 inciden- tal to .allotropism; in many cases it must bo 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 admit- ted. Although the mere term allotropism con- veys 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 sub- stances, is of preeminent importance. Im- portant contributions to our knowledge of isom- erism have been made in modern times by But- lerow, Kekule, Erlenmeyer, and Gibbs. (See AIXOTKOPISM.) ISOMETRIC PROJECTION (Gr. lao(, equal, and fiirpov, measure), a species of drawing, used chiefly by engineers, in which the perspective plane of the paper must be imagined as making equal angles with the three principal dimen- sions of the figure, and the eye at an infinite distance. Thus lines in the three principal di- rections will be drawn on the same scale, and that scale the same for all parts of each line. ISOMORPHISM (Gr. lao (, equal, and popM, form), in chemistry, the property possessed by certain bodies of replacing each other in com- pounds without causing in these an essential change of crystalline form. The bodies that thus replace each other possess themselves similar forms, and are said to be isomorphous. Familiar examples of this mutual replacement in minerals are of the protoxides of iron and manganese, and of lime and magnesia. Chlo- rine, bromine, and iodine possess this relation toward each other; also arsenic and phos- phorus, and the acids of these elements. The term, as proposed by Mitscherlich, strictly signi- fies similarity of form ; it is now applied to sub- stances which are not only similar in their crys- talline form, but are analogous in their chemi- cal composition. The study of isomorphism has greatly facilitated the classification of com- pounds and the determination of atomic weights. ISOPODS, a group of 14-footed crustaceans, so called because their thoracic feet, the three anterior in one series and the four posterior in another, are nearly equal; the branchiee are six pairs and abdominal. Some inhabit the sea, where they are generally parasitic on other animals; others are terrestrial, living in dark and damp places, like the wood lice and sow bug. The eyes are sessile or not placed upon stalks, and the head is distinct from the seg- ment bearing the first pair of feet. ISOTHERMAL LINES. See CLIMATE. ISPAHAN, or Isfahan (anc. Aspadana), a city of Persia, of which it was formerly the capital, in the province of Irak-Ajemi, 210 m. S. of Teheran, in lat, 32 39' N., Ion. 51 44' E. ; pop. probably not more than 60,000. It stands in