Page:The American Cyclopædia (1879) Volume IX.djvu/447

 ISOMERISM 433 of chlorine which has been kept from the light. Several other elements are known to he capa- ble 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 com- mon that some chemists have been led to be- lieve 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 confi- dent that it exists, as such, chemically combined in several oxides. Other chemists have referred the dissimilar varieties of certain compounds of phosphorus, arsenic, &c., to the allotropism of their elements. Berzelius long ago pointed out that the different states of sulphide of mercury, iodide of mercury, &c., were proba- bly to be attributed to a similar cause. Berthe- lot has advanced the opinion that the allotro- pic modifications of sulphur are intimately con- nected 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 com- pounds 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 ob- tained from compounds in which it plays the part of an electro-negative element, as in sul- phuretted hydrogen, it is susceptible of crys- tallization, and is soluble in bisulphide of car- bon, &c. Berthelot also states that the modi- fications of selenium exhibit a similar comport- ment, and has suggested that the different states of phosphorus may in like manner rep- resent respectively electro-negative (ordinary phosphorus) and electro-positive (red phos- phorus) conditions. It is worthy of remark that these views, which are of prime impor- tance in their bearing upon the theory of sub- stitutions, 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 de- serve the most careful consideration. The ap- parent relation between some of the phenome- na of allotropism and those exhibited by sub- stances 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 facil- ity at the instant when they escape from some of their combinations than at any other time) has been remarked by several chemists. In- timately 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 combi- nation, 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 ). Sev- eral 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 respective- ly 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 condi- tions of several bodies are themselves subject to certain variations. In support of this view may be instanced the great diversity of prop- erties 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, &c., 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 solu- tion 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 pre- cipitation 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 18- timately 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 com- ponents. 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 al- ready stated, even the allotropic conditions of substances are themselves liable. It would ap- pear indeed as if every substance, in each of