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The point of typical composition in the case of the crystals of Sb Zn” was still further marked in a most decided manner by a very remarkable property. It has already been stated that this compound has the power of decomposing water with rapidity at 100° C.; but this is true only of those crystals which have approximately the theoretical composition. During the course of my investigation I determined the quantity of hydrogen evolved by alloys of different composition during a given time, taking care, of course, that the circumstances should be the same in all cases; and I found that with the alloy containing 43 per cent of zinc, there is an immense maximum, confined at most between 2 per cent on either side, the alloy of 48 per cent yielding over nine times as much gas as an alloy of 50 per cent, although the crystals of the last were fully as definite as those of the first.*
It is evident from the above facts, that the points corresponding to the theoretical composition of the two compounds of zinc and antimony, are also points of maxima and minima of various properties. Now I have no doubt that the same truth will be found to hold in the mineral kingdom. In a mineral like tourmaline or mica, for example, the specimen having the exact theoretical composition may probably be discovered by examining a large number of specimens, and discussing their various physical properties. All the physical properties may be of value in this connection, such as lustre, hardness, specific gravity, specific heat, &c.; and no mechanical rules can be laid down. Much must depend on the discretion of the observer; and in any cases such properties will be selected as are best adapted to the circumstances of the case. In comparing different crystals of the same mineral, it is obviously important to select such as have been formed in a different matrix; for it is only with such that we should be led to expect great variations of composition. It is also evident that the phenomena would be complicated when there has been a substitution of isomorphous elements; and until the effect of such substitution on the physical properties can be traced, it will be necessary to select specimens of as uniform a constitution as possible.
With one other consideration I will close this paper. The principle which has been here discussed must modify materially our notion of a mineral species. The idea of a mineral species has hitherto involved chiefly two distinct characters:–first, a definite crystalline form; second, a constant general formula; and any important variation in either of these characters has been regarded as equivalent to a change of species. Rutile and anatase are regarded as different species, because their crystalline forms are slightly different, although both minerals have identically the same constitution; and again, magnetite and Franklin
* See Table in the memoir before cited.
ite, which have the same form, are regarded as different species, because they have a slightly different composition. It is true that the actual composition of a mineral may vary very greatly by the substitution of isomorphous elements, and yet, if the general formula remains constant, the species may not be changed. But the extent to which such substitution can be carried without changing the species is not so well settled among mineralogists as could be desired, and the same rule is not applied to all species. The difference between the varieties of garnet, for example, is as great as that between the species magnetite and Franklinite. Leaving, however, this point undetermined, all mineralogists have agreed that any essential change in the general formula was inconsistent with the idea of the same species. The result, however, of my investigation is to show that the general formula of a mineral species may vary also, or, as I should rather say, the general formula is not necessarily the actual formula of each given specimen, but only the typical formula of the species towards which the mineral tends, and which it would unquestionably reach if it could be several times recrystallized. According to this view, the general sormula represents not the actual constitution of the mineral, but only a certain typical consposition, which perhaps is never realized with any actual specimen. The fact that the composition of a mineral species may be modified by the substitution of isomorphous elements, was first established by Mitscherlich, and has long been an admitted principle in mineralogy. We must now, as I think, still further expand our idea of a mineral species, and admit that its composition may be modified by an actual variation in the proportions of its constituents. Thus it is that in mineralogy, as in other sciences, we are led to admit the truth of that maxim which every advance in true knowledge seems to verify, “Natura non facit saltus.” While the results of my investigations thus serve to render the idea of a mineral species less definite than before, I cannot but hope that they will tend ultimately to simplify the whole subject of mineralogy; for not only may we expect to reduce the number of mineral species, but also, by simplifying the general formulae of those which remain, to classify the whole with a greater precision than is now possible. To do this, however, implies a careful revision of the whole subject-matter of mineralogy on the principles above given, a labor of which few can appreciate the extent, except those who are familiar with the methods of physical research. The work cannot be done by any one person; and it is the chief object of the present paper to call the attention of mineralogists to the importance of the subject. I have not thought it necessary to dwell in this paper on the obvious distinction between the phenomena here in considera
tion, and those of isomorphism. It was shown in my previous memoir, that the variation in the composition of the crystals of Sb Ziro and Sb Zn” could not be explained by this principle; and the distinction between the two classes of phenomena has been still further illustrated by a recent investigation on the crystals formed in alloys of copper and zinc, made in my laboratory by Mr. F. H. Storer. These crystals, which are undoubtedly mixtures of isomorphous elements, give no indications whatever of points of typical composition,-thus illustrating not only the characters of an isomorphous mixture, but also the distinction between such a mixture and a true chemical compound. Admitting, then, the possibility of a variation of composition in a mineral species, independent of the phenomena of isomorphism, it becomes of importance to distinguish this new class of phenomena by a separate term; and I would propose for this purpose the word Allomerism. By this word I would designate a variation in the proportions of the constituents of a crystallized compound without any essential change in the crystalline form. If then, we also use the word typical to indicate the condition of definite composition, we may speak of those specimens of a mineral species which contain an excess of one or the other constituent, as allomeric variations from the typical composition. The degree of allomerism would then be measured by the excess of the allomeric constituent above the typical composition. Thus the crystals of Sb Zno containing 42:3 per cent of zinc would be said to have the typical composition; while those containing 55 per cent of zinc would be distinguished as an allomeric variety, the degree of allomerism in this instance amounting to 12 per cent, and zinc being the allomeric constituent. In the case of the mineral Discrasite, it is probable that no specimen having the typical composition has yet been analyzed. Those specimens whose analyses are given in Dana's ‘System of Mineralogy,’ are all probably allomeric varieties of the mineral, silver being the allomeric constituent, and the degree of allomerism varying from 4 to 7 per cent. It is unnecessary, however, to multiply examples, as the above are sufficient to illustrate the use of the term.
ART. XIX.-Notices of several American Meteorites; by . CHARLES UPHAM SHEPARD.
1. Nebraska Iron.—This very interesting mass, first noticed in a late number of the Proceedings of the St. Louis Academy of Sciences, was brought to St. Louis by the fur traders in the employ of C. P. Chouteau, Esq., about two years ago, and by him presented to the museum of the Academy. It was found near the Missouri River, between Council Bluff and Fort Union. It originally weighed about thirty-five pounds, but is now reduced to twenty-nine. Its shape was an oblong, compressed ovål, not unlike that of the Chesterville, South Carolina, iron-mass, which has been compared by me to the form of a thick, blunt edged fresh water clam (Unio). Its surface is as black and smooth as that of the Braunau Iron, from which however it differs in being more even and smooth, though it is not destitute of the usual indentations belonging to meteorites, but these are by no means aniform in their occurrence over the entire surface. The crust is everywhere extremely thin, amounting to scarcely more than a mere varnish; and what is very remarkable, is often insufficient to hide the Widmannstättian figures with which the body must have been covered before it entered our atmosphere. The lines are not equally displayed throughout, and indeed will generally require a single lens in order to be distinctly seen. Nor have they the same beautiful regularity as when obtained by etching upon a polished surface from the interior. They are moreover curiously knotted, so as to resemble, under the microscope, the blunted teeth of a fine saw blade. The configuration upon the etched plates of this iron resembles slightly that of the Texas mass, though the bars are much more rectilinear, and in this respect approach nearer to the new African irons described by me (those from Namaqua Land and Orange River). I observe, however, in this as well as in most other irons, that the fullest regularity of internal structure does not prevail, until some little depth from the outer surface or crust is reached. The Nebraska iron is quite free from earthy, plumbaginous or pyritic matter. Prof. Litton, of the St. Louis University, has recognized in it the presence of nickel; and is understood to be now engaged with its complete analysis. Its specific gravity is 7-735. The character of the surface renders it certain that this mass must be of very recent fall. I am indebted to the liberality of the Academy, and the kind offices of Nathaniel Holmes, Esq., its Secretary, for a fine slice of the mass, from which I have been enabled to make the foregoing observations. 2. Forsyth (Taney County, Missouri) Iron-My first information of this locality was derived, while on a visit to southeastern Missouri in April last, from N. Aubushon, Esq., of Ironton. He stated that a small specimen of curiously knitted, malleable ore, of a white color resembling silver, had been sent him two or three years ago by a person residing near the locality. Mr. Aubushon forwarded it to an assayer at Ducktown, Tenn., from whom he learned that it was composed of iron and nickel. On visiting St. Louis soon after, I was informed by Prof. Swallow, the State Geologist, that a specimen had also been transmitted to him by letter from the same place; and that Prof. Litton had found it to be composed of similar constituents. Prof. Swallow presented me a small fragment of his specimen, upon which I am able to offer a few remarks, awaiting the results of Prof. Litton's analysis, for fuller information. The mass evidently belongs to the rather rare group of amygdaloidal meteoric irons, in which, like those of Steinbach (Saxony) and Hainholz (Westphalia), the peridotic ingredient preonderates over the nickelic iron. Its specific gravity is 4.46. he iron is remarkable for its whiteness, while the peridot is of a well marked green color, and distinctly crystalline. No pyrites is visible in the very small fragments examined. It is reported that two considerable masses of this meteorite were found buried in the soil upon a hill-side; and that they are at present secreted under the belief that they contain silver. 3. Bethlehem (New York) Meteoric Stone, of Aug. 11, 1859.The only stone found from the great explosion heard over a large district of northwestern Massachusetts, and extending into the state of New York as far as ten miles west of the cities of Albany and Troy, was the little fragment, less in size than a pigeon's egg, of which an outline is here subjoined. I am imdebted to David A. Wells, Esq., the editor of the American Scientific Annual, for several interesting particulars concerning its discovery and properties. He was good enough to visit, at my request, the residence of Mr. Garritt Vanderpool (situated seven miles from Albany and one mile west of Bethehem church), where the stone fell, and to ascertain on the spot the facts respecting its descent. Mr. Wanderpool was at work near his house, and heard the explosion in common with other members of his family. About two minutes after, as it appeared to him, a stone, coming in an oblique course, struck the side of a waggon-house, glanced off, hit a log upon the ground, bounded again, and rolled into the grass. A dog lying in the doorway of the waggon-house sprang up, darted out and seized it, but dropped it immediately, probably on account of its warmth and sulphureous smell. Mr. Wells had two opportunities of inspecting the stone before it was sold to the State Cabinet in Albany. It was far from being entire when first picked up, no doubt having been broken by its contact with the house. On the second inspection, he noticed that one corner had been broken away, and other portions much marred through the use of knife blades upon its surface by the curious, who, in this rude way, had been led to investigate its peculiarities. About “one-half of it however,” he observes, “is covered with the peculiar dark colored crust of meteorites, and has a burnt appearance. This is so well marked that it at once establishes its identity as a meteoric stone. The other sides present