(6.) Lepidodendron and its allies, to which the spore-cases in question appear to belong, are evidently much less important to coal accumulation than Sigillaria, which cannot be affirmed to have produced spore-cases similar to those in question, even though the observation of Goldenberg as to their fruit can be relied on; the accuracy of which, however, I am inclined to doubt.

On the whole then, while giving due credit to Prof. Huxley and those who have preceded him in this matter, for directing attention to this curious and no doubt important constituent of mineral fuel, and admitting that I may possibly have given too little attention to it, I must maintain that Sporangite beds are exceptional among coals, and that cortical and woody matters are the most abundant ingredients in all the ordinary kinds; and to this I cannot think that the coals of England constitute an exception.

It is to be observed, in conclusion, that the spore-cases of plants, in their indestructibility and richly carbonaceous character, only partake of qualities common to most suberous and epidermal matters, as I have explained in the publications already referred to. Such epidermal and cortical substances are extremely rich in carbon and hydrogen; in this resembling bituminous coal. They are also very little liable to decay, and they resist more than other vegetable matters aqueous infiltration; properties which have caused them to remain unchanged and to resist the penetration of mineral substances more than other vegetable tissues. These qualities are well seen in the bark of our American white birch. It is no wonder that materials of this kind should constitute considerable portions of such vegetable accumulations as the beds of coal, and that when present in large proportion they should afford richly bituminous beds. All this agrees with the fact, apparent on examination of the common coal, that the greater number of its purest layers consist of the flattened bark of Sigillaria and similar trees, just as any single flattened trunk imbedded in shale becomes a layer of pure coal. It also agrees with the fact that other layers of coal, and also the cannels and earthy bitumens appear, under the microscope, to consist of finely comminuted particles, principally of epidermal tissues, not only from the fruits and sporecases of plants, but also from their leaves and stems. The same considerations impress us, just as much as the abundance of spore-cases, with the immense amount of the vegetable matter which has perished during the accumulation of coal, in comparison with that which has been preserved.

I am indebted to Dr. T. Sterry Hunt, for the following very valuable information, which at once places in a clear and precise light the chemical relations of epidermal tissue and spores

with coal. Dr. Hunt says--"The outer bark of the Cork tree and the cuticle of many if not all other plants consists of a highly carbonaceous matter, to which the name of suberin has been given. The spores of Lycopodium also approach to this substance in composition, as will be seen by the following, one of two analyses by Duconi,* along with which I give the theoretical composition of pure cellulose or woody fiber, according to Payen and Mitscherlich, and an analysis of the suberin of Cork, from Quercus suber, from which the ash and 2.5 per cent of cellulose have been deducted.+

This difference is not less striking when we reduce the above centesimal analyses to correspond with the formula of cellulose, CH,O,, and represent Cork and Lycopodium as containing 24 equivalents of carbon. For comparison I give the composition of specimens of Peat, Brown Coal, Lignite, and Bituminous Coal.‡

24 20 20

It will be seen from this comparison that, in ultimate composition, Cork and Lycopodium are nearer to Lignite than to woody fiber; and may be converted into coal with far less loss of carbon and hydrogen than the latter. They in fact approach closer in composition to resins and fats than to wood, and moreover like those substances repel water, with which they are not easily moistened, and thus are able to resist those atmospheric influences which effect the decay of woody tissue.”

I would add to this only one further consideration. The nitrogen present in the Lycopodium spores no doubt belongs to the protoplasm contained in them, a substance which would soon perish by decay; and subtracting this, the cell-walls of the spores and the walls of the spore cases would be most suitable material for the production of bituminous coal. But this suitableness they share with the epidermal tissue of the scales of *Liebig and Kopp, Jahresbuch, 1847-48. + Gmelin, Handbook, xv, 145. Canadian Naturalist, vi, 253.

strobiles, and of the stems and leaves of Ferns and Lycopods; and above all with the thick corky envelope of the stems of Sigillariæ and similar trees, which as I have elsewhere shown,* from its condition in the prostrate and erect trunks contained in the beds associated with coal, must have been highly carbonaceous and extremely enduring and impermeable to water. In short, if instead of "spore-cases,' we read "epidermal tissues in general, including spore-cases," all that Huxley has affirmed will be strictly and literally true, and in accordance with the chemical composition, microscopical characters and mode of occurrence of coal. It will also be in accordance with the following statement, which I may be pardoned for quoting from my paper on the Structures in Coal, published in 1859.

"A single trunk of Sigillaria in an erect forest, presents an epitome of a coal-seam. Its roots represent the Stigmaria underclay; its bark the compact coal; its woody axis, the mineral charcoal; its fallen leaves (and fruits), with remains of herbaceous plants growing in its shade, mixed with a little earthy matter, the layers of coarse coal. The condition of the durable outer bark of erect trees concurs with the chemical theory of coal, in showing the especial suitableness of this kind of tissue for the production of the purer compact coals. It is also probable that the comparative impermeability of the bark to mineral infiltration, is of importance in this respect, enabling this material to remain unaffected by causes which have filled those layers consisting of herbaceous materials and decayed wood, with pyrites and other mineral substances."

ART. XXXVIII. On a method of fixing, photographing and exhibiting the Magnetic Spectra; by ALFRED M. MAYER, Ph.D.

THE figures produced in iron-filings, when these are set in momentary vibration on a surface placed over a magnet, have received considerable attention from natural philosophers. The geometrical discussion of these spectra made by Lambert, Roget and others, have developed their symmetrical properties, and thereby have evolved the law of that action which emanates from the magnet. De Haldat has used them as a means of exploring the distribution and intensity of the effect of juxtaposed magnets variously arranged. But, above all, * Vegetable structures in Coal, Journ. Geol. Soc., xv, 626. Conditions of Accumulation of Coal; ib. xxii, 95. Acadian Geology, 197, 464.

"

See a neat Démonstration par le calcul des courbes magnétiques de la loi de l'inverse du carré de la distance," by M. Cellerier, published as a note on p. 592, vol. 1 of De la Rive's Traité d'Électricité.

have the researches of Faraday and W. Thomson on "the magnetic field" and on "the lines of magnetic force " given to these spectra- even when merely regarded as conventional symbolsan importance which has been fully shown; especially by Faraday, who was guided by their consideration to some of his most important discoveries. They have thus risen to so high a theoretical importance that a method which will fix them without danger of distortion, photographically reproduce them and readily serve to exhibit them to the largest audiences, will, I imagine, be acceptable to both investigators and lecturers.

The only process of fixing these spectra, known to me, is that practiced by De Haldat and Faraday, which, however, is but an application to the magnetic spectra of the method previously invented by Savart for preserving the Chladni figures of vibrating plates. In this process the spectra, produced in the usual manner either on glass or card-board, have pressed upon them a sheet of paper coated with mucilage, to which the filings adhere. In this operation of the transfer many particles are deranged from their positions and the figures are yet more distorted by the shrinkage of the wet paper, and are therefore not fit to serve in measures of precision; while the impressions cannot be exhibited with much more facility than the originals.

My process is as follows: a clean plate of thin glass is coated with a firm film of shellac, by flowing over it a solution of this substance in alcohol,* in the same manner as a photographic plate is coated with collodion. After the plate has remained a day or two in a dry atmosphere, it is placed over the magnet, or magnets, with its ends resting on slips of wood, so that the under surface of the plate just touches the magnet. Fine ironfilings, produced by "draw-filing" Norway iron, which has been repeatedly annealed, are now sifted uniformly over the film of lac by means of a fine sieve. The spectrum is then produced on vibrating the plate, by letting fall vertically upon it, at different points, a light piece of copper wire. The plate is now cautiously lifted vertically off the magnet and placed on the end of a cylinder of pasteboard, which serves as a support in bringing it quite close to the under surface of a cast-iron plate (I ft. diam.in. thick), which has been heated over a large Bunsen-flame. Thus the shellac is uniformly heated and the iron-filings, absorbing the radiation, sink into the softened film and are "fixed."

I generally allow the heat to act until the metallic lustre of the filings has disappeared, by sinking into the shellac, and the

The shellac dissolved in strong alcohol is allowed to stand a week or more. and the clear supernatant solution is then decanted.

film appears quite transparent. This degree of action is necessary when photographic prints are to be made from the plate, but when they are to be used as lantern slides I do not carry the heating so far. After the plate has cooled, it is allowed to fall upon its ends, on a table, so that any filings which have not adhered may be removed.

A short experience will give the proper strength of shellac solution to obtain a film so thick as just to be sufficient to hold the filings, and the requisite amount of heat to firmly cement them, without injuring the transparency of the film.

The plates can now serve (1) for the most accurate measures upon the magnetic-field; (2) for a photographic positive, which, in the printing-frame will produce the lines in white upon a dark ground, giving most beautiful and distinct impressions;* (3) or, if it is required to exhibit these figures to an audience, the plates are provided with glass covers, kept from touching the spectra by intervening slips of card-board, and there result "slides," in every way fit for giving a fine exhibition, when the images are projected upon a screen. I have thus obtained images, clear and sharp, of over 12 feet in diameter.†

By this process many plates have been produced; showing the action of single magnets of various forms, and of juxtaposed bars; as well as the effects of electric currents led by wires through holes drilled in the plates. Those exhibiting the inductive action of magnets on bars of soft iron and the interaction of magnets and electric currents are peculiarly interesting. An approximate representation of the resultant lines of the terrestrial magnetic action has been obtained by magnetizing equably tempered steel discs of from 2 ins. to 3 ins., and even more, in diameter. The magnetic axis or axes of these discs are predetermined by making them the continuations of the axes of very powerful electro-magnets, terminated with cones of soft iron with slightly rounded apices. The arcs of the great circles, including the terrestrial magnetic poles, having been calculated, the axes of the electro-magnets are inclined to that angle, while the steel disc is held close to their poles. On passing the current the disc is magnetized and we have an approximate representation of a section of the earth's magnetic effect. These results when viewed as photographic prints, or, as exhibited by the lantern, are so beautiful and instructive as to appear to me to warrant this

* Photographic prints from a series of eight of these plates I have presented to Harvard College; American Academy of Sciences; Sheffield Scientific School; Columbia College; Stevens Institute of Technology, Hoboken; Lehigh University, Pa.; American Philosophical Society; Franklin Institute; Peabody Institute, Balt.; Smithsonian Institution; Chicago Academy of Sciences; and to the University of Virginia-where they can be examined by the readers of this paper. Several of these are 16 ins. long by 10 wide.