SCIENCE. 



209 



ered who have passed beyond tribal society to national so- 

 ciety based on property, i.e., that form of society which is 

 characteristic of civilization. Some peoples ma} - not have 

 reached kinship society ; none have passed it. 



Nations with civilized institutions, art with palaces, 

 monotheism as the worship of the Great Spirit, all vanish 

 from the priscan condition of North America in the light of 

 anthropologic research. 



Tribes, with the social institutions of kinship, art with 

 its highest architectural development exhibited in the struc- 

 ture of communal dwellings, and polytheism in the worship 

 of mythic animals and nature-gods, remain. 



THE GENESIS OF CERTAIN IRON ORES.* 

 By Dr. T. Sterry Hunt, LL. D., F.R.S. 



Dr. Hunt began by considering the presence of iron, gener- 

 ally in a ferrous condition, in mineral silicates, in the crys- 

 talline rocks, and its liberation therefrom by the sub-aerial 

 decay of these as hydrous ferric oxide. This, as is well 

 known, is, by the agency of organic matter, again reduced 

 to ferrous oxide, which is dissolved in natural waters by 

 carbonic acid or some organic acid, from which solutions 

 it may be deposited either as hydrous peroxide (limonite, 

 etc.,) as carbonate (siderite), as silicate, or as sulphide 

 (pyrite, etc.), in all of which forms iron is found in sedi- 

 mentary deposits. As regards the formation of siderite, he 

 described experiments which show that solutions holding 

 five grammes of ferrous carbonate dissolved as di-carbonate 

 in a litre of water, are spontaneously decomposed in close 

 vessels at the ordinary temperature, and deposit two-thirds 

 of their iron as a white crystalline (hyd rated) mono-carbon- 

 ate, with liberation of carbonic-dioxide. This serves to 

 render more intelligible the reduction and segregation of 

 iron as siderite in earthy sediments, as long since pointed 

 out by W. B. Rogers, for the ores of the coal-measures. 



The intervention of soluble sulphates, and their reduc- 

 tion through organic agenq- to sulphides, determines the 

 formation of sulphide of iron in sediments. The genera- 

 tion of a bi-sulphide (pyrite or marcasite) was then dis- 

 cussed, and it was shown that the ferrous mono-sulphide, 

 which naturally is first generated, may fix a further portion 

 of sulphur and thus form a more stable compound. One 

 example of this is seen when recently precipitated hydrous 

 ferrous sulphide is brought in contact with a solution of a 

 ferric salt, which takes up a portion of the iron, leaving 

 sulphur free to unite with the undecomposed sulphide, and 

 form therewith a very stable higher sulphide of iron. Ex- 

 periments now in progress lead the writer to believe that 

 sulphur liberated from soluble sulphides may, in a similar 

 manner, unite with ferrous sulphide, and thus help us to 

 explain the generation of pyrites in nature, in the presence 

 of water, at ordinary temperatures. 



The changes of siderite and pyrite under atmospheric in- 

 fluences were next considered. The latter by oxidation 

 yields, as is well known, ferrous sulphate. Its frequent 

 conversion by sub-aerial decay into limonite was conceived 

 to be due to the intervention of water, holding carbonates, 

 which, conjointly with oxygen, changes it into hydrous 

 peroxide (limonite), which often retains the form of the 

 pyrites. The transformation of carbonate of iron into 

 hydrous peroxide is a familiar fact. 



Limonite ores may thus be produced in three ways. 

 They are sometimes formed by the peroxidation and preci- 

 pitation of dissolved ferrous salts, as in the so-called bog- 

 ores ; but more frequently from the alteration in situ of de- 

 posits of pyrite or of siderite. Such are the limonites 

 which mark the outcrops of beds or veins of pyrites in the 

 decayed crystalline rocks of the Blue Ridge. The similar 

 ores found in the decayed Taconic schists of the great Ap- 

 palachian valley can be shown to be due in some cases to 

 the alteration of included pyritous masses, and in others to 

 the alteration of similar masses of siderite, both of which 

 are found in the unaltered Taconic rocks, as, indeed, at 

 various other horizons in the geological scries. 



If wc take the specific gravity of pyrites at 5.0, we shall 

 find that its complete conversion into a limonite of sp. gr. 



* Read before the A. A. A. S., Boston, 1880. 



4.0 would be attended with a contraction of only 2.7 hun- 

 dredths, while if the limonite have a sp. gr. of 3.6, there 

 would be an augmentation of 10.7 p. c. With siderite of 

 sp. gr. 3.6, on the contrary, its conversion into limonite of 

 the same density would result in a contraction of 19.5 p. c, 

 and into lemonite of sp. gr. 4.0 to a contraction of 27.5 p. c. 

 The evidences of this contraction may be seen in the struc- 

 ture of the limonite derived from siderite. The process 

 operates from the surface of the masses, often resulting in 

 the production of geodes. Their structure will generally 

 serve to distinguish the sideritic from the pyritic limonites. 

 These differences were illustrated in the history of various 

 iron ores in the Appalachian valley, and it was further 

 pointed out that the pyritic limonites, other circumstances 

 being equal, should be freer from phosphorus than those 

 derived from siderite, since the native carbonates almost 

 always contain phosphates, from which pyritous deposits 

 are comparatively free. The source of limonites thus be- 

 comes a question of importance to the metallurgist. In 

 conclusion it wus pointed out that deposits of manganese 

 ores are, in some cases at least, generated by the alteration 

 in situ of manganous carbonates, by a process analogous 

 to that by which limonite is produced from siderite. — 



MICROSCOPY. 



NEW CELL FOR OPAQUE OBJECTS. 



I desire to call the attention of the microscopists and 

 preparers of objects generally to the new rubber cell for 

 opaque slides, recently devised by me. A considerable 

 experience in mounting opaque slides during the past few 

 years has convinced me that much of the labor incident to 

 it could be avoided, if a cell of suitable material and shape 

 could be produced at a nominal cost. This, I think, has 

 now been attained, and I take pleasure in submitting one 

 lor which I claim convenience, cheapness, and general 

 utility. With it the amateur can produce a slide fully as 

 perfect, and with as great a degree of neatness as can the 

 professional. The cell is of hard rubber, highly polished, 

 and of attractive shape ; the base is solid, thus giving a 

 black back-ground of rubber ; around the top is a ledge 

 fitted to receive a one-half inch cover glass ; this, being 

 secured by a little shellac or any similar cement, com- 

 pletes the mounting. The cell may be attached to a glass 

 slip by any cement, before or alter preparation. For ex- 

 changes it offers superior advantages, inasmuch as the cell, 

 with objects enclosed, may be sent through the mails in- 

 dependent of the glass slips, the recipient attaching them. 

 In this way a saving is made in postage, and no risk of loss 

 by slips being broken in transit. 



They will solve the problem which often perplexes the 

 student or collector who is crowded for cabinet room. 

 Many objects for future reference may be mounted in this 

 simple cell, numbered and put away without a slide, a 

 cabinet drawer holding two hundred of them, while but 

 forty slides could be accomodated in the same space. 



The above sectional view conveys a good idea of its 

 shape, the dotted line indicating the position of the thin 

 glass cover. 



1 have made arrangements to have them supplied by the 

 following firms at thirty cents per dozen, five cents extra 

 on single dozens to cover cost of postage and box, and they 

 may be obtained from the parties mentioned below or from 

 the subscriber. In remitting small sums three cent postage 

 samps mav be used. 



Geo. S. Woolman, No. 116 Fulton St., New York ; Jas. 

 W. Queen & Co., Chestnut St., Philadelphia; Bausch & 

 Dransfield, Arcade, Rochester, N. Y. ; W. H. Bullock, 

 No. 126 So. Clark St., Chicago, Ills. 



In conclusion I would add that I have had these rubber 

 cells prepared without regard to any pecuniary gain to 

 myself, hoping they may prove an aid to those engaged in 

 microscopical research. 



H. F. Atwood, 

 No. 50 Hamilton Place, Rochester, N. Y. 



[We have seen a sample of Mr. Atwood's rubber cell, 

 and consider it a very perfect arrangement for opaque 

 objects.— Ed.] 



