TABLE A.—Showing the Temperature produced by Solar Radiation at Noon, for each degree of Latitude, when the Earth is in Aphelion. Northern Hemisphere : TABLE B.-Showing the Temperature produced by Solar Radiation of the Earth's orbit; also the gradual DIMINUTION of Temperature during the first half, and the gradual INCREMENT of Temperature during the second half-year :— TABLE C.-Temperatures produced by Solar Radiation, June 26, 1871, compared with the entered in the Table constructed 1870, for corresponding Zenith distances. Mean discrepancy = 90'70 5.86 Temperatures. 026° Fah. : 1'31 85.95 'II 85.76 0.92 85.58 0'74 85.43 0'59 84'90 0'06 84.86 84.99 85.07 85.83 86.03 I'19 87.22 *38 87'47 2.63 88.71 3.87 88.95 4'II 90'01 5'17 9016 5°32 90 66 5.82 his recent work "Le Soleil" (p. 267). “During a great number of observations made at Rome," says the author, "the difference between the two temperatures (that indicated by the thermometer exposed to the sun and that of the surrounding casing), was 12'06° (21'70° Fah.); during days when the sky was clearer, it rose to 14°." Consequently, the highest temperature indicated by the instrument referred to, was 25'2° Fah., against 66'04°, which is the true maximum solar intensity in the latitude of Rome. It will be seen then, that, by exposing the bulb of the thermometer in the manner pointed out, it is possible to reduce the temperature produced by solar radiation to 0'38 of the actual temperature. It will be proper to observe with reference to the accompanying tables-constructed in accordance with the result of investigations continued winter and summer during three years-that the opinion expressed by the Director of the Roman Observatory, respecting solar intensity at different seasons, is wholly at variance with the facts established by my numerous observations. The question was raised last summer whether the high temperature during the "heated term" would not charge the atmosphere with an additional amount of vapour capable | of retarding the passage of the heat rays, thus rendering the figures entered in my tables to some extent unreliable. Accordingly, during the solstice June 26, 1871, the sky being very clear, the actinometer was put in operation for the purpose of ascertaining with critical nicety whether the atmosphere which had been loaded with vapour for several weeks previously possessed any unusual property tending to check the heating power of the sun's rays. The observations were made late in the afternoon under great zenith distance and increased atmospheric depth, in order to subject the heat rays to an additional retardation from the supposed vapours. The result is recorded in Table C, by which it will be seen that the reduction of temperature was only o'26° Fah., a difference too small to call for any explanation. The result of the observations made during midwinter are equally conclusive with reference to the permanency of solar energy at all seasons. Among others may be mentioned that of January 17, 1871, the zenith distance being 61° 30', the actinometer remained perfectly stationary at 58 73° Fah., from 12h. 10m. to 12h. 20m. P.M. The table just referred to shows that on June 26, 1871, the actinometer indicated 53.08° when the sun's zenith distance was 61° 30'. Hence during midwinter the temperature proved to be 58.73°- 5308-565° higher, for corresponding zenith distance, than during the summer solstice. By reference to Table B it will be seen that owing to the diminished distance between the sun and the earth, the increment of temperature on January 17, ought to have been 5'75°, discrepancy o'1° Fah. In the face of such facts it is idle to contend that the temperature produced by solar radiation under corresponding zenith distance and a clear sky, varies from any other cause than the varying distance between the sun and the earth. Of course there are many regions in which the sun, in consequence of local peculiarities, but seldom acts with maximum energy. Alaska, for instance, is hardly ever favoured with a full amount of solar heat; nor does Rome, we are now informed by the Italian physicist, receive maximum solar heat excepting during winter, owing, it may be imagined, to the absorptive power of the atmosphere of the Campagna during summer. Without entering the field of speculation, let us consider that the established diminution of solar heat on the ecliptic, nearly 18° Fah., proves the existence of a powerful retarding medium, and points to the presence of a permanent mass of aqueous matter in the higher regions of the atmosphere; necessary, it may be urged, to regulate terrestrial temperature and render vegetable life possible under the destructive vicissitudes of heat and cold, inevitable in the absence of a permanent regulator. The assumption that the supposed mass of aqueous matter is nearly invariable, and at all times present, can alone account satisfactorily for the remarkable fact that, whenever a clear sun is presented, either by the opening of the clouds or by their disappearance, the actinometer indicates the same temperature, subject only to the variations depending on the sun's zenith distance, and the varying position of the earth in its orbit. The variation of temperature produced by the latter cause is entered in Table B, for every fifth day in each month. This table, an extract from a more elaborate one showing the temperature for every day in the year, the meteorologist will find indispensable to harmonise observations made at different seasons. It may be mentioned that the attempt to construct a curve, the ordinates of which would determine the temperature for different zenith distances, at first met with apparently insuperable difficulty. The result of observations made at different seasons under the most favourable circumstances, failed to produce a regular curve until the change of temperature corresponding with the varying distance between the sun and the earth was determined and introduced in the calculation. This at once harmonised the previously conflicting observations, and rendered the task easy of perfecting the curve, and obtaining ordinates consistent with the observed temperature produced by solar radiation at different seasons and different zenith distance. Regarding Table A, it will suffice to state that it is based upon our acquired knowledge of the temperature produced by solar radiation at given zenith distances when the earth is in aphelion. Evidently if we know that, for instance, when the sun's zenith distance is 43° the temperature is 6c57° Fah., we know also that this is the temperature at noon on the Arctic Circle, the latter being 43° from the ecliptic at the summer solstice. Again, the North Pole being 66° 30′ from the ecliptic at the same time, we find by referring to the figures entered in the table of zenith distances and temperatures (previously published) that the depth of atmosphere to be penetrated by the rays when the sun is 66°, 30′ from the zenith, is 2 444 times greater than on the ecliptic; and that, therefore, the radiant intensity, as shown in the table, is reduced from 67 20° at the tropic of Cancer to 49′91° Fah. at the pole. Possibly it may be found necessary to introduce a correction for the difference of atmospheric density in the higher latitudes; but at present I deem it inexpedient to complicate the matter by applying a correction which obviously cannot affect the general result. THE following are the lecture arrangements for 1871-72 at the Royal Institution, Albemarle Street:-Prof. Tyndall, F.R.S. : six lecturcs on "Ice, Water, and Air," on December 28, 30, 1871; January 2, 4, 6, 9, 1872. Dr. W. Rutherford, F. R.S. E.: ten lectures on "The Nervous and Circulatory System," on Tuesdays, January 16 to March 19. Prof. Odling, F.R.S.: ten lectures on "The Chemistry of Alkalies and Alkali Manufacture," on Thursdays, January 11 to March 21. Mr. W. G. Clark, late Public Orator: six lectures on "The History of Dramatic Literature, Ancient and Modern," on Saturdays, January 20 to Feb. 24. Mr. Moncure D. Conway: four lectures on "Demonology," on Saturdays, March 2 to 23. THE following Lectures to Women, on the Elements of Physical Science, will be delivered during the ensuing term, in the Lecture Theatre of the South Kensington Museum, by Professors Huxley, Guthrie, and Duncan. Professor Duncan: ten lectures on "Elementary Physiography," commencing on Saturday the 18th November, and ending on the 20th December; Saturdays and Wednesdays at 2.30. Professor Guthrie: fifteen lectures on "Elementary Physics and Chemistry," commencing on Wednesday the 10th January, and ending on Wednesday the 28th Feb. ruary; Wednesdays and Saturdays, at 2.30. Professor Huxley ten lectures on Elementary Biology," commencing on Saturday the 2nd March, and continued on Saturdays only at 2.30 P. M., on the 9th, 16th, 23rd March; 13th, 20th, 27th April; 4th, 11th, 18th May. A CLASS for the teaching of Natural Science has been formed at the College for Women, at Hitchin. Until very recently, classics and mathematics were almost exclusively the subjects brought under the consideration of the students; but a demand for the teaching of Natural Science has arisen, and under the advice of Prof. Liveing, of Cambridge, the subject of Chemistry has been taken to begin with. Prof. Liveing is on the list of lecturers at the College for Women, but in consequence of the weak state of his health-the result of overwork-he is unable to undertake the teaching himself. The actual professor at Hitchin is Mr. Hicks, Natural Science Lecturer at Sidney Sussex College, Cambridge. The lecturer gives one lecture a week, illustrated by experiments; and Mrs. Whelpdale, a lady who has had experience in teaching the subject, also gives supplementary teaching once a week. This lady works under the direction of Mr. Hicks, and acts as a tutor preparing for the lectures. So far as this has been worked, the plan seems to answer exceedingly well. The apparatus considered by Prof. Liveing and Mr. Hicks to be indispensable, has been provided by the college, but the authorities would be glad to make it more complete. Prof. Liveing has kindly promised to lend from Cambridge some of the It is quite more expensive things which are not in constant use. evident, however, that until there is a completely furnished laboratory, with all the appliances requisite for the study of Physical Science, the efforts made for the teaching of such science must be, to a certain extent, partial. It is to be hoped that funds will be forthcoming from some of the friends of the higher education of women to furnish the means for all that is needed in the new college building near Cambridge, to which the College for Women will, in time, be removed. President of the Royal College of Physicians of Edinburgh. He has been a professor of the University of Edinburgh since 1822, and is the author of a work on Poisons, which, although written many years since, is still a standard authority; and of a highly esteemed treatise on Materia Medica. Sir Robert Christison is a Crown Member of the General Medical Council, and took a leading part in framing the authorised edition of the British Pharmacopaia issued by the Council. Recently, as a mark of especial esteem and respect from his colleagues in the University of Edinburgh and other friends, his brst was sculptu.cd by subscription, and placed in the library of the University-an honour which, according to the British Medical Journal, had not previously been conferred on any professor during life. IN the year 1872 there will be open for competition, at St. John's College, Cambridge, four minor scholarships, two of the value of 70l. per annum, and two of 50l. per annum, together with three exhibitions of 50l. per annum, tenable on the same terms as the minor scholarships, and two of 40l. per annum, tenable for four years. The examination of candidates for the above-mentioned scholarships and exhibitions will commence on Tuesday, the 9th of April, 1872. The examination will consist of three mathematical papers and four classical papers. Besides the nine minor scholarships or exhibitions above mentioned, there will be for competition an exhibition of 50l. per annum for proficiency in natural science, the exhibition to be tenable for three years in case the exhibitioner has passed within two years the previous examination as required for candidates for honours, otherwise the exhibition to cease at the end of two years. The candidates for the Natural Science Exhibition will have a special examination on Friday and Saturday, the 12th and 13th of April, 1872, in (1) chemistry, including practical work in the labora tory; (2) physics, viz., electricity, heat, and light; (3) physiology. They will also have the opportunity of being examined in one or more of the following subjects--(4) geology, (5) anatomy, (6) botany, provided that they give notice of the subjects in which they wish to be examined four weeks prior to the examination. No candidate will be examined in more than three of these six subjects, whereof one at least must be chosen from the former group. It is the wish of the master and seniors that excellence in some single department should be specially regarded by the candidates. Candidates must send their names to one of the tutors (Rev. S. Parkinson, Rev. T. G. Bonney, and Mr. J. E. Sandys), fourteen days before the commencement of the examination. The minor scholarships are open to all persons under twenty years of age, whether students in the university or not, who have not yet commenced residence in the university or who are in the first term of their residence. TRINITY COLLEGE, Cambridge, offers one or more of its foundation scholarships, of the value of 80l. per annum each, for proficiency in the Natural Sciences. The examination will commence on April 5, and will be open to all undergraduates of Cambridge or Oxford, as well as to persons, under twenty-one, who are not members of the Universities. Further information may be obtained from the Rev. E. Blore, tutor of the college. THE first course of Cantor Lectures of the Society of Arts for the ensuing session will be "On the Manufacture and Refining of Sugar," by C. Haughton Gill, and will consist of four lectures to be delivered Monday evenings November 27, and December 4, 11, and 18. AT the late examination for the Natural Science Mcderatorship in Trinity College, Dublin, the first senior moderatorship was awarded to Phineas Simon Abraham, the second to Charles B Ball; the junior moderatorships were given to R. D. Purefoy and W. J. Smyly. The subjects for examination were-Comparative and Physiological Anatomy, Zoology, Botany, Physical A BARONETCY has been conferred on Prof. Christison of Edinburgh in recognition of his well-earned position at the head of the profession in Scotland. Prof. Christison already holds the appointment of Honorary Physician to the Queen in Scotland, and is President of the Royal Society of Edinburgh. He has received the honorary doctorate of Oxford, and has been twice | Geography, and Paleontology. THE annual general meeting of the Royal Horticultural Society of Ireland was held in Dublin on the 9th of November. The report of the Council was most satisfactory, and the treasurer's account showed a balance on the year to the credit of the society of upwards of 1,060. Of this sum 1,000l. was added to the reserve fund. In addition to the usual early Spring, Summer, and Autumn shows it was resolved to hold in October next a grand international fruit show, which we hope will be attended with success. MR. JOHN RUSKIN has lately presented a valuable collection of minerals and fossils to the High School, Nottingham. Among the former are two hundred metalliferous ores, including some rare specimens from Hungary, a hundred choice silicates, the principal varieties of fluor spar, calcite, and barytes, some agates, and a series of fine gems. The fossils are mainly from the Cretaceous Rocks of Kent and Sussex. ON Saturday last Sir William Stirling Maxwell was elected Rector of the University of Edinburgh. THE great Aquarium at the Crystal Palace, of which we recently gave a full description and drawing, was formally opened to the public on Friday evening last by a soirée. THE GEOGNOSY OF THE APPALACHIANS III. THE direct formation of the crystalline schists from an aqueous magma is a notion which belongs to an early period in geological theory. De la Beche, in 1834,† conceived that they were thrown down as chemical deposits from the waters of the heated ocean, after its reaction on the crust of the cooling globe, and before the appearance of organic life. This view was revived by Daubrée in 1860. Having sought to explain the alteration of paleozoic strata of mechanical origin, by the action of heated waters, he proceeds to discuss the origin of the still more ancient crystalline schists. The first precipitated waters, according to him, acting on the anhydrous silicates of the earth's crust, at a very elevated temperature, and at a great pressure, which he estimated at two hundred and fifty atmospheres, formed a magma, from which, as it cooled, were successively deposited the various strata of the crystalline schists. This hypothesis, violating, as it does, all the notions which sound theory teaches with regard to the chemistry of a cooling globe, has, moreover, to encounter grave geognostical difficulties. The pre-Silurian crystalline rocks belong to two or more distinct systems of different ages, succeeding each other in discordant stratification. The whole history of these rocks, moreover, shows that their various alternating strata were deposited, not as precipitates from a seething solution, but under conditions of sedimentation very like those of more recent times. In the oldest known of them, the Laurentian system, great limestone formations are interstratified with gneisses, quartzites, and even with conglomerates. All analogy, moreover, leads us to conclude that even at this early period life existed at the surface of the planet. Great accumulations of iron-oxyd, beds of metallic sulphids, and of graphite, exist in these oldest strata, and we know of no other agency than that of organic matter capable of THE Session of the Institution of Civil Engineers commenced on the 14th inst., and the annual general meeting "to receive and deliberate upon the report of the Council on the state of the Institution, and to elect the officers for the ensuing year," will be held on Tuesday, the 19th of December. At the same time the members have been reminded of the obligation entered into on election to promote the public and scientific obligations contemplated in the Royal Charter of Incorporation granted to the institution by preparing, or aiding in the preparation of, original communications for reading at the meetings, by frequent attend-generating these products. ance at the meetings and occasionally taking part in the discussion, and by presenting to the library copies of reports and scientific treatises not already in the collection. It has also been notified that the qualifications of candidates seeking admission into the institution must in all cases be set forth with the utmost precision and in considerable detail, in order to enable the Council, upon whom the classification involves, and the members, with whom the subsequent election rests, to form a correct opinion as to the nature of the practice, the extent of the experience, and the degree of responsibility of every candidate. The casualties which have occurred among the members of this body during the last three months include the death of Field-Marshal Sir John Burgoyne, G.C.B., &c., honorary member; of Messrs. Joseph Hamilton Beattie, John George Blackburne, Robert Benson Dockray, Albinus Martin, and Josiah Parkes, members; and of Messrs. Arthur Field, Edward Mosely Perkins, and Henry Beadon Rotton, associates. This has reduced the total number of members of all classes from 2,009, at which it stood on the 1st of August last, to 2,000, comprising 14 honorary members, 725 members, 1,056 associates, and 205 students. During the period referred to the ordinary general meetings have been suspended, so that there has been no ballot for new members. MR. BROTHERS has made a photograph eight inches in diameter of one of Mr. Proctor's star maps, containing nearly fifty thousand stars. The more marked constellations are just distinguishable upon a background, which appears to be shaded with innumerable minute points representing smaller stars. The increase of intensity in the shading is very evident upon certain parts of the picture. The whole represents the heavens as we should see them if the pupils of our eyes were a little more than two inches in diameter. DR. J. B. PETTIGREW, F.R.S., will deliver a course of twelve lectures on physiological and pathological subjects at the Royal College of Surgeons, Edinburgh. Bischof had already arrived at the conclusion, which in the present state of our knowledge seems inevitable, that "all the carbon yet known to occur in a free state can only be regarded as a product of the decomposition of carbonic acid, and as derived from the vegetable kingdom." He further adds, “living plants decompose carbonic acid; dead organic matters decompose sulphates, so that, like carbon, sulphur appears to owe its existence in a free state to the organic kingdom." As a decomposition (deoxidation) of sulphates is necessary to the production of metallic sulphids, the presence of the latter, not less than that of free sulphur and free carbon, depends on organic bodies; the part which these play in reducing and rendering soluble the peroxyd of iron, and in the production of iron ores, is, moreover, well known. It was, therefore, that, after a careful study of these ancient rocks, I declared in May, 1858, that a great mass of evidence "points to the existence of organic life, even during the Laurentian or so-called azoic period."|| This prediction was soon verified in the discovery of the Eozoon Canadense of Dawson, the organic character of which is now admitted by all zoologists and geologists of authority. But with this discovery appeared another fact, which afforded a signal verification of my theory as to the origin and mode of deposition of serpentine and pyroxene. The microscopic and chemical researches of Dawson and myself showed that the calcareous skeleton of this foraminiferal organism was filled with the one or the other of these silicates in such a manner as to make it evident that they had replaced the sarcode of the animal, precisely as glauconite and similar silicates have, from the Silurian times to the present, filled and injected more recent foraminiferal skeletons. I recalled, in connection with this discovery, the observations of Ehrenberg, Mantell, and Bailey, and the more recent ones of Pourtalès, to the effect that glauconite or some similar substance occasionally fills the spines of Echini, the cavities of corals and millepores, the canals in the shells of Balanus, and even forms casts of the holes made by burrowing sponges (Clionia) and Address of Prof. T. Sterry Hunt on retiring from the office of President of the American Association for the Advancement of Science; abridged from the "American Naturalist," concluded from p. 34. + Researches in Theoretical Geology, pp. 297-300. Etudes et expériences synthétiques sur le Metamorphisme, pp. 119-121. worms. The significance of these facts was further illustrated by showing that the so-called glauconites differ considerably in composition, some of them containing more or less alumina or magnesia, and one from the tertiary limestones near Paris being, according to Berthier, a true serpentine.* These facts in the history of Eozoon were first made known by me in May 1864, in the American Journal of Science, and subsequently more in detail, February 1865, in a communication to the Geological Society of London. + They were speedily verified by Dr. Gümbel, who was then engaged in the study of the ancient crystalline schists of Bavaria, and who soon recognised the existence, in the limestones of the old Hercynian gneiss, of the characteristic Eozoön Canadense, injected with silicates in a manner precisely similar to that observed by Dawson and myself. Later, in 1869, Robert Hoffmann described the results of a minute chenical examination of the Eozoön from Raspenau, in Bohemia, confirming the previous observations in Canada and Bavaria. He showed that the calcareous shell of the Eozoon examined by him, had been injected by a peculiar silicate, which may be described as related in composition both to glauconite and to chlorite. The masses of Eozoön he found to be enclosed and wrapped around by thin alternating layers of a green magnesian silicate allied to picrosmine, and a brown non-magnesian mineral, which proved to be a hydrous silicate of alumina, ferrous oxyd, and alkalies, related in composition to fahlunite, or more nearly to jollyte. § Still more recently, in the course of the present year, Dr. Dawson detected a mineral insoluble in acids, injecting the pores of crinoidal stems and plates in a paleozoic limestone from New Brunswick, which is made up of organic remains. This silicate which, in decalcified specimens, shows in a beautiful manner the intimate structure of these ancient crinoids, I have found by analysis to be a hydrous silicate of alumina and ferrous oxyd, with magnesia and alkalies, closely related to fahlunite and to jollyte. The microscopic examinations of Dr. Dawson show that this silicate injected the pores of the crinoidal remains and some of the interstices of the associated shell-fragments, before the introduction of the calcite which cements the mass. I have since found a silicate almost identical with this, occurring under similar conditions in an Upper Silurian limestone said to be from Llangedoc in Wales. Gümbel, meanwhile, in the essay on the Laurentian rocks of Bavaria, in 1866, already referred to, fully recognised the truth of the views which I had put forward, both with regard to the mineralogy of Eozoon and to the origin of the crystalline schists. His results are still farther detailed in his Geognost. Beschreibung des östbayerischen Grenzegebirges, 1868, p. 833. Credner, moreover, as he tells us, had already from his mineralogical and lithological studies, been led to admit my views as to the original formation of serpentine, pyroxene, and similar silicates (which he cites from my paper of 1865, above referred to**), when he found that Gumbel had arrived at similar conclusions. The views of the latter, as cited by Credner from the work just referred to, are in substance as follows :-The crystalline schists, with their interstratified layers, have all the characters of altered sedimentary deposits, and from their mode of occurrence cannot be of igneous origin, nor the result of epigenic action. The originally formed sediments are conceived to have been amorphous, and under moderate heat and pressure to have arranged themselves, and crystallised, generating various mineral species in their midst by a change, which, to distinguish it from metamorphism by an epigenic process, Gümbel happily designates diagenesis. It is unnecessary to remark that these views, the conclusions from the recent studies of Gumbel in Germany and Credner in North America, are identical with those put forth by me in 1860. At the early periods in which the materials of the ancient crystalline schists were accumulated, it cannot be doubted that the chemical processes which generated silicates were much more active than in more recent times. The heat of the earth's crust was probably then far greater than at present, while a high temperature prevailed at comparatively small depths, and thermal waters abounded. A denser atmosphere, charged with carbonic acid gas, must also have contributed to maintain, at the earth's Amer. Jour. Sci. II. xl. 360, Report Geol. Survey Canada, 1866, p. 231, and Quar. Geol. Jour. XXI. 71. Amer. Jour. Sci. II. xxxvii. 431. Quar. Geol. Jour. XXI. 67. 1 Proc. R. Bavar. Acad. for 1866, and Canadian Naturalist, N. S., III. 81. Jour. für Prakt. Chem. May, 1869, and Amer. Jour. Sci. ÍII. i. 378. Amer. Jour. Sci. III. i. 379. Hermann Credner; die Gleiderung der Eozoischen Formationsgruppe Nord Amerikas. Halle, 1869. **That in the Quar. Geol. Jour. XXI. 67. surface, a greater degree of heat, though one not incompatible with the existence of organic life.* These conditions must have favoured many chemical processes, which, in later times, have nearly ceased to operate. Hence we find that subsequently to the eozoic times, silicated rocks of clearly marked chemical origin are comparatively rare. In the mechanical sediments of later periods certain crystalline minerals may be developed by a process of molecular re-arrangementdiagenesis. These are, in the feldspathic and aluminous sediments, orthoclase, muscovite, garnet, staurolite, cyanite, and chiastolite, and in the more basic sediments, hornblendic minerals. It is possible that these latter and similar silicates may sometimes be generated by reactions between silica on the one hand and carbonates and oxyds on the other, as already pointed out in some cases of local alteration. Such a case may apply to more or less hornblendic gneisses, for example; but no sediments, not of direct chemical origin, are pure enough to have given rise to the great beds of serpentine, pyroxene, steatite, labradorite, &c., which abound in the ancient crystalline schists. Thus, while the materials for producing, by diagenesis, the aluminous silicates just mentioned, are to be met with in the mud and clayrocks of all ages, the chemically formed silicates capable of crystallising into pyroxene, tale, serpentine, &c., have only been formed under special conditions. The same reasoning which led me to maintain the theory of an original formation of the mineral silicates of the crystalline schists, induced me to question the received notion of the epigenic origin of gypsums and magnesian limestones or dolomites. The interstratification of dolomites and pure limestones, and the enclosure of pebbles of the latter in a paste of crystalline dolomite, are of themselves sufficient to show that in these cases, at least, dolomites have not been formed by the alteration of pure limestones. The first results of a very long series of experiments and inquiries into the history of gypsum were published by me in 1859, and further researches, reiterating and confirming my previous conclusions, appeared in 1866.+ In these two papers it will, I think, be found that the following facts in the history of dolomite are established, viz.: first, its origin in nature by direct sedimentation, and not by the alteration of non-magnesian limestones; second, its artificial production by the direct union of carbonate of lime and hydrous carbonate of magnesia, at a gentle heat, in the presence of water. As to the sources of the hydrous magnesian carbonate, I have endeavoured to show that it is formed from the magnesian chlorid or sulphate of the sea or other saline waters in two ways:-first, by the action of the bicarbonate of soda found in many natural waters; this, after converting all soluble lime-salts into insoluble carbonate, forms a comparatively soluble bicarbonate of magnesia, from which a hydrous carbonate slowly separates; secondly, by the action of bicarbonate of lime in solution, which, with sulphate of magnesia, gives rise to gypsum; this first crystallises out, leaving behind a much more soluble bicarbonate of magnesia, which deposits the hydrous carbonate in its turn. In this way for the first time, in 1859, the origin of gypsums and their intimate relation with magnesian limestones were explained. It was, moreover, shown that to the perfect operation of this reaction, an excess of carbonic acid in the solution, during the evaporation, was necessary to prevent the decomposing action of the hydrous mono-carbonate of magnesia upon the already formed gypsum. Having found that a prolonged exposure to the air, by permitting the loss of carbonic acid, partially interfered with the process, I was led to repeat the experiment in a confined atmosphere, charged with carbonic acid, but rendered drying by the presence of a layer of dessicated chlorid of calcium. As had been foreseen, the process under these conditions proceeded uninterruptedly, pure gypsum first crystallising out from the liquid, and subsequently the hydrous magnesian carbonate. I This experiment is instructive as showing the results which must have attended this process in past ages, when the quantity of carbonic acid in the atmosphere greatly exceeded its present amount. As regards the hypotheses put forward to explain the supposed dolomitisation of previously-formed limestones by an epigenic process, I may remark that I repeated very many times, under varying conditions, the often-cited experiment of Von Morlot, who claimed to have generated dolomite by the action of sulphate of magnesia on carbonate of lime, in the presence of water at a Amer. Jour. Sci. II. xxxvi. 396. † Amer. Jour. Sci. II xxxviii. 170, 365; xlii. 49. 1 Proceedings Royal Institution, May 30, 1867, and Canadian Naturalist, New Series, III. 231. |