attention was called to a statement by Sir W. Armstrong, to the effect that Mr. Siemens had suggested as an explanation of the otherwise anomalous advantage of forcing air into the boiler of a steam-engine, that the air may prevent, in a great measure, the condensation at the surface of the cylinder. It would thus seem that Mr. Siemens has already suggested the probability of the fact which is proved in this investigation. I am not aware, however, that any previous experiments have been made on the subject, and therefore I offer these results as independent testimony of the correctness of Mr. Siemens's views as well as of my own. GEOLOGICAL SOCIETY. [Continued from vol. xlvi. p. 329.] February 5, 1873.-Warington W. Smyth, Esq., F.R.S., VicePresident, in the Chair. The following communication was read: 1. "On the Oolites of Northamptonshire.-Part II." By Samuel Sharp, Esq., F.S.A., F.G.S. In the first part of this memoir the succession of beds in the neighbourhood of Northampton was shown to be as follows: The Great Oolite limestone of this section has been confounded, even up to the present time, with a limestone (frequently Oolitic) which occurs between Kettering and Stamford, is prevalent about the latter town, extends through Rutland and Lincolnshire (where it attains a thickness exceeding 200 feet) and into Yorkshire, which limestone has been distinguished by Mr. Judd as the " Lincolnshire limestone." The object of the author was to show that these two limestones were distinct, and that while the former was of the Great Oolite period, the latter as certainly belonged to the Inferior Oolite; and in citing evidence in proof of this position upon stratigraphical and paleontological grounds, he gave a general account of the geology of the northern division of Northamptonshire, illustrating his description by the exhibition of numerous fossils gathered from the various beds and localities referred to. Between Northampton and Kettering, the Great Oolite limestone is the surface rock; and intersecting valleys upon that line, and the escarpment of the Ise valley, a mile east of Kettering, exhibit this sequence of beds : Great Oolite. Limestone. Upper Estuarine Clays. Inferior Oolite.... {Ferruginous Beds Clay. .... Northampton And this section, with the successive superaddition of Great Oolite Clay, Cornbrash, Kelloway Rock, and Oxford Clay, is continued due east across the country to the valley of the Nene, and on into Huntingdonshire. Upon the same Ise escarpment, about a mile north-east of Kettering, the thin end of the wedge of the Lincolnshire limestone is seen to come in; and this sequence, for the first time, is presented : J Limestone. Upper Estuarine Clays. Clay. The same sequence, with the occasional superaddition of the Great Oolite Clay, was shown to be repeated upon the western escarpment of the Ise, at Glendon, Barford Bridge, near Rockingham, at Weekly, and at Geddington (the Lincolnshire limestone increasing in thickness at every advance), and to occur over and over again upon innumerable escarpments in the counties of Northampton, Rutland, Lincoln, and York, offering unmistakable and incontrovertible evidence of the true stratigraphical position of the Lincolnshire limestone. The author described the section in the ancient quarries at Weldon, whence is obtained the highly reputed freestone of that name. He exhibited a diagram of the horizontal section from Rockingham, on the Welland valley (which marks to the north-west the line of division between Northamptonshire and Rutland), through Weldon, by Oundle, and across the Nene valley into Huntingdonshire. It was shown by this diagram that, although the Lincolnshire limestone has a thickness of some 30 feet at Weldon, it thins out very rapidly eastwards, being nowhere found within three miles of Weldon, and being altogether absent from the Nene escarpment at Oundle, which presents, with this exception, the whole series of beds from the Oxford Clay to the Upper Lias Clay inclusive. The author successively described sections in the Lincolnshire limestone :—at Kirby, where the equivalent of the Colley weston slate is quarried, this being the extreme western point at which it has been found; at Deene, Wakerley, Morcot (where the whole series from the Cornbrash to the Upper Lias inclusive occurs, the Lincolnshire limestone attaining to a thickness of 60 feet), Ketton (where the famous Ketton freestone is quarried), and Colley weston. At the last-named place the well-known calcareo-arenaceous slate has been quarried for more than 350 years; and the numerous quarries occupy an area exceeding a mile in diameter. Here everywhere may be noted the position of this slate-bed, passing the Lincolnshire limestone series, and immediately overlying the Lower Estuarine Sands. Some three miles north-east of the Colley weston slate quarries is the high ground of the Stamford Open Field, the summit of which is about 200 feet above the level of the river Welland. In this one hill-mass occur (and may be observed) the whole series of beds from the Cornbrash to the Upper Lias inclusive, the Lincolnshire limestone having reached a thickness of 75 feet, and being the most important bed of the section. The limestone which occurs beneath the Great Oolite Clay and above the Upper Estuarine series high up upon the escarpment of Stamford Field, occurs also, at a level some 100 feet lower, at Bemisthorpe, and at Danes' Hill and Essendine, in cuttings of the Great Northern Railway, the only record of the sections of which is to be found in Professor Morris's paper in the Society's Journal for 1853. This limestone has not been considered to be Great Oolite at all; but the author showed, by a comparison of the whole group of fossils obtained from both beds, that this limestone was identical with the Great Oolite limestone of the Northampton district. The author described the peculiar effect of a fault occurring south of the Welland at Stamford, by which the Upper Lias capped by the Northampton Sand has been thrown up to an elevation overtopping the town. Upon a severed and subsided mass the important suburb of St. Martin's has been built, and a peculiar repetition of beds has resulted-Upper Lias, Northampton Sand, Colleyweston Slate, and Lincolnshire Limestone being in a double sequence encountered upon an ascent of the escarpment. The fact was stated that the Colley weston slate was found near the foot of the escarpment (whence the beautiful Astropecten Cotteswoldiæ, var. Stamfordensis, Wright, was obtained by the author in 1853), and again upon the summit of the escarpment, at a distance of one third of a mile, at an increased elevation of 150 feet. Other anomalous results of the same fault, which extends some miles eastward, were described. The area of the old "Barnack Rag" quarries was referred to. These were in work in the time of the Romans, but had been exhausted for 400 years. The stone (Lincolnshire limestone) was the building-stone over a large district in ancient times; and its excellence was proved by time. The escarpment south of the Nene valley at Wansford presents the same sequence as that of the Stamford Field-Cornbrash to the Upper Lias inclusive. The railway-tunnel is excavated in the Lincolnshire limestone. East of this, at Castor and Water Newton, and west of the same point at Elton, the Lincolnshire limestone thins away, and the two estuarine series again come together in vertical contact. Along the Nene valley towards Northampton the southern escarpment presents the complete sequence of beds from the Oxford Clay to the Upper Lias inclusive (minus the Lincolnshire limestone), as far as Thrapstone. Here the Oxford Clay and Cornbrash part company and trend southward, the other members of the series (Great Oolite Clay and limestone, Upper Estuarine, Lower Estuarine, and ferruginous beds of Northampton sand, and Upper Lias) continuing on to the Northampton district. The author contended that the stratigraphical and palæonto logical evidence combined to establish the position which he assumed in his introduction-that the Great Oolite limestone of Northampton was identical with what he considered was Great Oolite at Stamford, and that the "Lincolnshire limestone" formation, and a member of the Inferior Oolite series. was a distinct He had confidence that he had shown that the series of beds in the north-eastern portion of the northern division of Northamptonshire comprised all the beds between the Oxford Clay and Upper Lias inclusive, including the Lincolnshire limestone-and that those of the south-western portion comprised the same sequence, excluding the Lincolnshire limestone. He considered that the Great Oolite Clay represented the Forest Marble and the Bradford Clay of the West of England; that the Great Oolite limestone was nearly equivalent to the Great Oolite of Bath and the Cotteswolds, and to the upper beds of Minchinhampton; that the Upper Estuarine might be nearly identical with the Upper Plant Shale of Yorkshire, but more certainly with the Stonesfield slate of Oxfordshire; that the Lincolnshire limestone was nearly synchronous with the grey limestone of Yorkshire (Inferior Oolite), and probably with the lower portion of the Am. Humphriesianus zone of the west of England, but extending a little below this zone; that the Lower Estuarine answered to the Lower Plant Shale of Yorkshire, but had no representative in the west; that the upper portion of the ferruginous beds of the Northampton Sand was nearly upon the same horizon as the Glaizedale and Dogger beds of Yorkshire and the Am. Murchisona zone of the west; and that the lower portion of the Northampton sand was represented by the Am. opalinus zone and the Midford Sand. XI. Intelligence and Miscellaneous Articles. ON SOME PHENOMENA OF ILLUMINATION. BY A. LALLEMAND. ALL LL the effects of illumination observed in diaphanous bodies traversed by the light of the sun, either natural or polarized, are readily explained if it be admitted that the vibratory motion of the æther in penetrating the transparent medium, encounters a resistance in virtue of which the vibrations are propagated laterally in such a manner that along any direction oblique to the incident ray the motion of the æthereal particle represents the projection of that of the æther on its passage from the luminous pencil, and if, on the other hand, it be also admitted that the molecules of the medium, absorbing a portion of the vis viva of the ather, vibrate in their turn and propagate in the æthereal fluid the complex vibrations which constitute natural light. The illumination therefore results from two effects superposed; and the light emanating from it is formed of two sorts of rays: the one sort, always of the same colour as the incident rays, are polarized either partially or completely, according as the incident pencil is neutral or polarized; the others, the refrangibility of which is often inferior to that of the exciting rays, have the properties of natural light, and determine a general property of bodies which has been called fluorescence. In the case of opaque bodies this property corresponds to what is more usually designated by "the proper colour of the body." I have just said that fluorescence is a general phenomenon of transparent bodies. In fact, by operating on the purest liquids (such, for example, as are obtained by condensing a gas like sulphurous acid, cyanogen, &c.), it is proved that the illumination is not extinguished when we look in a direction normal to the pencil and to the plane of polarization of the incident light; or if we view natural light through a double prism, one of the two images of the luminous ray never disappears completely. Nevertheless in studying under better conditions some crystallized substances, I have been able to recognize that quartz and rock-salt, when very pure, exhibit not the slightest trace of fluorescence and are illuminated very cleanly. To succeed in this experiment, it is necessary to polarize a large pencil of solar rays with a Foucault normal-face prism, and to concentrate it with a quartz lens cut parallel to the axis, of 35-40 centims. focus. The principal sections of the polarizer and the lens are made to coincide; and then their movements are rendered conjoint. If the pencil thus concentrated pass through the quartz either in the state of an ordinary or of an extraordinary ray, a very distinct white trail is observed in the plane of polarization, which a Nicol completely extinguishes. On looking in a direction normal to the plane of polarization, there is no illumination; there remains not the least trace of fluorescence. When the solar ray traverses the quartz along the optic axis, the dispersion of the plane of polarization has for its result the giving equal illumination around the ray, and polarization is complete only in a direction normal to the pencil. In this case chromatic illumination should be observed, similar to that of sirup of sugar and all solutions with rotatory power; but with quartz it is not manifest ; indeed we know that the mixed tints, much weakened, all affect a uniform grey tint which the eye cannot distinguish. Very pure rock-salt is illuminated also like quartz, and is not fluorescent. It is not the same with Iceland spar. All the specimens I have examined are illuminated with orange-red more or less brilliant; but this coloured illumination is the same in the plane of polarization and normal to that plane; it is not extinguished by a Nicol, when the emergent rays, which necessarily undergo double refraction, remain superposed. This red light is due solely to fluorescence; and the polarized illumination resembling that of quartz is not appreciable. A peculiarity which I must notice is, that when the incident solar thread is not polarized and traverses the spar rhombohedron so as to give two pencils well separated, the fluorescence due to the ordinary ray appears more vivid and of a deeper red than that of the extraordinary ray; at least this is what I have distinctly observed in two strongly fluorescent specimens. The fluorescence of spar has besides been studied by M. Becquerel with the phosphoroscope; and I only confirm, by another method, his observations. Colourless fluor-spar combines with more intensity the two distinct properties of quartz and of spar: it gives a white illumination, very bright in the plane of polarization, and a violet |