evolved in the dark. Hence we may conclude, that the light does not effect a permanent modification, either of the chlorine or hydrogen, but that the combination produced by the light must depend on photo-cheinical action affecting only the increasing attractions of the chemically active molecules.

All the curves representing the increase of the induction under various conditions have a common form, and a point of flexure at which the maximum increase occurs. In order to determine whether this common property of the curves arises from the general mode of action of affinity, or whether the light plays an essential part, we have made experiments upon idio-chemical induction, i. e. action in which pure chemical attractions alone effect the alteration. For this purpose we employed a dilute aqueous solution of bromine with tartaric acid, which mixture, when left to itself in the dark, undergoes decomposition, hydrobromic acid being formed. By determining the amount of free bromine contained in the liquid at different times, we became acquainted with the rate at which the decomposition occurred. Analysis showed that the amount of hydrobromic acid formed was not the same in equal spaces of time; and curves representing this increase were found to have the form obtained for the photo-chemical induction. Hence the cause of this maximum increase appears not to lie in any peculiar property of light, but rather in the mode of action of affinity itself.

One of the many interesting applications of the laws of photochemical induction relates to the phenomena of photography. As an instance of this application we cite the remarkable observations of Becquerel, which induced him to assume the existence of certain rays which can continue, but not commence, chemical action. In order to explain the phenomenon observed by the French physicist, we do not need to suppose the existence of a new property of light, as the facts are easily explained by the laws of photo-chemical induction; and we are satisfied that these relations, which we have examined only in the case of chlorine and hydrogen, occur in a slightly modified form in other photo-chemical processes.

Having determined in this part of our investigation the most important phenomena of photo-chemical induction, we shall in the next section consider the laws which regulate the chemical action of light after the induction is completed.

II. "Observations on Glaciers." By JOHN TYNDALL, Esq., F.R.S., and THOMAS H. HUXLEY, Esq., F.R.S. Received January 15, 1857.

(Abstract.)

On the 6th of June, 1856, certain views were advanced by one of us on the origin of slaty cleavage, and soon afterwards his attention was drawn by the other to the observations of Prof. J. D. Forbes on the structure of glacier ice, as suggesting the idea that the ice structure might be due to the same cause as the slate cleavage. On consulting the observations referred to, the lecturer at once perceived the probability of the surmise, and the consequence was a joint visit for a few days to the glaciers of Grindelwald, the Rhone, and the Aar. The subject being a physical one, it was followed up by the physicist on his return from the Continent. Reading, reflection, and experiment extended the inquiry until it embraced the main divisions of the problem of glacial structure and motion; and the paper now submitted to the Society contains an account of the investigation.

The first division is devoted to the consideration of certain phenomena connected with the motion of glaciers. The power of glaciers to accommodate themselves to the sinuosities of the valleys which they occupy, and the resemblance of their motion through such valleys to the motion of a river, suggest ideas which find their clearest expression in the viscous theory of glacier motion, propounded by Prof. J. D. Forbes. Numerous appearances indeed seem to favour this notion. The aspect of many glaciers as a whole, their power of closing up crevasses, and of reconstructing themselves after having been precipitated down glacial gorges; the bendings and contortions of the ice, the quicker movement of the central portion of the glacier where it is uninfluenced by the retardation of the banks,—are all circumstances which have been urged with such ability as to leave the viscous theory without any formidable competitor at present. To these may be added, the support which the theory derived from its apparent competency to explain the laminar structure of the ice,-a structure which it is affirmed is impossible of explanation upon any other hypothesis.

Nevertheless this theory is so directly opposed to our ordinary experience of the nature of ice, as to leave a lingering doubt of its truth upon the mind. To remove this doubt it is urged, that the true nature of ice is to be inferred from experiments on large masses, and that such experiments place the viscosity of ice in the position of a fact rather than in that of a theory. It has never been imagined that the bendings and contortions, and other evidences of apparent viscosity exhibited by a glacier, could be made manifest on hand specimens of ice. In the present paper, however, this is shown to be possible. Spheres of ice are described as being flattened into cakes, and squeezed into transparent lenses. A straight prism of ice six inches long, is described as having been passed through a series of moulds augmenting in curvature, and finally coming out bent into a semi-ring. A piece of ice is placed in a hemispherical cavity and is pressed upon by a protuberance not large enough to fill the cavity, and is thus squeezed into a cup. In short, every observation made upon glaciers, and adduced by writers upon the subject in proof of the plasticity of ice, is shown to be capable of perfect imitation with hand specimens in the laboratory.

These experiments, then, demonstrate a capacity on the part of small masses of ice which has hitherto been denied to them by writers upon this subject. They prove to all appearance that the substance is even much more plastic than it has hitherto been supposed to be; but the real germ from which these results have sprung, is to be found in a lecture given by Mr. Faraday at the Royal Institution in 1850, and reported in the 'Athenæum' and 'Literary Gazette' for that year. Mr. Faraday then showed, that when two pieces of ice, at a temperature of 32° Fahr., are placed in contact, they freeze together by the conversion of the film of moisture between them into ice. The case of a snow-ball was referred to as a familiar illustration of the principle: when the snow is below 32°, and therefore dry, it will not cohere, whereas when it is in a thawing condition, it can be squeezed into a hard compact mass. During one of the hottest days of last July, when the temperature was upwards of 100° Fahr. in the sun, and more than 80° in the shade, a number of pieces of ice placed loosely together in a window in the Strand, were observed by one of us to be frozen together; and he subsequently caused pieces of ice to freeze together under hot water. Hence the thought arose,

that if a piece of ice, a straight prism for example, were placed in a bent mould and submitted to pressure, it would break, but the continuance of the force would bring its severed surfaces to reunite, and that thus the continuity of the mass might be re-established. Experiment, as we have seen, completely confirmed this surmise; the ice passed from a continuous straight bar to a continuous bent one; the transition being effected, not by a viscous yielding of the particles, but through fracture and regelation.

All the phenomena on which the idea of viscosity has been founded, are brought by experiments similar to the above into harmony with the demonstrable properties of ice. In virtue of these properties the glacier accommodates itself to its bed; crevasses are closed up, and the broken ice of a cascade, such as that of the Talèfre or the Rhone, is recompacted to a solid continuous mass. But if the glacier effects its movement in virtue of the incessant fracture and regelation of its parts, this process will in all probability be accompanied by an audible cracking of the mass, and thus a noise of decrepitation may be expected to be heard, which would be absent if the motion were that of a viscous body. It is well known that such noises are heard, and they thus receive a satisfactory explanation*.

*It is manifest that the continuity of the fractured ice cannot be completely and immediately re-established after rupture; it is not the same surfaces that are regelated, and hence the new contact cannot be perfect throughout. After rupture, the surfaces of fracture will enclose for a time capillary fissures, and thus the above theory is in harmony with the known structure of glacier ice. Since the paper was presented to the Society, I (on January 30th) made the following experiments bearing upon this point:-A piece of ordinary ice was taken, and a cavity hollowed in it was filled with an infusion of cochineal; the ice was perfectly impervious to the liquid, which remained in it for half an hour without penetrating it in the slightest degree. A piece of the same ice was subjected to a gradually increasing pressure. Flashes of light were seen to issue from it at intervals, which indicated the rupture of optical continuity, and a low, and in some instances, almost musical crackling was heard at the same time. Relieved from the pressure, the ice appeared continuous to the naked eye, but a cavity being formed and the cochineal infusion placed within it, the coloured liquid immediately diffused itself through the capillary fissures, producing an appearance accurately resembling the drawings illustrative of the infiltration experiments of M. Agassiz on the glacier of the Aar.

To account for a “bruit de crépitation" heard upon the Aar glacier, M. Agassiz

The next division of the paper treats of the veined or laminar structure of glacier ice, which Prof. J. D. Forbes, in his earlier writings, compared to slaty cleavage. His theory of the structure is perhaps the only one which has made any profound impression, and it may be briefly stated as follows. Owing to the quicker flow of the centre of a glacier, a sliding of the particles of ice past each other takes place; in consequence of this sliding, fissures are produced, which, when filled with water and frozen in winter, produce the blue veins of the glacier. To account for the obliquity of the veins to the sides of the glacier, a drag towards the centre is supposed to take place, producing a differential motion which results in the formation of fissures. But at the centre of the glacier this drag towards it cannot be supposed to exist; and to account for the veined or laminated structure of the centre, which, under normal conditions, is transverse to the length of the glacier, it is supposed that the thrust behind meeting an enormous resistance in front, produces a differential motion in a direction approximating to the vertical, and that in consequence of this motion fissures are produced, which, when filled and frozen, produce, as in the other cases, the blue veins. In the present paper it is observed that the only fact connected with this theory, is that of differential motion in the direction of the length of the glacier. Beyond this, all is conjecture. It

refers to an observation which might be made on a fine day in summer, and which would show the air within the glacier ice escaping from its surface.. M. Agassiz supposes the ice to be diathermanous, and that thus the sun-beams get through it and heat the air-bubbles which it encloses, causing the air to expand, rupture the ice, and escape in the manner observed. The observation is an interesting one, whatever difficulty we may find in the explanation. An experiment made to-day (January 31) appears to me to account for the observation in a satisfactory manner. Snow having fallen, I was early at work compressing it; and on removing a plate of it from the press, I noticed, as the surface melted, a sparkling motion produced by the escape of the air enclosed within the mass. To imitate the action of the sun upon the glacier, an iron spatula was heated, and on bringing it near the surface of the compressed snow, the jumping of the surface, caused by the issue of air through the film of water which covered it, was greatly augmented. On removing the spatula the motion subsided. To a similar action on the part of the sun melting the surface of the glacier, and thus liberating by degrees the air-bubbles with which the ice is filled, the observation of M. Agassiz is in all probability to be referred.-J. T.