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that seen in all matter when very minute particles are suspended in a liquid so as to allow freedom of motion. It is not seen to advantage if the diameter of the bubbles is more than one ten-thousandth of an inch; when it is about one fifty-thousandth, they move to and fro in the most surprising manner, and with such rapidity that the eye can scarcely follow them.

The examination of the diamond led Messrs. Sorby and Butler to correct various theories which had been put forth by Sir David Brewster; one of them, that which that eminent philosopher put forth to the effect that certain black specks which were surrounded by a black cross, when examined with polarized light, were minute cavities. It is impossible to say whether they are cavities or enclosed crystals; but it is rather more probable that they are crystals; the forms are exactly those of crystals; they depolarize light; have much less refractive power than that of the diamond; and as the inclined planes totally reflect the transmitted light, they thus look quite black. It is, doubtless, this circumstance which makes many of the smaller enclosed crystals to appear like mere black specks; the other curious fact has been determined, viz. that the enclosed crystals have exercised a pressure on the surrounding diamond, not by increasing in size, but by checking the uniform contraction of the diamond. The researches of Sir David Brewster in the cavities noticeable in the topaz, and of other eminent observers, demonstrate that these phenomena have much in common with what occurs at a lower temperature in the case of the liquid enclosed in the sapphire, and that they are of great importance in connexion with the origin of fluid-cavities. Since they became full of liquid at a comparatively low temperature, it was not unreasonable to suppose that the minerals in which they occur must have been formed where the heat was scarcely above that of the atmosphere; but these facts seem to show that the occurrence of such fluid-cavities is quite reconcilable with a very high temperature; for it is obvious that if at a great depth below the surface highlycompressed gaseous carbonic acid, greatly heated, was enclosed in growing crystals, it might condense on cooling so as to more or less completely fill the cavities with the liquid acid.

The cavities in emeralds are very interesting in connexion with this subject, and also furnish strong evidence against the opinion that the liquid was not present when the crystals were formed, but penetrated into the fluidcavities at a subsequent period, either filling vacant spaces or removing and replacing the material of glass cavities as suggested by Vogelsang. On the whole, the various facts described in this paper seem to show that the ruby, sapphire, and emerald were formed at a moderately high temperature, and under so great a pressure that water might be present in a liquid state. The whole structure of the diamond is so peculiar that it can scarcely be looked upon as positive evidence of a high temperature, though not at all opposed to this supposition. The absence of fluid-cavities containing water or a saline solution does not by any means prove that there was no water, because the fact of its becoming enclosed in crystals depends so much on their individual nature. At the same time the finding of fluid-cavities containing what seems to be merely liquid carbonic acid is scarcely reconcilable with the presence of more than a very little water in either a liquid or a gaseous form. The curved or irregular form of the fluid-cavities is no proof that the minerals were ever in a soft state, since analogous facts are seen in the case of crystals deposited from solution.

Professor Tyndall has prosecuted with his usual zeal those remarkable researches which have made his name famous, "On the Blue Colour of the Sky, the Polariza

tion of Sky-light, and on the Polarization of Light by Cloudy Matter generally;" and many new points have been established by his successful investigations. Certain it is that the blue colour of the sky and the polarization of sky-light have been the two great standing enigmas of meteorology. The apparatus worked by Professor Tyndall we have already noticed last year. It may be, however, well again to state that it is a glass tube about a yard long, and from two and a half to three inches in diameter. The vapour to be examined is introduced into this tube, and the condensed beam of the electric lamp is permitted to act on it until the neutrality or activity of the substance has been declared.

The first object is to make the chemical action of light upon vapours visible. For this purpose substances have been chosen, one at least of whose products of decomposition under light shall have a boiling point so high that as soon as the substance is formed it shall be precipitated. By graduating the quantity of this vapour, this precipitation may be rendered of any degree of fineness, forming particles, some of which are distinguishable by the naked eye, while others probably lie far beyond the reach of our highest microscopic powers. There can be no doubt that particles thus obtained exhibit but a very small portion of the length of a wave of light. In all cases, if the vapours of the liquids employed are sufficiently attenuated, the visible action (visible, however, only under a powerful beam of light, surrounded by complete darkness) commences with the formation of a blue cloud, which blue cloud differs altogether from the finest ordinary clouds, and perhaps occupies an intermediate space between these visible clouds and cloudless vapour. Numerous experiments were then made; the decomposition of carbonic acid by light was attempted; a plate of tourmaline was placed between the eye and the blueish cloud; thin plates of selenite or quartz were arranged between the Nicol prism and the blueish cloud; and benzol, bisulphide of carbon, iodide of allyl, and various other substances were brought under examination.

One grand and uniform result was obtained, viz. that a vapour which, when alone or mixed with air in the experimental tube, assists the act of light, may, by placing it in proximity with another gas or vapour, be caused to exhibit under light, vigorous, if not violent action. This carbonic acid gas, which, diffused in the atmosphere, resists the decomposing action of solar light, when placed in contiguity with the chlorophyl in the leaves of plants has its molecules shaken asunder. Professor Tyndall adds, after full details of experiments which it is impossible to describe popularly, that the notion of the colour of the sky being due to the action of finely-divided matter, thus rendering the atmosphere a turbid medium through which we gaze into the darkness of space, dates as far back as Leonardo da Vinci. Sir Isaac Newton, too, conceived that the colour was due to exceedingly small water particles acting as thin plates. Goethe's experiments in connexion with this subject are well known and exceedingly instructive. One very striking observation of Goethe's refers to what is technically called "chill" by painters, a result doubtless due to extremely fine particles of varnish interposed between the eye and a dark background. Lastly, it has been well determined that all liquids have motes in them sufficiently numerous to polarize sensibly the light, and beautiful effects may be produced by very simple artificial devices. When, for example, a cell of distilled water is placed in front of the electric lamp, and a slice of the beam is permitted to pass through it, scarcely any polarized light is discharged, and scarcely any colour is produced by a plate of selenite. But if, while Team is passing through it, a bit of soap be agitated in the water above the

beam, the moment that the infinitesimal particles reach the beam, the liquid sends forth literally almost perfectly polarized light; and if selenite be employed, vivid colours flash into existence. A still more brilliant result is obtained with mastic dissolved in a great excess of alcohol. The selenite rings constitute an extremely delicate test as to the quantity of motes in a liquid. Commencing with distilled water, for example, a thickish beam of light is necessary to make the polarization of its motes sensible. A much thinner beam suffices for common water; while with Bruche's precipitated mastic a beam too thin to produce any sensible effect with most other liquids suffices to bring out vividly the selenite colours.

To Professor Moseley we are indebted for a very clever paper "On the mechanical possibility of the descent of Glaciers by their own weight," in which he pointed out most graphically the curious irregularities which have been noticed by various observers in the descent of glaciers from high altitudes. Thus he showed that all parts of a glacier do not descend with a common motion; it moves faster, for instance, at its surface than deeper down; and at the centre of its surface than at its edges. Thus, if a transverse section of the ice were made, it would be found that it was moving differently at every point of its motion. This fact, which was first noticed by M. Rendu, Bishop of Annecy, has been fully confirmed by the measurements of Agassiz, Forbes, and Tyndall. There is a constant displacement of the particles of the ice over one another and alongside one another, to which is opposed that force of resistance which is known in mechanics by the name of shearing force. By the property of ice called regelation, when any surface of ice so sheared is brought in contact with another similar surface, it unites with it, so as to form of the two one continuous mass. Thus a slow displacement of shearing, by which different similar surfaces are continually being brought into the presence and the contact of one another, exhibits all the phenomena of the motion of glacier ice. Now in the case of any metal the weight is not sufficient to set the shearing forces at work; hence, were the Mer de Glace filled with cast-iron instead of ice there is no reason to suppose that there would be any descent of the iron.

The forces which oppose themselves to the descent of any glacier are:1. The resistance to the sliding motion of one part of a piece of solid ice over the surface of another, which is taking place continually throughout the mass of the glacier by reason of the different velocities with which the different parts move. This kind of resistance may be called (for convenience) shear, the unit of shear being the pressure in lbs. necessary to overcome the resistance of shearing of one square inch, which may be presumed to be constant throughout the mass of the glacier. 2. The friction of the super-imposed laminæ of the glacier, which move with different velocities on one another, which is greater in the lower ones than the upper. 3. The resistance to abrasion or shearing of the ice at the bottom of the glacier and on the sides of its channel, caused by the roughnesses of the rock, the projections of which insert themselves into its mass and into the cavities in which it moulds itself. 4. The friction of the ice in contact with the bottom and the sides so sheared over or abraded. The details the professor enters into in support of his general view are too long and too intricate for insertion here. It is enough for us to add his final conclusion, in which, having examined his data, we thoroughly concur, viz. that the weight alone of a glacier is insufficient to account for its descent ;-that it is necessary to conceive, in addition to its weight, the operation of some other and much greater force, which must also be such as would produce those internal and molecular dis

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placements, and those strains which are actually observed in glacier ice, and which must, therefore, be present in every part of the glacier.

Mr. Crookes has contributed a paper "On the Measurement of the Luminous Intensity of Light," in which he shows that the measurement of this intensity is a problem which, though repeatedly attempted, has not been as successfully accomplished as in the case of other radiant forces. The problem being clearly susceptible of division into the absolute and the relative, what we want to obtain is the first, but, at present, we are apparently far from procuring or constructing a photometer analogous to a thermometer in fixity of standard and facility of observation. The probable course towards the attainment of this object is shown in the observations of M. Becquerel, Sir John Herschel, and others, on the chemical action of the solar rays, and on the production thereby of a galvanic current capable of measurement by a delicate galvanometer. The measurement of a chemical beam of light is as distinct from photometry proper as is the thermometric registration of the heat-rays constituting the other end of the spectrum. What we want is a method of measuring the intensity of those rays which are situated at the intermediate parts of the spectrum, and which produce in the eye the sensation of light and colour. Experiments made some years since convinced Mr. Crookes that it is not merely the ultra-violet invisible rays which are valuable for photography, but that some of the most highly luminous rays of light are capable of exerting chemical action; and this position was ultimately proved by him by means of a combination of certain chemical compounds.

It is very likely that the further carrying out of these experiments may lead to the construction of a photometer capable of measuring the luminous rays; it being remembered that the proportion of red, yellow, green, and blue rays is always invariable in white light (for, if this were not so, the light would not be white, but coloured), from which it follows directly that a satisfactory method of measuring one set of the components of white light will give all the information we want, just as in an analysis of a definite chemical compound the chemist is satisfied with an estimation of one or two constituents only, and from these is able to calculate the others. Methods based on the previous considerations would supply us with what may be termed an absolute photometer, the indication of which would be always the same for the same amount of illumination, and would require no standard light for comparison. A relative photometer is one in which the observer has only to determine the relative illuminating powers of two sources of light, one of which is kept as uniform as possible, the other being the light whose intensity is to be determined. The first thing to be aimed at is an absolutely uniform source of light, and this is most difficult to obtain. In the ordinary process of photometry the standard used is a candle, defined by Act of Parliament as a sperm candle of six to the pound, burning at a rate of 120 grains per hour:" hence the meaning of such terms as "12-candle gas," "14-candle gas." The difficulty is to obtain candles truly made, containing refined sperm mixed with a small portion of wax, and wicks of the best cotton, each made of three cords plaited, and each cord itself again of seventeen strands. Again, according to the quality of the sperm in richness or hardness, so will also vary the plaiting and number of the strands; and, further, experience shows that, supposing all the previous conditions to be satisfactorily attained, the illu minating power of the candle will be found to vary with the temperature of the place where it has been kept, the time which has elapsed since it was made, and

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the temperature of the room where the experiment is tried. The "Parliamentary candle," therefore, may be pronounced a failure wherever accurate results are required. The general principle on which the illuminating powers of different substances have hitherto been tested depends on the optical law that the amount of light which falls upon a given surface varies inversely with the square of the distance between the source of light and the object illuminated. In practice, however, this method is not sufficiently accurate to be used, except for the roughest approximations. The rest of Mr. Crookes' paper is devoted to details of a process he has invented to get rid of the "Parliamentary candle." This is, however, too abstruse and minute for extraction here.

The Earl of Rosse, F.R.S., has communicated a paper "On the Radiation of Heat from the Moon," which is too mathematical for a popular notice. The general result, however, is to show conclusively that the moon's heat is capable of being detected with certainty by the thermopile; no inconsiderable quantity of heat reaching the earth by radiation from the moon. The points to be determined were:-1. The heat which, coming from the interior of the moon, does not vary with the phase. 2. That which falls from the sun on the moon's surface and is at once reflected regularly and irregularly. 3. That which, falling from the sun on the moon's surface, is afterwards radiated as a heat of low refrangibility.

Mr. Ellery, of the Observatory, Melbourne, in a letter to the President of the Royal Society, gives an interesting account of the safe arrival of the great telescope we noticed at some length last year, and of the means which have been already taken to set it up and to make it available for its intended use. He states it had, on the whole, travelled perfectly round the Cape, his words being that "the principal or more delicate portions of the instrument came out in good order; the specula are still in their coats of varnish, and their surfaces appear to be in perfect good order. Some of the large castings and portions of the gearing had got rusted, but not to an injurious extent. The piers were completed on New Year's morning, and form a magnificent piece of masonry; the stone employed being the grey basalt so common here (called blue stone'), in blocks from one to three tons in weight each. The building we have finally decided on is built of stuccoed brick-work, eighty feet long by forty wide. Forty feet in length is taken up by the telescope-room, which is covered by a ridged roof of iron travelling on rails on the walls, and moving back on the other forty feet of the building, leaving the telescope in the open air. The back forty feet is covered by a fixed roof lower than the moveable one, and will contain a polishing and engine-room, a capacious library, and an office for the observer. The cost of piers, building, and roof will be 17007. The Government, with hard economy in all other directions, have acted very liberally about this work."

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Dr. Radcliffe, M.D., has contributed a very able paper, entitled "Researches in Animal Electricity," containing a description of certain instruments now employed for the first time in researches of this kind, the chief subjects of inquiry being the electrical phenomena which belong to nerve and muscle in a state of rest; those which mark the passing of nerve and muscle from a state of rest into that of action; the motor phenomena ascribed to the action of the "inverse" and "direct" voltaic currents, and electrotonus. The instruments used were Sir W. Thomson's reflecting galvanometer, Latimer Clarke's potentiometer, and some new electrodes devised by the author. The last, which is an ingenious adaptation of the idea on which Wheatstone's bridge is based, is an extremely delicate instru

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