tions the more diathermic liquids exhibited a remarkable and anomalous deportment towards radiant heat. This observation led him to make a subsequent investigation, the results of which are summed up as follows:— When the more diathermic liquids are introduced between two parallel plates of rock-salt separated by a very small interval, the rays from an artificial source are found to be more freely transmitted than when air intervenes between the plates. For a space of 02 inch wide the increased transmission amounts with bichloride of carbon to about 12 per cent., with bisulphide of carbon to 9 per cent., and with chloroform to 4.5 per cent. This effect disappears and less heat is transmitted (1) when the transcalency of the liquid diminishes; e. g. the same thickness of sulphuric ether intercepts 30 per cent. of the heat previously passing through the empty cell; (2) when the distance between the plates is increased beyond, say, 10 of an inch in the case of bisulphide, and of an inch in the case of bichloride of carbon. The increased transmission by these liquids reappears, however, in thicker layers when plane parallel glass plates are substituted for rock-salt, and continues, apparently indeed augmenting, as the depth of the cell increases, so far as the experiments were carried. Bisulphide of carbon, poured into a cell with glass sides 12 inch apart, increases the heat falling on the pile 6 per cent., and bichloride of carbon a still larger amount. Altering the temperature or nature of the source, the size of the aperture in a screen behind the cell, or the position of the cell, makes no material change in these results. But altering the character, or augmenting the thickness, of the walls of the cell has considerable influence. For example, if the cell-walls be of glass, increasing their thickness from one- to three-tenths of an inch raises the heat falling on the thermoscope 6 per cent. when equal depths of the selfsame liquid are poured into the cell. Again, by merely changing the parallel sides of the same cell from rock-salt to precisely similar plates of glass, the very same liquid can be shown to intercept a certain quantity of the heat falling on the thermo-pile in the one case, and to augment that quantity in the other-the difference amounting to upwards of 10 per cent. of the total radiation through the empty cell. The explanation of the foregoing facts may be traced to two main causes. The increased transmission noticed with films of the more diathermic liquids chiefly arises from the reduction or abolition of the reflection taking place from the interior surfaces of the walls of the cell, owing to the optical density of the liquid introduced being nearer to the cell-walls' than that of the medium it replaces. But in glass cells of considerable depth, retaining the former explanation, the augmented heat there observed is probably mainly due to an effect of the refraction of divergent rays by plane surfaces; this gives rise to a concentration of the beam, which become sensible when accompanied by a great transcalency of the liquid in the cell. In similar cells with rock-salt ends the effect is not observed, probably on account of such cells sifting the beam far less than glass, and thus permitting a higher absorption of the liquid. Nevertheless even with rock-salt cells the causes alluded to must necessarily render, to a certain extent, incorrect the precise absorption hitherto attributed to liquids. These sources of error in determining the true absorption of a liquid or solid can, however, be avoided by employing truly parallel rays; and these are best obtained from the sun. On the Thermal Resistance of Liquids. By FREDerick Guthrie, F.R.S.E. If we wish to get an insight into the specific resistances of the elements and into the law connecting thermal resistance and chemical constitution, we must examine liquids rather than solids, because while the former are essentially homogeneous, the latter are never without structure, and seldom even without texture. To examine the conductivity of a liquid, it must be either heated from above or cooled from below, in order that convection may be avoided. If the liquid be contained in a vessel, the difference between the conductivities of the liquid itself and of the containing vessel will also introduce convection. In spite of the labours of many able physicists, these difficulties have hindered the prosecution of this direction of research. The chief numerical results are those obtained by Despretz in his well-known examination of the conductivity of water. In order to homologate the thermal and electrical phenomena, the term thermal resistance is used in preference to conducting-power or conductivity. The zero of thermal resistance is supposed to exist when two bodies of unequal temperature are in actual contact. If a substance is interposed between the hotter and colder bodies in such a manner that the heat can only pass between them by means of conduction through the interposed substance, then the difference between the quantity of heat which passes when the bodies are in contact and the quantity which passes when the third substance is interposed, is equal to the quantity of heat intercepted by that substance, and is a measure of its resistance. The instrument used for this purpose is the "Diathermometer;" its construction is as follows:-Two conical brass vessels having thin polished platinum bases are fastened in a stand in such a manner that their axes are in the same vertical straight line; their bases are opposed to one another and are parallel. The apices of both cones are made tubular. The lower cone is screwed into the stand, its neck is fitted with a cork and tube, which dips into water, and which carries a scale. The lower cone and tube form an air-thermometer. The upper cone is moveable vertically, its motion being commanded by a micrometer-screw, which is so divided as to allow of the adjustment of the cone to the 0·005 of a millimetre in vertical direction. The parallel bases of the two cones are adjusted horizontally by a spirit-level and levelling screws in the stand. The upper cone carries a cork, through which pass two tubes, one reaching to 0.5 millimetre of the bottom of the cone, the other opening just below the cork. A current of water may thus be made to flow through the upper cone. A large vessel of water is maintained at any required constant temperature by means of a thermostat. Screens intervene between this vessel and the diathermometer. By means of a siphon and flexible tubes, a current of warm water of known temperature is allowed to pass through the upper cone, commencing at any given moment. The zero or minimum resistance is found by wetting the bases of the cones with a little mercury and bringing them into contact (the air-film is thus excluded), and passing water of a known temperature for a given time through the upper cone. If the calibre of the tube of the lower cone is known, we can, by observing the linear depression of the column of water in it, calculate the number of heatunits which enter the air of the lower cone by taking into account its capacity, the specific heat of moist air, and the pressure to which the latter is subjected. If, now, the cones be separated to a known distance and a liquid be introduced between them, it is supported in its position by adhesion and cohesion. When water is passed through the upper cone under the same conditions as in the experiment when the cones were in contact, a less number of units of heat enter the air of the lower cone (as is shown by the smaller amount of depression in the thermometer-tube). This diminution in the number of heat-units is called the resistance of the liquid under the special conditions. As the area of the base of the cones is known, we can calculate the resistance of a rectangular prism of known base and height of a liquid for a given time at a given temperature, and for a given temperature-difference at its two extremities. The loss of heat suffered by the water in passing from the reservoir was estimated at all temperatures and allowed for. The absolute errors due to the absorbtion and radiation of heat by the brass of the lower cone and to the accumulation of heated air in its upper portion, affect equally the determination of the minimum resistance and that of the interposed liquid; consequently they do not affect the result, which is their difference. It was shown by measuring the time required to produce any effect on the lower cone, as also by interposing paper disks in the liquid between the cones, that the diathermancy of the liquids at the temperature-differences employed was either nothing or so small as to be negligible. Special experiments made by colouring the base of the upper cone showed that there was no convection. The resistance (measured by the number of heat-units arrested in a given time) of a cubic millimetre of about twenty chemically pure liquids was determined under the conditions-temperature of liquid=20°-17 C., temperature-difference =10° C. for water Specific resistance. 1.0 The column of specific resistance is obtained by dividing the resistances of the other liquids by that of water. Of all liquids, with the exception of mercury, water has the least resistance. Of bodies belonging to the same series, those have the greatest specific resistance which have the greatest molecular complexity. Experiments were made to determine the rate at which heat of different temperatures travelled through water. Through 3 millimetres of water the first effect of the heat was manifested in 11 when the temperature-difference was 5.8 9 15.8 25.8 It appeared from numerous experiments made in this direction that for the above thickness of water the time for the production of the first effect is diminished about 1" for every increase of 5° C. in the temperature-difference. With regard to the time required for the heat to penetrate different thicknesses of water, it was found that the time increases more rapidly than the thickness. Thus for a temperature-difference of 10° C. The quantities of heat passing through various thicknesses of water in a given time were also determined; and it was found that the resistance was not by any means proportional to the thickness. Thus if the resistance through On examining aqueous saline solutions, it was found that their resistances were always greater than that of water, even when the metal in solution was a good conductor, and when the solutions were saturated. From the experiments made in this direction, it is concluded that the solution of a salt in water affects the resistance of the latter only either by displacing some of it and altering its specific beat. LIGHT. Certain facts bearing on the Theory of Double Refraction. By A. R. CATTON. On Actinometry. By LOUIS BING. The writer describes a series of experiments which he made for the purpose of ascertaining the actinic power of light. He shows that the transmission of actinism through a transparent medium varies with different intensities of light in such a manner that instruments constructed for actinometric purposes by means of a transparent medium are practically almost useless. He describes an actinometer which he constructed, and which consists of a single rectangular tube, at one end of which light for measurement is admitted, and to one side of which sensitive 1868. 2 paper is applied. He says, "The principles upon which this instrument is founded are (1)" That diffused light, on entering a tube at one end only, varies in intensity within the tube inversely as the squares of the distances from the aperture where light enters. (2) "That any number of tubes, whatever their magnitudes, contain the same intensity of light if the ratios of their diameters to their lengths are equal, and if we absorb the light that may be reflected from their sides." He then describes experiments which he made by means of tubes of equal diameter, and of lengths varying by a semidiameter and also with tubes of various magnitudes, the ratios of their diameters to their lengths being equal. By inserting small mica-actinometers at the bases of those tubes, and exposing them thus situated simultaneously to the action of light, the writer gains results which demonstrate the above principles. Observations on the Atmospheric Lines of the Solar Spectrum in High Latitudes. By GEORGE GLADSTONE, F.C.S., F.R.G.S. This paper was explanatory of some diagrams which the author had prepared of the atmospheric lines in the solar spectrum, from observations taken by him during a recent voyage along the north-west coast of Norway. The author stated that what are known by observers of the solar spectrum as the "atmospheric lines" are certain dark lines or bands, which make their appearance under certain conditions, and sometimes even attain a considerable development. These lines, or bands, appear to be due to the presence of some substances in the earth's atmosphere, as they are always most prominent when observing the sun through a long reach of air (as at sunrise or sunset), while they are scarcely visible when the sun is high above the horizon. The observations, of which drawings were exhibited, were taken in the months of June and July last, from the deck of the vessel when off the coast near Stavanger, and at the entrances to the Trondhjem and Namsen fjords; the latter being in 64° 30' north latitude, in which parallel the sun skirts the horizon for a long time, thus affording very favourable opportunities for observation. It appears that in those regions the red end of the spectrum is very brilliant, so that with the small portable spectroscope he distinctly recognized, on two occasions, the remarkable line A. The observations went to show that the atmospheric band grows in width and intensity as the sun approaches the horizon, and that what in certain states of light, or of the atmosphere, appear to be bands of shade are under other circumstances broken up into lines. Under some conditions the red rays suffer very little diminution of light up to a certain point, when they are suddenly cut off; while under others the obscuration takes place more gradually, and the visible spectrum is much longer. The length of the spectrum, however, in no case affects the width between the respective lines, which remains always the same, but is entirely due to more or less of the extremities being altogether lost in darkness. On the Value of the Hollow Wedge in examining Absorption Spectra. By Dr. J. H. GLADSTONE, F.R.S. The usual way of examining absorption of light by a coloured liquid is to place it in a test-tube behind a narrow slit, and to disperse the line of light by means of a prism. The black or shaded bands due to the absorption may thus be easily noted; but of course they represent only one particular thickness of the liquid. Now the number of these bands often varies, and the extent of them always varies with the depth of the liquid traversed by the light, or, if it be a solution, with the quantity of colouring-matter dissolved. A great advantage is obtained by substituting a hollow glass wedge for the test-tube, and so arranging it before or behind the slit that the narrow line of light examined shall have traversed all thicknesses from, perhaps, two centimetres to nothing. Thus the varying absorption at different depths is seen all at once, and can be easily represented in a diagram which becomes characteristic of the particular substance. The value of this mode of TRANSACTIONS OF THE SECTIONS. examining absorption spectra was illustrated by an extreme case-permanganate of potassium; ordinary drawings of the bands were made for four different thicknesses, and the same were expressed in Mr. Sorby's ingenious notation of numerals and dashes and dots, which four figures bore little or no apparent relation to one another, but they were explained by, and included in, the figure obtained when the hollow wedge was employed. Figures were also exhibited showing the absorption of light by cruorine, hæmatine, and cineal in alum, which illustrated in various ways the importance of observing the effect of the varying thicknesses of these substances. The author suggested the use of the hollow wedge as far back as the Cheltenham Meeting in 1856, and exhibited there, and published afterwards, many diagrams of absorption spectra thus obtained. The reason why other observers have made little use of it was believed to be twofold-it is too simple to please some, requiring no apparatus beyond a common window-shutter, the wedge, and a good prism, while it demands a little more thought in adjustment than the test-tube does, seeing it also bends the ray of light. It may be advantageously placed in front of spectroscopes of the ordinary form; and as the prismatic analysis of transmitted light is becoming a matter of great importance, it seems desirable to adopt the best methods. Sur une action particulière de la lumière sur les sels d'argent. ELECTRICITY, MAGNETISM. On a further development of the Dynamo-Magneto-Electric Machine. At the Meeting last year the author brought before the Section one of his small dynamo-magneto machines, the first that had been made upon that principle. The author has since constructed a much larger machine, and it may be interesting now to give some particulars respecting it. The object in constructing it was to supply a good electric light for the purpose of lecture demonstrations. It is constructed upon the double armature principle, both armatures being placed end to end, so that their magnetic axes cross each other at right angles. The short armature contains 108 feet of very stout copper wire, and sends its currents into 240 lbs. of copper wire surrounding the electromagnet, exciting a large amount of magnetism in the body of the machine. And as the second armature is also made to revolve between the poles of this electromagnet, a sufficient effect is produced at the two ends of the 312 feet of very stout copper wire (which is wound upon it) to produce a good electric light from the carbon-poles of the regulator. But in order to make that light sufficiently continuous, it is requisite that the armatures should revolve from 1800 to 2000 revolutions per minute; but as the armatures have to be magnetized and demagnetized twice during each revolution, there would be in the latter case 4000 flashes of light per minute. Now it has been shown that every time iron becomes magnetized it is elongated, and again shortened when demagnetized. At every alteration, therefore, of the condition of the iron some small amount of heat must be devolved, and would increase to such an extent that, if unchecked, it would in the course of time be so great as to destroy the insulation of the wire. The author did not wish it to be inferred that the sole cause of heat is due to the elongation of the iron. Doubtless the electric currents passing through the wire would produce heat; but he believed the quantity produced by that means would be small as compared with that produced by the elongation of the iron itself. The author gave the following reasons for entertaining this opinior. One of these magneto-machines driven by steam-power was lately used in connexion with a large inductorium, and after a few hours it was found that the copper or primary 2* |