state, and the spectrum of this gas being yet unknown, the observ ations of M. Lielegg have served to fill up a gap in the series of spectra produced by the gases in ignition. The apparition and the disappearance of some of the luminous fixed lines is closely connected with the metallurgical operation. At the moment the decarbonization of the iron is nearly terminated, the spectral lines undergo essential modifications. The apparition of a group of lines and of an isolated line in the violet-blue portion of the spectrum marks a particular reaction, during which the soft iron is being formed, and these lines disappear sooner than all the others; their appearance and disappearance serve therefore to indicate the termination of the process.

The same indefatigable observer has also ascertained that the spectrum of the colour of sea water is deprived of its red portion at small depths, and successively of the yellow and green, for the greater depths, until it appears of a violet-blue. In trying to ascertain whether the same was the case in glaciers, he has made some interesting experiments in an artificial grotto in the Grindenwald glacier. This cavern is 100 mètres deep, transparent in its walls, through which the solar light penetrated. The light was of a fine blue tint, the red being extremely weak, so that in the grotto human countenances assumed a cadaverous aspect. On looking towards the entry, at a certain distance in the cavern, it appeared to be lit up with a red light, doubtless the effect of contrast. The thickness of the superposed mass was not enough to show a greater effect than the almost complete absence of the red, and a great diminution of the yellow. The ice was said to be 15 mètres thick, but was probably less; it was perfectly compact and limpid, but with a few air-bubbles.

M. Felix Lucas concludes from theoretical considerations that the luminous distance at which the electric spark is visible is greater than that of a permanent light, the apparent intensity of which would be 250,000 times that of the spark. The light actually employed to illuminate modern lighthouses gives a brilliancy equal to 125 carcel lamps. An electric spark possessing the illuminating power of the 200th part only of a carcel burner is superior as to its power of projecting light. Hence we can conceive the immense effect of a warning light composed of intermittent flashes of the electric spark proceeding from a strong Leyden battery. M. Lucas states that, in an experiment made in a laboratory two apparatuses were established, one voltaic battery equal to 125 carcel lamps, and another spark-battery equivalent to only the 1-2000th part of a carcel lamp. The photometer (such as is employed in the lighthouse administration) showed a marked superiority in favour of the spark.

Photographers will read with interest the announcement by M. Prat of the discovery of a compound of silver more sensitive to light than the chloride. This chemist has been for some time past investigating the chemical constitution of fluorine compounds and the isolation of fluorine. M. Prat starts from the fact that the fluorides are really oxyfluorides; that the fluoride of calcium, for example, is formed of two equivalents of calcium, one of oxygen, one of fluorine; and that, in consequence, the true equivalent of fluorine is 29-5, and not 19. In order to obtain fluorine, it is only necessary to treat the fluoride of calcium with chlorate of potassium, or, what is better, perchlorate of potassium, for it is only with this last salt that the reaction takes place. Oxygen is disengaged, and a gas is produced, which silver absorbs, giving rise to a fluoride of silver, insoluble in water, soluble in ammonia, from which it is precipitated by nitric acid, and which is altered by the action of light more rapidly than the chloride of silver; the formula of the real chloride is Ag Fl, whilst that of the soluble fluoride of chemists is Ag Fl, Ag O.

Several new instruments, suitable for the observation of different organs of the eye, have been described by M. Robert Houdin. They serve also for the examination of entoptic images, or the shadows thrown on the retina by intra-ocular bodies. Seven instruments of this class have been invented by M. Houdin; these are: 1, the Iridoscope, for the manifestation of entoptic images; 2, the Diopscope, by the aid of which the inversion of the images on the retina are determined; 3, the Pupilloscope, demonstrating in a magnified form the dilations and contractions of the pupil; 4, the Pupillometer, which gives the diameter of the pupil to within a quarter of a millimetre; 5, the Diopsimeter, for measuring the extent of the field of vision; 6, an Optometer, for the use of any persons who wish to determine the distance of distinct vision; 7, the Retinoscope, an instrument with which one can see the vesicular group in his own eye.

In all stereoscopes there is an optical arrangement by which the right eye sees an image of one picture and the left eye that of another. These images ought to be apparently in the same place, and at the distance of most distinct vision. In ordinary stereoscopes these images are vertical; the observer has to place his eyes near two apertures, and he sees the united images, as it were, behind the optical apparatus. Professor J. Clerk Maxwell, F.R.S., has recently had made by Messrs. Elliott, Brothers, a real-image stereoscope, in which the observer stands at a short distance from the apparatus, and looks with both eyes at a large lens, the image appearing as a real object close to the lens. The stereoscope consists of a board about two feet long, on which is placed:-1, a vertical frame, to

hold the pair of pictures, which may be an ordinary stereoscopic slide turned upside-down; 2, a sliding piece near the middle of the board, containing two lenses of six feet focus, placed side by side, with their centres about one inch and a quarter apart; 3, a frame, containing a large lens of about eight inches focal length, and three inches diameter. The observer stands with his eyes about two feet from the large lens. With his right eye he sees the real image of the left-hand picture formed by the left-hand lens in the air close to the large lens, and with the left eye he sees the real image of the other picture formed by the other lens in the same place. The united images look like a real object in the air, close to the larger lens. This image may be magnified or diminished at pleasure, by sliding the piece containing the two lenses nearer to or farther from the picture.

HEAT.-Herr C. Sching has investigated the subject of fusible silicates, and the temperature required for forming and melting the same. He finds by application of a thermo-electric pyrometer that silicates are formed and melted at the same temperature, and that the formation of the silicates depends more on time than on temperature, i. e. it depends, in fact, on the conducting power of heat which the materials composing the silicates possess. He also finds the temperature required for melting metals and metallurgical products to be lower than usually stated, 1,431-1,445°, for melting the same. Sching now finds that a temperature of a glass furnace in operation is only 1,100-1,250° C.; that crystal glass is worked at 833°, and becomes completely liquid at 929°. A Bohemian green glass tube softens at 769°, and becomes liquid at 1,052°. Pure limestone loses its carbonic acid by heating for several hours at a temperature of 617-675°. An increase of the temperature will shorten the time.

Mr. C. Tomlinson, F.R.S., in a communication to the 'Chemical News,' has stated his opinion that the Camphor Storm-Glass is useless as a meteorological instrument. The frequent reference made to it by the late Admiral Fitzroy gave an almost official sanction to its use, and induced some instrument makers to manufacture it largely, and even to attach it to the ordinary barometer and thermometer. This led Mr. Tomlinson to examine the storm-glass with some care. One was made on a large scale in a quart bottle, placed on the window ledge, and a journal of its behaviour kept during some months. The conclusion arrived at was that the storm-glass is not acted on by light, or atmospheric electricity, or wind, or rain, &c., but solely by variations in temperature; that it is, in fact, a rude kind of thermoscope, vastly inferior to an ordinary thermometer, and has no meteorological value whatever.

VOL. V.

I

The subject of the transparency of red- or white-hot metals was referred to in our last Chronicles. It is generally believed amongst scientific men that the supposed phenomenon is merely an optical delusion. A correspondent of the 'Chemical News' has, however, adduced an observation on the opposite side. He says a few weeks ago he went over some steel works in the North of England, and there the manager spoke of it as a well-known fact that steel at a white heat was transparent. In proof of this he showed that when the molten metal was being poured out, the edge of the crucible appeared to be distinctly visible through the molten metal. This could only be seen directly the crucible was taken out of the furnace before it had cooled in the least.

Mr. A. E. Fletcher, Government Inspector of Alkali Works for the Western District, has constructed a most useful instrument for measuring the velocity of a current of air. He uses it for measuring the speed of air in flues and chimneys. The construction of the apparatus is based on the fact that a current of air, passing across the open end of a straight tube, causes a partial vacuum in it. An application of this principle is seen in a small toy in common use, in which a liquid is made to ascend several inches in a vertical tube, by blowing through another tube across its open end; it rises by virtue of the partial vacuum caused by the current of air which crosses it. If then a straight tube is inserted through a hole in the brickwork of a chimney or flue, so that the current of air in the flue passes across its open end, a 'partial vacuum will be formed in it, greater or less in proportion to the velocity of the current. A tube in such a position will, however, communicate a suction arising from that of the chimney itself, besides that suction produced by the current of air passing across its open end, and for the present purpose these two must be distinguished. To effect this, two tubes should be inserted in the chimney, one of them having a straight, and the other a bent end, the bend to be turned so as to meet the current of air; both tubes are open. In each of these tubes will be experienced the partial vacuum due to the suction of the chimney itself. In the straight tube, however, this will be increased by the suction caused by the passage of the current of air across its open end, while in the case of the bent tube this will be diminished by the pressure caused by the current of air blowing into it. The difference therefore between the suction in the two tubes will be due to the action of the current of air in the chimney, and it remains only to measure this difference in order to measure the velocity of the current itself. After many trials, Mr. Fletcher adopted the following plan for measuring this suction. The tubes were connected with a U-Tube, and means were adopted for accurately seeing and measuring its slightest indications. In the first place, the limits were increased until they were no longer small tubes of

about 0-4 inch internal diameter, but cylinders of 4 inches diameter; these were connected at the bottom by a small tube. Thus the power, exerted by the pressure communicated through the connecting tubes, operating on the extended surface of the liquid in the cylinders, was increased a hundred-fold over that operating in the smaller U-Tube; but the friction could only have been increased ten-fold, giving therefore a ten-fold increase of delicacy. In order to observe accurately the rise and fall of the liquid in the cylinders, floats were introduced, on each of which was engraved a very fine horizontal line; and to measure accurately the comparative elevation or depression of these two lines, a finely divided scale and vernier were added, working with a delicate screw adjustment. With this it is possible to measure an elevation or depression of Tooth inch, which is sufficiently accurate for the purpose in view.

On trying now to apply the instrument so constructed, and attempting to measure very minute variations of pressure, failure still seemed imminent; for although the motion of the water in the increased limbs of the U-tube could be measured to both inch, the water refused to move except under pressures exceeding that which would be indicated by so small a column: in other words, the water seemed to stick in the cylinders. After substituting ether for water the action of the manometer was quite satisfactory; the lines on the floats always returned exactly to their original position after any disturbance, and its indications could be relied on to Tooth inch.

By the aid of this ether manometer the speed of any current of air in flues or chimneys can be measured by simply boring a hole one inch in diameter through the brickwork, and inserting two tubes, one with a bent, the other with a plain end as already described, and making the necessary observation of the floats; and in this operation neither soot, heat, nor corrosive vapours can prove any hindrance.

So sensitive is the apparatus, that on a windy day the effect of each successive gust of wind is observable, as it causes variations in the draught of the chimney. The instrument may be used as a wind gauge, by fixing through the roof of an observatory a small vertical pipe presenting a plain open end to the wind. The lower end of this pipe brought down into the observatory and connected with the ether manometer, would communicate the varying pressures due to the varying speed of the wind.

ELECTRICITY.-M. Rondel has examined a phenomenon which has been noticed more than once by workers with induction coils. If while the current of a pile passes through the primary wire of a coil, one of the extremities of the secondary wire is brought near one of the extremities of the iron core, sparks can be drawn of