Dec 



[880] 



NATURE 



any other part. The stratifications produced by electric dis- 

 charges through flames may be similarly explained ; and these 

 researches have an obvious bearing on the structure of Lichten- 

 berg's well-known figures. 



MONS. Mercadier has been devoting some attention to the 

 subject of the pholophone, and more particularly to the produc- 

 tion of sounds by the simpler forms of the instrument, in which a 

 selenium receiver with its electrical connections is dispensed 

 with. The musical photophone — or, as M. Mercadier chooses 

 to style it, the radiophone — may be described as a sort of 

 optical siren, in which a rotating disk pierced with holes is 

 interposed in the path of a period of rays of light, causing 

 intermittences of regular period varying with the speed of the 

 disk. Our readers will remember that sucli a beam falling on a 

 simple disk of metal or of hard rubber throws it into vibration, and 

 it emits a note corresponding in pitch with the frequency of the 

 intermittences of the liglit. In Prof. Bell's actual instrument 

 this " siren " was a heavy dislc of brass pierced with holes. M. 

 Mercadier prefers a disk of black paper gummed upon a glass 

 disk in order to get rid of the whistling sounds which even a 

 gentle cuiTent of air produces on the brass disk. It may be 

 noted in passing that ^i. Duboscq has independently constructed 

 similar disks. The receiving disks were fixed in a suitable 

 holder at the end of a short india-rubber hearing-tube. M. 

 Mercadier finds that when opaque disks of zinc, copper, and 

 other substances are employed to receive the beams, very little 

 difference in the loudness of tlie sounds can be perceived, 

 whether the disks are polished or not. But the thickness of the 

 disks is of great importance, thin ones answering much better 

 than those a little thicker. With transparent lamina; such as 

 glass and quartz, M. Mercadier obtains strong effects, whereas 

 Prof. Bell found only feeble results with these substances. The 

 degree of polish is here unimportant also ; but a film of smoke or 

 white paint, or of metallic silver on the front of the disk, dimi- 

 nishes its powers, while, on the contrary, the loudness is .augmented 

 by blackening the back of the disk. M. Mercadier employed as 

 sources of light the lime-light and flames of petroleum fed with 

 oxygen. 



Herr F. Klocke has lately discovered an anomalous pro- 

 perty in hyposulphite of lead in respect of its action on polarised 

 light. This substance usually exhibits circularly polarised light : 

 but Klocke has found that plates cut perpendicularly to the 

 optic axi--, when viewed in the field of a polariscope by parallel 

 rays of light, appear unequally bright, Ijeing divided by dark 

 bands into si.-c sectors, of which opposite pairs are equally bright. 

 In convergent light, moreover, the ordinary ring-figure of a 

 uniaxial ci-ystil is not seen, but instead there appears in each 

 sector a figure of the form characteristic of the ordinary biaxial 

 crystal, and having the plane of the optic axes perpendicular to 

 the neighbouring edge of the crystal. The expkanation of this 

 curious phenomenon appears to be that there is some anomaly 

 in the molecular structure of the crystals, by virtue of which the 

 six portions are compressed equally each in direction perpen- 

 dicular to the neighbouring face of the prism. 



In the Vienna Bericlilc for June, 18S0, Victor von Lang 

 describes a form of dichroiscope, in which a small improvement 

 upon the common form has been made. Usually the small 

 square aperture tlirough v^ hich light is admitted to the rhomb of 

 spar is fi.ved rigidly to the tubular holder of the latter. In the 

 new form the square aperture is cut in a diaphragm fastened to 

 an outer tube, which can be rotated round the iinier. The 

 advantages gained in permitting the rhomb of spar to be turned 

 independently of the aperture are obvious. A plano-convex 

 lens of small magnifying power is added as usual as an eyepiece 

 at the other end of the rhomb. 



M. Amagat has experimented on the compressibility of 

 oxygen gas in an apparatus in which the working fluid for trans- 

 mitting the pressure was mercury. Since the experiments of 

 Regnault it has been commonly assumed that the absorption of 

 the gas by mercury at high pressures and temperatures rendered 

 inexact any such experiments. M. Amagat however finds that 

 the absorption is almost insensible, an oxygen manometer and a 

 nitrogen manometer giving identical indications for several days, 

 even with temperatures varying up to 100°. 



MM. Hautefeuille and Chappuis have continued their 

 researches on the liquefaction of ozone, which they have lately 

 liquefied in the "presence of carbonic acid. They believe the 

 point of liquefaction of ozone to be very near that of carbonic 



acid; and on mixing ozonised o.xygen with carbonic acid and 

 submitting it in a capillary tube to a slow pressure at a tempera- 

 ture of - 23' (obtained by the evaporation of methylic chloride), 

 they obtained a liquid separated by a distimt meniscus from the 

 gas. This liquid was of a clear blue tint, as was the compressed 

 gas above it. If the substance is then allowed to expand gently 

 and immediately compressed, the liquid becomes much more 

 blue, OH ing to the greater proportion of liquefied ozone. The 

 blue tint thus characteristic of ozone under pressure proves it to 

 be present in the gases which result when the silent electric 

 discharge is passed through carbonic acid gas for some hours. 



Edison has lately patented a " webcrmctcr." This is an 

 instrument for measuring the amount of electric current flowing 

 through a circuit, or in other words a meter for electric currents 

 to tell the number of webers that have been supplied. The 

 name is at least in accordance with the inventor's usual abundant 

 ingeniosity. 



In the Complcs ixiidus M. Gouy publishes an extract from a 

 memoir presented by him to the Academic des Sciences, on the 

 propagation of light. In this memoir he proposes to examine 

 the particular case of propagation of luminiferous waves, in 

 which, while the direction of the propagation of the movement 

 is constant, the intensity of the waves or of the source of light 

 varies. This problem, which has doubtless been suggested to 

 the author by considerations derived from the photophone, affects 

 the whole question of tlie measurement of the velocity of light, 

 whether by the methods of occultations of Ra?mer and Fizeau, 

 or by that of aberrations (in the rotating mirror), as devised by 

 Foucault. Tlie former case only is treated of in M. Gony's paper. 

 Setting aside at first the case of dispersive media, and restricting 

 the question to isotropic media, M. Gouy investigates mathe- 

 matically whether the velocity of propagation of the amplitude is 

 the same as that of the wave, and finds that this is the case only 

 for those waves for which the differential equations contain no 

 terms beyond those of the second order — those in which the 

 vibration has virtually attained to the steady condition. For such 

 waves moreover in dispersive media the amplitude is not propa- 

 gated with the same velocity as the waves themselves, but the 

 amplitude itself varies according to a complex function of the 

 wave-length according to an ascertainable periodic law. If we 

 remember rightly, a similar hydrodynamic investigation of the 

 rate of propagation of waves in water was made some years ago 

 by Prof. Osborne Reynolds, with the result that the effective 

 wave-front only travelled at half the velocity of the steady 

 waves. The inference is that that which physicists usually term 

 " the velocity of light " is only the rate of propagation of the 

 wave-front, which is slower than the true velocity, the retardation 

 being greatest for the vibrations of greatest wave-length. 



Another new property of selenium is claimed as the disco- 

 very of M. Blondlot. He states that when selenium is rubbed 

 upon platinum, each metal being connected w ith a terminal of 

 a capillary electrometer, a current is observed. This current, 

 which is observed to pass, through the electrometer from the 

 platinum to the selenium, appears therefore to differ from the 

 tribo-electric currents discovered by Becquercl, and which were 

 always in the same direction as the thermo-electric currents 

 which would have been produced had the surfaces of friction been 

 directly heated. The true thermo-electric current of a selenium- 

 platinum pair is, according to M. Blondlot, from selenium to 

 platinum through the heated junction. One curious point stated 

 by M. Blondlot is that no indication whatever is obtained upon 

 the capillary electrometer by friction between two metals, or 

 between two insulators, or' between a metal and an insulator. 

 The electrometer in the selenium experiment indicated a differ- 

 ence of potential about equal to that of one Daniell's cell. 



Magnus and Tyndall found carbonic acid to have a con- 

 siderable absorbent action on radiant heat. Dr. Lecher {Il'ieii. 

 Acad. Am. ) has lately made new observations, especially as to ab- 

 sorption of solar radiation by the carbonic acid in the atmosphere. 

 Experiments with a gas-lamp and glass cylinder first shewed 

 that carbonic acid in a length of 214 mm. gave passage to 94'S 

 per cent, of the radiation; 536 mm. 93"8 per cent. ; 917 mm. 

 Sg'o per cent. At Greifenstein, outside of Vienna (chosen for 

 pure air), the sun's rays also w ere proved to undergo consider- 

 able weakening in passage through 'carbonic acid gas. A layer 

 of this gas one metre thick' absorbed about 13 per cent, when 

 the sun had an altitude of 59° ; the number however diminished 

 in proportion as the sun got Ijwer. This shows that the absorp- 

 tion of solar radiation by carbonic acid is selective, and that the 



