706 



PHYSICS IN 1899. 



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gy. It follows that monatomic molecules 

 uld not show increase of specific heat. Mer- 

 cury and cadmium have been found to show a 

 decrease, for which the author accounts by the 

 diminution of mutual molecular action due to 

 increased distance apart caused by the rise of 

 temperature. 



Long Heat Wave*. Rubens and Aschkraasa 

 (Wiedemann's Annalen, LXVII, 2, p. 459) have 

 succeeded, by using quartz prisms of small re- 

 fracting angle, in obtaining waves of 5> ^, for 

 which quartz has a refractive index of 2.18. At- 

 tempts to obtain longer waves failed, because the 

 radiation of an absolutely black body beyond 

 60 n is less than T cn>W of the total ener gy- 



Light. Its Nature. Rovida (Rivista Scien- 

 tifica, XXX, p. 225) rejects Garbasso's conclusion 

 that monochromatic light is due to a simple vibra- 

 tion, and regards it as doubly damped. The spec- 

 tral " line " for each color must still have some 

 breadth, else it would not be visible, but this 

 indicates continuous damping in function of time 

 or of displacement. The former, when consider- 

 able, gives rise to widening of spectral lines: the 

 latter, to anomalous dispersion. The " continu- 

 ity " of the spectrum resembles the imaginary 

 continuity of water in hydrodynamics. 



Absorption. Fessenden (American Associa- 

 tion) notes that the flash of smokeless powder 

 may be distinguished at a distance by the use 

 of an absorptive color screen. The flash, he says, 

 proves to be essentially red, whereas the diffused 

 light coming from the landscape at large is very 

 weak in the red ^consequently when a red screen 

 is used the illumination of the general landscape 

 is greatly reduced, and the gun flash appears with 

 undiminished brightness. The contrast becomes 

 sufficient to enable the observer to readily locate 

 the flash. 



Refraction. Hallwachs (Isis Society. Dresden) 

 has investigated the relation between the refrac- 

 tive index of various substances and their degree 

 of concentration, in order to discover whether 

 this is influenced at all by dissociation. Experi- 

 ments with a double-trough refractometer, using 

 the author's differential method with grazing in- 

 cidence, show that the influence, if any, is too 

 small for measurement. Le Blanc and Rohland 

 had previously reported the existence of such an 

 influence in the case of brome-cadmium. Besides 

 this substance, Hallwachs used sugar and di- and 

 trichloracetic acids, with their potassium salts. 

 Bender (Wiedemann's Annalen, LXVIII, 2, p. 

 343) has determined the refractive index of pure 

 water for the hydrogen lines by means of Pul- 

 frich's refractometer, in which a cube takes the 

 place of the prism. Between 10 and 40 this 

 index is the same as that for water containing 

 air. Hallwachs (ibid., January) measures small 

 differences of refractive index without an inter- 

 ference refractometer by using glancing incidence 

 on a partition separating the two liquids to be 

 compared. By this method the refractive index 

 with respect to water may be found to within 

 one millionth of its value. 



Total Reflection. Ketteler (Wiedemann's An- 

 nalen, LXVII, 4, p. 879) states that the so-called 

 " glancing ray " which, according to Voigt, pene- 

 trates the second medium in total reflection, must 

 be explained on different principles from those 

 given. The energy of such a ray must be zero, 

 and, though there is wave motion at the boundary 

 of the two media, no' train of waves is propa- 

 gated into the second medium. (See also Polari- 

 zation, below.) 



Diffraction, Rayleigh (Philosophical Maga- 

 zine, XLVII, p. 375), in a mathematical investiga- 



tion into the weakening of light by passage 

 through a medium containing small particles,. 

 takes up the connected question whether the light 

 from the sky can be explained by diffraction from 

 the molecules of air themselves or whether it 

 must be produced by suspended particles, solid 

 or liquid. He concludes that even if there are 

 no foreign particles the light scattered from the 

 molecules of air would produce a blue sky, though 

 somewhat darker than the actual sky. 



Dispersion. Becquerel (Comptes Rendus 

 CXXVIII, p. 145), in a study of the anomalous 

 dispersion of sodium vapor, finds that for wa\e. 

 with frequencies slightly higher than either of the 

 lines D or D 2 the indices of refraction are nega- 

 tive that is, these waves travel faster in sodium 

 vapor than in a vacuum. 



Polarization. Geigel (Wiedemann's Anna 1m. 

 LX1V, 4, p. 098) has observed polarization of 

 the emergent beam in total reflection (ordinarily 

 unnoticeable) by using a hexagonal prism in 

 which a beam is totally reflected a large number 

 of times. Polarization in the plane of incid -n< < 

 appears after the forty-fourth reflection. The au- 

 thor believes that the component normal to tin- 

 plane of incidence emerges into the rarer medium 

 and is lost. Light of all visible wave lengths be- 

 haves in the same manner. Beckenkamp (ibid., 

 LXVII, 2, p. 474) finds that the rotation of the 

 plane of polarization in certain crystals is inde- 

 pendent of the orientation of the crystals but pro- 

 portional to the length of the ray. He applies 

 to the case a mechanical explanation de\i-M-d to 

 account for certain pyro-electric phenomena, ac- 

 cording to which three ether currents emulate 

 in normal planes around the molecule, and at 

 all places where the current path changes the 

 ether pressure is above or below the normal. 

 Borel (Comptes Rendus, CXXVIII, p. 1095) has 

 studied the magnetic rotatory polarization of 

 quartz discovered by Becquerel. The natural ro- 

 tational power of the mineral makes this diffi- 

 cult, but it was accomplished by using a com- 

 bination of right-handed and left-handed quart /-. 

 The rotation increases with the temperature at 

 a rate that increases with the refrangibility. 



Npcctroscopj/. Hamy (Comptes Rendus, 

 CXXVIII, p. 1380) describes a method for measur- 

 ing the wave lengths of standard spectral line- by 

 interference fringes. He has designed what lie 

 calls a "simplifier," with which he can suj>i 

 one or more members of a doublet or a triplet 

 without appreciably reducing the intensity of the 

 remaining light. Gramont (Comptes Rendus, 

 CXXVIII, p. 1564) has devised a method to re- 

 duce a spectrum observed in an ordinary spectro- 

 scope to any desired micrometric scale. In his. 

 spectroscope the micrometer is mounted in an 

 extra collimator, and its image is reflected into 

 the telescope by a face of the prism. Lenses ittj 

 the extra tube permit the observer to enlarge <>r 

 reduce the micrometer scale. The prism can be 

 turned out of the position of minimum devia- 

 tion, so that its dispersion in any given region 

 can be altered. Thus an unknown spectrum can 

 be mapped in terms of a known one. Liveing 

 (Proceedings of the Cambridge Philosophical So- 

 ciety, X, p. 38) has observed by using the very 

 hot flame of cyanogen in oxygen two spectral 

 lines of mercury hitherto obtained only in the 

 electric arc. The emission of these in conditions 

 where high temperature is the only stimulus 

 shows, according to the author, that part of the 

 heat communicated to the mercury is transformed 

 into vibrations that affects the ether. The true 

 inference from the peculiar ratio of the special 

 heats of mercury vapor appears to be that, at 



