532 



PHYSICS, PROGRESS OF, IN 1901. 



become a keynote. The acoustic scale is there- 

 fore the true scale for melody, as well as harmony. 



Transmission through Porous Materials. F. L. 

 Tufts (American Journal of Science, May) has 

 studied this question by using cylindrical vessels 

 filled with shot of various sizes, from 4.37 down 

 to 1.22 millimeters in diameter. The results indi- 

 cate the law that the resistance offered by gran- 

 ular material to the to-and-fro motion of a sound- 

 wave is proportional to the thickness of the mate- 

 rial, other things being equal. Hence the relation 

 between the resistance and the thickness is the 

 same as holds for direct currents or air. 



Heat. Expansion. Lemeray (Comptes Ren- 

 dus, Dec. 31, 1900) concludes from experiment 

 that if pieces of different metals have equal vol- 

 umes at absolute zero, their volumes will again be 

 equal to one another at their respective melting- 

 points. Thus the product of the coefficient of 

 linear expansion by the absolute temperature 

 will be constant. The author gives a diagram 

 which shows that in the case of sixteen metals 

 considered the conclusion is verified with reason- 

 able accuracy. 



Radiation. Thiesen and also Michelson state 

 that for any given wave-length the energy of 

 radiation increases indefinitely with the tempera- 

 ture, or, in other words, that there is no limit 

 to the amplitude of the vibration which gives 

 rise to that particular ray. On the other hand, 

 W. Wien considers that with rising temperature 

 the energy of every wave-length approaches a 

 finite limit. Lummer and Jahnke (Annalen der 

 Physik, October, 1900) endeavor to obtain a gen- 

 eral formula from which the two contending for- 

 mulae could be deduced by altering the constants. 

 The equation so obtained makes Thiesen's view 

 the only admissible one. 



Light. Photometry. J. Violle, in a paper read 

 at the Paris International Electrical Congress 

 (abstract in The Electrician, Sept. 28, 1900), gives 

 the following account of the various photometric 

 standards: 



I. Incandescence standards. (1) The platinum 

 international standard is satisfactory provided 

 the materials used are pure. Petavef prefers the 

 Violle standard. The uncertainty relatively to 

 the intensity of the light emitted by the melted 

 platinum under normal conditions does not ex- 

 ceed 1 per cent. 



(2) Lummer and Kurlbaum propose to use in- 

 candescent platinum below the temperature of 

 fusion and in such condition that ^ of the energy 

 radiated should be absorbed by a layer of water 

 2 centimeters thick; but a serious objection is 

 that this temperature is too low and the light 

 emitted is too red. 



(3) The crater of the positive carbon in the 

 electric arc may be taken as a standard, but a 

 difficulty arises, from the fact that when the arc 

 hisses the most brilliant part is displaced. 



(4) Incandescence lamps have been introduced 

 as intermediate standards in photometry. In or- 

 der that the luminous intensity may be constant, 

 very precise regulation of the potential at the 

 terminals is necessary. 



II. Flame standards. (1) The Hefner stknd- 

 ard, which uses amyl acetate and requires very 

 careful regulation of the height of the flame, is 

 the legal standard in Germany. Its intensity is 

 greater in winter than in summer owing to the 

 presence of water vapor, and the red tint of the 

 flame and its feeble luminous intensity are grave 

 faults. 



(2) The pentane standard excels the Hefner 

 lamp in having a more vigorous flame, but re- 

 quires more attentive regulation. The influence 



of humidity is the same as in the Hefner lamp, 

 and it is also affected by pressure. 



(3) In the apparatus of Dibdin the air is car- 

 buretted by passing over pentane in a reservoir, 

 which forms the base of the apparatus. The pro- 

 portion of air and of pentane depends on the tem- 

 perature of the room, and the degree of humidity 

 must also have a great influence. The Methven 

 standard, on the other hand, has a diaphragmed 

 flame, carburetted with pentane and fed by or- 

 dinary illuminating gas, and gives better results. 



(4) Among various flame standards lately pro- 

 posed, but not yet adopted in practise, are many 

 acetylene standards. The light from acetylene 

 differs little from that given out by melted plati- 

 num. 



A bolometric study by Clayton, Sharp, and 

 Turnbull has revealed unexpectedly large varia- 

 tions in the total emission of the different stand- 

 ards. In the English candle the variations may 

 attain 4G.5 per cent, of the mean intensity; the 

 maximum variation for the German candle is 24 

 per cent. The variations of the Hefner standard 

 may reach 22.6 per cent., but they may be re- 

 duced to 2 per cent, by careful regulation. The 

 Carcel lamp may have extreme variations of 18.2 

 per cent., but the changes are slow and the varia- 

 tion may remain for thirty-five minutes inferior 

 to 0.8 per cent. The authors conclude that the 

 Carcel lamp is the most constant of the standards 

 they have examined. 



Constitution of White Light. O. M. Corbino 

 (Comptes Rendus, Aug. 26), after stating Gouy'S 

 opinion that the constituents of the spectrum' of 

 white light are components of a single complex 

 vibration, and are capable of mutual interference, 

 and that of Carvallo, who regards the separate 

 radiations as completely independent, and there- 

 fore incapable of interference, asserts that the 

 latter view is supported by spectroscopic observa- 

 tion of beats produced by two pencils of white 

 light, one of which is slightly altered in period. 



Velocity. Perrotin (Comptes Rendus, Nov. 5, 

 1900) describes experiments made at the Obser- 

 vatory of Nice to determine anew the velocity of 

 light. The method adopted is that of the toothed 

 wheel of Fizeau as modified by Cornu. In pre- 

 liminary observations between two stations near- 

 ly 12 kilometers apart the value found is 299.90 

 0.08 for the velocity in vacuo expressed in thou- 

 sands of kilometers. About 1,500 measurements 

 were taken during more than a year, making use 

 only of absolutely still images, and taking ob- 

 servations withov : precipitation so as to atten- 

 uate or eliminate systematic errors, especially 

 physiological ones. The source of light was the 

 filament of an electric lamp of 16 candle-power. 



The " Gray-Glow." O. Lummer criticizes ths 

 law of Draper that all bodies begin to emit light 

 at the same temperature. According to the more 

 recent researches the first sensation of light as the 

 temperature of a body is gradually raised in a 

 dark room is the " gray-glow," now known to be 

 due to the rods in the retina, but this can scarce- 

 ly be called emission of light, since the human eye 

 can not then bring the body to a focus, the at- 

 tempt only serving to make the body invisible 

 again. The first real sensation of light takes 

 place at a much higher temperature, which varies 

 with the body emitting it, so that here Draper's 

 law does not apply. Whether it applies to the 

 " gray-glow " is very doubtful. The author has 

 observed the gray-glow as low as 360 C. 



Absorption. Koenigsberger (Annalen der Phys- 

 ik, April) has experimentally determined the in- 

 fluence of temperature on the absorption of light 

 by various solids, and finds that in solids having 



