182 



SCIENCE. 



[Vol. IV., No. 82. 



tion is adequate, the friction is found to be nearly 

 independent of the load, and much smaller than is 

 usually supposed, giving a coefficient as low as a 

 thousandth. When the layer of oil is well formed, 

 the pressure between the solid surfaces is really 

 borne by the fluid; and the work lost is spent in 

 shearing, that is, in causing one stratum of the oil to 

 glide over another. 



The nature of gaseous viscosity, as due to the dif- 

 fusion of momentum, has been made clear by the 

 theoretical and experimental researches of Maxwell. 

 A flat disk, moving in its own plane between two 

 parallel solid surfaces, without contact, is impeded by 

 the necessity of shearing the intervening layers of 

 gas ; and the hinderance is proportional to the velocity 

 of the motion and to the viscosity of the gas, so that, 

 under similar circumstances, this effect may be taken 

 as a measure, or rather definition, of the viscosity. 

 From the dynamical theory of gases, to the develop- 

 ment of which he contributed so much, Maxwell 

 drew the startling conclusion that the viscosity of 

 a gas should be independent of its density; that 

 within wide limits the resistance to the moving disk 

 should be scarcely diminished by pumping out the 

 gas, so as to form a partial vacuum. Experiment 

 fully confirmed this theoretical anticipation, — one 

 of the most remarkable to be found in the whole his- 

 tory of science, — and proved that the swinging disk 

 was retarded by the gas as much when the barome- 

 ter stood at half an inch as when it stood at thirty 

 inches. It was obvious, of course, that the law must 

 have a limit; that at a certain point of exhaustion 

 the gas must begin to lose its power; and Lord Ray- 

 leigh remembers discussing with Maxwell, soon after 

 the publication of his experiments, the whereabouts 

 of the point at which the gas would cease to produce 

 its ordinary effect. His apparatus, however, was 

 quite unsuited for high degrees of exhaustion; and 

 the failure of the law was first observed by Kundt 

 and Warburg, at pressures below one millimetre of 

 mercury. Subsequently the matter has been thor- 

 oughly examined by Crookes, who extended his ob- 

 servations to the highest degrees of exhaustion, as 

 measured by MacLeod's gauge. Perhaps the most 

 remarkable results relate to hydrogen. From the 

 atmospheric pressure of seven hundred and sixty 

 millimetres, down to about half a millimetre of mer- 

 cury, the viscosity is sensibly constant. From this 

 point to the highest vacuum, in which less than a mil- 

 lionth of the original gas remains, the coefficient of 

 viscosity drops down gradually to a small fraction 

 of its original value. 



Such an achievement as the prediction of Maxwell's 

 law of viscosity has, of course, drawn increased at- 

 tention to the dynamical theory of gases. At the 

 same time, the theory presents serious difficulties; 

 and we can but feel, that, while the electrical and 

 optical properties of gases remain out of relation to 

 the theory, no final judgment is possible. 



In optics, attention has naturally centred upon the 

 spectrum. By the use of special photographic meth- 

 ods, Abney has mapped out the peculiarities of the 

 invisible rays lying beyond the red with such success 



that our knowledge of them begins to be comparable 

 with that of those visible to the eye. Equally im- 

 portant work has been done by Langley, using a 

 refined invention of his own, based upon the princi- 

 ple of Siemens' s pyrometer. Interesting results have 

 also been obtained by Becquerel, whose method is 

 founded upon a curious action of the ultra-red rays 

 in enfeebling the light emitted by phosphorescent 

 substances. One of the most startling of Langley's 

 conclusions relates to the influence of the atmos- 

 phere in modifying the quality of solar light. By the 

 comparison of observations made through varying 

 thicknesses of air, he shows that the atmospheric 

 absorption tells most upon the light of high refran- 

 gibility ; so that, to an eye situated outside the atmos- 

 phere, the sun would present a decidedly bluish tint. 



Cornu has made use of the fact that the refrangi- 

 bility of a ray of light is altered by a motion of the 

 luminous body to or from the observer to determine 

 whether a line is of solar or atmospheric origin. For 

 this purpose a small image of the sun is thrown upon 

 the slit of the spectroscope, and caused to vibrate two 

 or three times a second, in such a manner that the 

 light entering the instrument comes alternately from 

 the advancing and retreating limbs. As the sun is 

 itself in rotation, and thus the position of a solar 

 spectral line is slightly different according as the light 

 comes from the advancing or from the retreating 

 limb, a line due to absorption within the sun ap- 

 pears to tremble, as the result of slight alternately 

 opposite displacements. But, if the seat of the ab- 

 sorption be in the atmosphere, it is a matter of 

 indifference from what part of the sun the light 

 originally proceeds ; and the line maintains its posi- 

 tion in spite of the oscillation of the image upon the 

 slit of the spectroscope. 



The instrumental weapon of investigation, the 

 spectroscope itself, has made important advances. 

 The magnificent gratings of Rowland are a new power 

 in the hands of the spectroscopists, and, as triumphs 

 of mechanical art, seem to be little short of perfec- 

 tion. 



The great optical constant, the velocity of light, 

 has been the subject of three distinct investigations 

 by Cornu, Michelson, and Forbes. As may be sup- 

 posed, the matter is of no ordinary difficulty, and it 

 is therefore not surprising that the agreement should 

 be less decided than could be wished. From their 

 observations, which were made by a modification of 

 Fizeau's method of the toothed wheel, Young and 

 Forbes drew the conclusion that the velocity of light 

 in vacuo varies from color to color, to such an extent 

 that the velocity of blue light is nearly two per cent 

 greater than that of red light. Such a variation is 

 quite opposed to existing theoretical notions, and 

 could only be accepted on the strongest evidence. 

 Mr. Michelson, whose method (that of Foucault) is 

 well suited to bring into prominence a variation of 

 velocity with wave-length, has recently repeated his 

 experiments with special reference to the point in 

 question, and has arrived at the conclusion that no 

 variation exists, comparable with that asserted by 

 Young and Forbes. The actual velocity differs little 



