414 



NA TURE 



[August 28, 1884 



go far towards upsetting some ideas hitherto widely admitted. 

 When the lubrication 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 1/1000. 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 causirtg one stratum of the oil to glide over another. 



In order to maintain its position, the fluid must possess a cer- 

 tain degree of viscosity, proportionate to the pressure ; and even 

 when this condition is satisfied, it would appear to be necessary 

 that the layer should be thicker on the ingoing than on the out- 

 going side. We may, I believe, expect from Prof. Stokes a 

 further elucidation of the processes involved. In the meantime, 

 it is obvious that the results already obtained are of the utmost 

 value, and fully justify the action of the Institution in devoting 

 a part of its resources to experimental work. We may hope 

 indeed that the example thus \\ isely set may be follow ed by other 

 public bodies associated with various departments of industry. 



I can do little more than refer to the interesting observations 

 of Prof. Darwin, Mr. Hunt, and M. Forel on ripplemark. The 

 processes concerned would seem to be of a rather intricate cha- 

 racter, and largely dependent upon fluid viscosity. It may be 

 noted indeed that most of the still obscure phenomena of hydro- 

 dynamics require for their elucidation a better comprehension of 

 the laws of viscous motion. The subject is one which offeis 

 peculiar difficulties. In some problems in which I have lately 

 been interested, a circulating motion presents itself of the kind 

 which the mathematician excludes from the first when he is treat- 

 ing of fluids destitute altogether of viscosity. The intensity of 

 this motion proves, however, to be independent of the coefficient 

 of viscosity, so that it cannot be correctly dismissed from con- 

 sideration as a consequence of a supposition that the viscosity 

 is infinitely small. The apparent breach of continuity can be 

 explained, but it shows how much care is needful in dealing with 

 the subject, and how easy it is to fall into error. 



The nature of gaseous viscosity, as due to the diffusion of 

 momentum, has been made clear by the theoretical and experi- 

 mental researches of Maxwell. A flat disk moving in its own 

 plane between two parallel solid surfaces is impeded by the 

 necessity of shearing the intervening layers of gas, and the mag- 

 nitude of the hindrance is proportional to the velocity of the 

 motion and to the viscosity of the gas, so that under similar cir- 

 cumstances this effect may be taken as a measure, or rather 

 definition, of the viscosity. From the dynamical theory of gases, 

 to the development 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 re- 

 sistance to the moving disk should be scarcely diminished by 

 pumping out the gas, so as to form a partial vacuum. Experi- 

 ment fully confirmed this theoretical anticipation — one of the 

 most remarkable to be found in the whole history of science, and 

 proved that the swinging disk was retarded by the gas, as much 

 when the barometer 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 I remember 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 ob- 

 served by Kundt and Warburg, at pressures below I mm. of 

 mercury. Subsequently the matter has been thoroughly exa- 

 mined by Crookes, who extended his observations to the highest 

 degrees of exhaustion as measured by MacLeod's gauge. Per- 

 haps the most remarkable results relate to hydrogen. From the 

 atmospheric pressure of 760 mm. down to about ^ mm. of 

 mercury the viscosity is sensibly constant. From this point to 

 the highest vacua, in which less than one-millionth of the original 

 gas remains, the coefficient of viscosity drops down gradually to 

 a small fraction of its original value. In these vacua Mr. 

 Crookes regards the gas as having assumed a different, ultra- 

 gaseous condition ; but we must remember that the phenomena 

 have relation to the other circumstances of the case, especially 

 the dimensions of the vessel, as well as to the condition of the 

 gas. 



Such an achievement as the prediction of Maxwell's law of 

 viscosity has of course drawn increased attention to the dynamical 

 theoi y of gases. The success which has attended the theory in 

 the hands of Clausius, Maxwell, Boltzmann, and other mathe- 

 maticians, not only in relation to viscosity, but over a large part 



of the entire field of our knowledge of gases, proves that some 

 of its fundamental postulates are in harmony with the reality of 

 Nature. At the same time it presents serious difficulties ; and 

 we cannot but feel that, while the electrical and optical properties 

 of gases remain out of relation to the theory, no final judgment 

 is pos-ible. The growth of experimental knowledge may be 

 trusted to clear up many doubtful points, and a younger genera- 

 tion of theorists will bring to bear improved mathematical 

 weapons. In the meantime we may fairly congratulate our- 

 selves on the possession of a guide which has already conducted 

 us to a position which could hardly otherwise have been 

 attained. 



In optics attention has naturally centred upon the spectrum. 

 The mystery attaching to the invisible rays lying beyond the red 

 has been fathomed to an extent that, a few years ago, would 

 have seemed almost impossible. By the use of special photo- 

 graphic methods Abney has mapped out the peculiarities of this 

 region with such success that our knowdedge of it begins to be 

 comparable with that of the parts visible to the eye. Equally 

 important work has been done by Langley, using a refined in- 

 vention of his own based upon the principle of Siemens' pyro- 

 meter. This instrument measures the actual energy of the 

 radiation, and thus expresses the effects of various parts of the 

 spectrum upon a common scale, independent of the properties of 

 the eye and of sensitive photographic preparations. 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 atmosphere 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 refrangibility ; so that to an eye situated outside the 

 atmosphere the sun would present a decidedly bluish tint. It 

 would be interesting to compare the experimental numbers with 

 the law of scattering of light by small particles given some years 

 ago as the result of theory. The demonstration by Langley of 

 the inadequacy of Cauchy's law of dispersion to represent the 

 relation between refrangibility and wave-length in the lower 

 part of the spectrum must have an important bearing upon 

 optical theory. 



The investigation of the relation of the visible and ultra-violet 

 spectrum to various forms of matter has occupied the attention of 

 a host of able workers, among whom none have been more suc- 

 cessful than my colleagues at Cambridge, Profs. Liveirg and 

 Dewar. The subject is too large both for the occasion and for 

 the individual, and I must pass it by. But, as more closely 

 related to optics proper, I cannot resist recalling to your notice 

 a beautiful application of the idea of Doppler to the discrimina- 

 tion of the origin of certain lines observed in the solar spectrum. 

 If a vibrating tody have a general motion of approach or reces- 

 sion, the waves emitted from it reach the observer with a fre- 

 quency which in the first case exceeds, and in the second case 

 falls short of, the real frequency of the vibrations themselves. 

 The consequence is that, if a glowing gas be in motion in the 

 line of sight, the spectral lines are thereby displaced from the 

 position that they would occupy were the gas at rest — a principle 

 which, in the hands of Huggins and others, has led to a deter- 

 mination of the motion of certain fixed stars relatively to the 

 solar system. But 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. 

 This displacement was, I believe, first observed by Thollon ; 

 but what I desire now to draw attention to is the application of 

 it 1 iy Cornu 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 re- 

 treating limbs. Under these circumstances a line due to absorp- 

 tion within the sun appears 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 position in spite of the oscillation of the image upon 

 the slit of the spectroscope. In this way Cornu was able to 

 make a discrimination which can only otherwise be effected 

 by a difficult comparison of appearances under various solar 

 altitudes. 



