ADDRESS. 15 



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 un- 

 suited for high degrees of exhaustion, and the failure of the law was 

 first observed by Kundt and Warburg, as pressures below 1 mm. of 

 mercury. Subsequently the matter has been thoroughly examined by 

 Crookes, who extended his observations to the highest degrees of ex- 

 haustion as measured by MacLeod's gauge. Perhaps the most remark- 

 able 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 theory of gases. 

 The success which has attended the theory in the hands of Clausius, 

 Maxwell, Boltzmann, and other mathematicians, not only in relation to 

 viscosity, but over a large part of the entire field of oui- 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 possible. 

 The growth of experimental knowledge may be trusted to clear up many 

 doubtful points, and a younger generation of theorists will bring to bear 

 improved mathematical weapons. In the meantime we may fairly con- 

 gratulate ourselves 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 photographic methods Abney has 

 mapped out the peculiarities of this region with such success that our 

 knowledge 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 invention of his own based upon the principle of Siemens' 

 pyrometer. This instrument measures the actual energy of the radia- 

 tion, 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 



