ADDREJSS. 



15 



the 

 ler 

 liev 

 the 

 Ihe 



of 



lisc 



a 



3n, 



ich 



|ty 



iat 

 id 



I romcmbei" discussing with Maxwell, soon after the publication of his 

 exiicrimcnts, the whereabouts of tho point at which the gas would cease 

 to produce its ordinary ett'ect. His apparatus, however, was (juite un- 

 suited for high degrees of exlianstion, and the failure of tho law was 

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

 mercury. Subsequently tho matter has been thoroughly examined by 

 Orookcs, 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 tho atmospheric jiressuro of TOO 

 mm. down to about ^ mm. of mercury the viscosity is sensibly constant. 

 Fiom this jioint to the highest vacua, in which less than one-millionth of 

 the original gas ronuiins, tho coefficient of viscosity drops down gradually 

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

 regards tho gas as having assumed a different, ultra-gaseous, condition ; 

 but we must remember that tho phenomena have relation to the Lher 

 circurustancos of the case, especially the dimensions of the vessel, as well 

 as to the condition of tho gas. 



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

 lias, of course, drawn increased attention to the dynamical theory of gases. 

 Tho success whioh has attended the theory in the hands of Clausius, 

 Maxwell, Jioltzmann, and other mathematicians, 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 

 tho 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 

 'ioubtful points, and a younger generation of theorists will br'ng to bear 

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

 gratulate ourselves on the possession of a guide Avhich has already 

 conducted us to a position which could hardly otherwise have been 

 attained. 



In Optics attention has naturally centred upon the spectrum. Tho 

 mystery attaching to the invisible rays lying beyond the red has been 

 fathomed to an extent that, a fev/ 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 bo 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 efi'ects 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 



