Rayleigh's Address to the British Association. 103 



ber 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, 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 exhaustion as measured by MacLeod's 

 gauge. Perhaps 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 con- 

 dition ; 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 dyna- 

 mical 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 our knowledge of gases, proves that 

 some of it3 fundamental postulates are in harmony with the reality 

 of nature. At the same time it presents serious difiiculties, 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 congratulate ourselves on the posses- 

 sion 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 



