THE RECENT PROGRESS OF PHYSICAL SCIENCE. 751 



theoretical anticipation — one of tlie 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 Max- 

 well, soon after the publication of his experiments, the whereabout of 

 the point at which the gas would cease to produce its ordinary effect. 

 His apparatus, however, was quite unsuited for high degrees of ex- 

 haustion, and the failure of the law was first observed by Kundt and 

 Warburg, at pressures below one millimetre of mercury. Subse- 

 quently the matter has been thoroughly examined by Crookes, who ex- 

 tended his observations to the highest degrees of exhaustion as meas- 

 ured by MacLeod's gauge. Perhaps the most remarkable results re- 

 late to hydrogen. From the atmospheric pressure of 760 millimetres 

 down to about one half millimetre 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 viscos- 

 ity 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 our 

 knowledge of gases, proves that some of its fundamental postulates 

 are in harmony with the reality of Nature. At the same time, it pre- 

 sents serious difiiculties ; and we can not but feel that, while the elec- 

 trical 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 mathe- 

 matical weapons. In the mean time 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 centered 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 al- 

 most 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 Lang- 



