4 



The Mechanical Action of Radiation, 



gravitation." Ilis first paper was sent to the Royal Society 

 in May, 1873. ''Experiments were thereupon instituted 

 with a view to make tlie action more sensihle and to eli- 

 minate sources of error." 



By ingeniously devised apparatus Dr. Crookes was able 

 to subject light bodies to the action of radiant beat and 

 light in a vacuum perfect, doubtless, beycnd precedent. 

 While the exhaustion proceeded he found the motion of 

 his slender balance beam to be toward the source of heat, 

 until, a very high degree having been attained, the motion 

 turned the other way, the light body receded from the 

 source of radiation as if driven by a delicate repulsion. 

 (See Qaarterly Journal of Science, 1875.) His second paper 

 communicated to the Royal Society the interesting disco- 

 very of the mechanical eftect of radiation in a vacuum, 

 and Reptdslon by Radiation is a phrase describing a new- 

 found fact in physical science for which we are indebted 

 to this capital research of Dr. Crookes. 



These experiments of the English scientist, pronounced, 

 by the President of the Mathematical and Physical Section 

 of the British Association, to be among the most interest- 

 ing in the whole range of physical science, have attracted 

 much attention to the phenomena. Prof Dewar in Scot- 

 land and Herr ]^eesen in Germany have made valuable 

 additions to the experimental data. 



Prof Dewar employed a novel means of obtaining the 

 necessary vacuum. When the pump refused to reduce the 

 tension of the rarefied gas, the residue was removed and 

 the vacuum perfected by the absorbent power of charcoal. 

 The vacuum thus obtained, like that of Mr. Crookes, for- 

 bade the passage of the induction spark, and so sensitive 

 was his instrument that " an ordinary lucifer-match held 

 at a distance of four feet produced instant action, which 

 was observed b}' mea?is of a telescope." {Nature, xii, 217.) 

 Of his results more is to be said as w^e proceed. Herr 



