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XL VI. Resonance Experiments with the Longest Heat- Waves. 

 By R. W. Wood, Professor of Experimental Physics, 

 Johns Hopkins University, and Adams Research Fellow of 

 Columbia University *. 



[Plate III.] 



IN the autumn of 1910 H. Rubens and R. W. Wood suc- 

 ceeded in isolating and measuring the longest heat-waves 

 known at the time, by a method based on the circumstance 

 that the refractive index of quartz is much higher for these 

 waves than for light- waves and the shorter heat-waves. 

 A description of the method, which may be termed " focal 

 isolation/' will be found in my ' Physical Optics,' 2nd edition 

 (1912) j also Phil. Mag. Feb. 1911. The waves were emitted 

 from a Welsbach mantle and had a wave-length of over one- 

 tenth of a millimetre (112 /a), and Rubens and von Baeyer 

 subsequently obtained by the same method waves of greater 

 length (0*3 mm.) from the quartz-mercury arc. As the shortest 

 electric waves obtained by von Baeyer have a length of about 

 2 mm., it will be seen that the gap between the electric and 

 optical spectrum is very nearly filled up. 



The very great length of these heat-waves makes it 

 possible to try experiments analogous to the experiments 

 made by various investigators on the action of metallic 

 resonators on electromagnetic waves. I have already com- 

 menced an investigation of this subject, and the results 

 obtained up to the present time will be reported in this 

 paper. 



The first experiments were made with very minute particles 

 of metallic copper deposited on quartz plates. The metal can 

 be obtained from the chemists in the form of a very fine 

 powder, and if some of this is violently shaken in a tall glass 

 jar, the finest particles remain suspended in the air. If the 

 jar is then inverted over the quartz plate, and allowed lo 

 stand for a few minutes, a beautifully regular deposit settles 

 on the plate. Treating the plate to the process four or five 

 times builds up a deposit which is almost opaque to light, 

 the slight amount which struggles through being of a greenish 

 colour. To my surprise I found that even these very heavy 

 deposits, which by actual measurement transmitted only two 

 or three per cent, of visible light, were perfectly transparent 

 to the long heat-waves. The same amount of metal, in fact 

 a much less quantity, in the form of a uniform film would be 



* Communicated by the Author. 



