1920] 



on Low Temperature Studies 



265 



Typical Results of Transmissive Power. 



Table II. exemplifies the simplest manner of deducing the pro- 

 portionate transmissions of typical substances. These substances are 

 enumerated in the first column, and their thicknesses in the second ; 

 the third and fourth columns give the actual displacements of the 

 manometer, in two sets, with the " black body '' at 15° C. and 100° C, 

 and the fifth column gives the proportionate transmissions, based on 

 the readings given in the first line with unscreened radiation. The 

 first two substances were equally transmissive at several thicknesses. 



The next, rubber, became very absorptive by the time a thickness 

 of 0*6 mm. was reached, although very transmissive when stretched to 

 the form of a transparent membrane 0"02 mm. thick. Crystalline 

 quartz 2"3 mm. thick was more transmissive than fused quartz of 

 1*5 mm.— viz. 16 per cent, and 12 per cent, respectively. Glass, 

 even thinner than the rubber membrane, was still able to absorb 

 two-thirds of the incident radiation, and the absorption of mica 

 increased at an even more rapid rate with thickness. In this connec- 

 tion very thin films of water and alcohol, two very absorptive bodies, 

 were found to be quite fairly transmissive, as were also collodion 

 films, especially when thin enough either to appear black or to show 

 the first order colours by reflected light. The thin liquid films were 

 measured between polished plates of silver chloride, while the 

 collodion films were picked up by rings from the surface of water.* 



Proc. Roy. Inst., xxi. p. 787. 



