234 



HEAT. 



small unglazed earthen pans were placed, about l inch creep, filled with 

 boiled soft water. Evaporation and radiation, acting together, cooled the 

 water below freezing, and ice was formed. Dr. Wells pointed out that 

 as the formation was most successful on calm clear nights, the effect was 

 chiefly due to radiation. Probably evaporation would aid the first cool- 

 ing until the dew-point was reached, any subsequent cooling being due 

 to radiation. 



Transparency and Opacity. Substances differ as markedly in 

 their transparency for dark radiations as for light, and all the phenomena 

 of selective absorption are repeated with these longer waves. 



Many bodies which are transparent to light are opaque to long wave 

 radiations, but the opacity differs with the source, that is, with the 

 quality, or wave-length, of the radiation received. Thus, Langley finds 

 that glass is more or less transparent to all the radiations reaching us 

 from the sun. But Melloni found that a plate of glass about 2 mm. 



from Gasholder through 

 Drying Tubes' 



Tube Containing Vapour closed 

 by Rocksalt Plates 



FlO. 138. TyndalTs Experiment on Emission and Absorption of Gases. 



thick would absorb half the radiation from an argand burner, while in 

 front of a Leslie cube at 100 it was as efficient a screen as a plate of 

 metal. 



Again, it was found by Melloni that rock-salt transmits a very large 

 proportion of dark radiations, and it was formerly supposed to be trans- 

 parent to all kinds. But Balfour Stewart showed that a plate of cold 

 rock-salt was exceedingly opaque to the radiations from hot rock-salt. 



Lampblack, which is opaque to nearly all radiations, has been found 

 by Langley (Phil. Mag., 5th series, xxi. p. 403) to transmit some of very 

 great wave-length. Tyndall has found that a solution of iodine in 

 bisulphide of carbon, though quite opaque to light, is transparent to 

 many long wave radiations. A very thin plate of ebonite is also trans- 

 parent to long wave radiations, and if placed in front of the condenser of 

 a lantern, allows enough radiation to pass through to heat the hand if 

 held at the focus. 



A thin ebonite prism spreads the invisible long-wave radiation into a 

 spectrum, which is quite sensibly detected by the radio-micrometer. 



Radiation and Absorption by Gases and Vapours. Tyndall 



