EFFECT OF SURROUNDING MEDIUM. 



149 



was found by Rubens and Aschkinass 1 for water-vapor, which behaves 

 like the liquid in its properties for absorbing heat-rays), beyond which 

 there is great opacity. In fig. 104 is given the transmission curve of a 

 column of 75 cm. of water-vapor (due to Rubens and Aschkinass, loc. cit.), 

 from which it will be noticed that there is a quite transparent region from 

 8 to 15 /*. They found that the heat waves at 51 \x were entirely absorbed, 

 while at 80 \x theory (see Drude, Optik, p. 359) predicts a band of metallic 

 reflection. Moreover, water differs from most other substances in that its 

 great opacity is due to numerous small absorption bands. Consequently 

 its absorption coefficient is smaller than that of a substance like quartz 

 which has bands of metallic reflection at 8.5, 9.02, and 20.75 /* Hence, 

 there is no objection to saying that "the whole question of the formation 



100% 



10 II 12 13 14- 15 16 



Fig. 104. Transmission of water vapor. 



of anchor ice depends upon admitting that the long heat-waves can pene- 

 trate freely through the water," for the maximum radiation of a body at 

 a temperature of o C. lies in the region of the spectrum extending from 

 wave-lengths 8 to 20 p., and it is here that water has its greatest transparency 

 for long heat-waves. 



It is difficult to conceive of a more complex form of radiation than the 

 one here involved. According to Provost's theory of exchanges, when two 

 bodies are at different temperatures the hotter receives energy from, and 

 imparts energy to, the colder by radiation, and vice versa. In the case of 

 the river, when the sky is clear, the water is radiating into space which is 



1 Rubens & Aschkinass: Ann. der Phys. (3), 64, p. 584, 1898; also Ann. der Phys., 65, 

 p. 251, 1898. 



