The Sea-water and its Physical and Chemical Properties 51 



The magnitude of the molecular viscosity was eariier attributed some importance 

 in the biological and dynamic processes in the sea, but it has since been recognized 

 that processes in oceanic currents are always turbulent and the coefficient of turbulent 

 viscosity is considerably larger than the coefficient of molecular viscosity. This has 

 very much reduced the importance of the latter. 



(d) Surface tension. Krummel (1907) investigated the dependence of the surface 

 tension on the temperature and the salinity; it decreases with rising temperature and 

 with decreasing salinity. Fleming and Revelle (1939) have taken more recent values 

 to derive the equation 



surface tension in dyn/cm^ = 75-64 - 0-144/ + 0-0399 CI. 



Impurities in the water always lead to a considerable reduction and this must be taken 

 into consideration for surface waters of the sea. 



7. The Optical Properties of Sea-water 



(a) The Extinction of Incoming Radiation 

 Parallel radiation entering a layer of sea-water is gradually weakened in three ways: 



(1) By absorption by the pure sea- water. 



(2) By scattering by the pure sea-water. 



(3) By scattering, diffraction and reflection by suspended particles in the water 

 (impurity of sea- water). 



The last two factors do not change the form of the energy but divert a part of the 

 radiation from its original direction. A beam of radiation of wavelength A passing 

 through a distance dx in water is reduced in intensity by an amount dl which is pro- 

 portional to the intensity and to the distance ^.v travelled through the water, so that 

 dl = —Kidx. K the extinction coefficient (cm~^) is dependent on the wavelength A. 

 If the intensity of the radiation is /q when x = 0, then for a distance ,v 



I = I,e-^\ 



The reduction in intensity of the radiation is often characterized in practice by the 

 extinction E for a layer of thickness 1 m and is given as a percentage of the incident 

 radiation 



£■= 100 ("l - ^ 



The transmission D may also be used, and gives the percentage of the incident radia- 

 tion passing through a layer of fixed thickness 



i) = 100 - - 100 e--^^ 



Detailed measurements have been made of the extinction coefficient for water over 

 the whole spectral region from 0-186/x in the ultraviolet to 8-5 ju. in the infra-red. 

 The spread of 2-3% in the values obtained in different series of measurements are 

 largely due to the difficulty of preparing "pure water". Dietrich (1939) has given a 

 comparison of the older measurements of Aschkinas (1895) and more recent values 

 by Kreusler (1901), Sawyer (1931) and Collins (1925, 1933) from which the values 

 shown in Table 1 8 have been abstracted. 



