PHYSICAL OCEANOGRAPHY 253 



but when the red rays, for instance, are cut off, a piece of red 

 paper will look black. 



The usual method of studying the transparency of the water Transparency 

 is to lower a large white disc, noting the depth at which it of^ea-water. 

 disappears from view. The degree of transparency is found 

 to vary greatly, for in the clear dark-blue water in the middle 

 of the ocean near the tropics the white disc can sometimes 

 be seen as far down as 50 metres below the surface, or even 

 more, while in those places where rivers bring down large 

 quantities of detritus from the land the disc may occasionally be 

 invisible a couple of metres beneath the surface. The enormous 

 quantities of small plankton organisms inhabiting the upper 

 layers may also render the water relatively opaque. The 

 penetration of light thus varies according to circumstances, but 

 few direct observations of the light-intensity have as yet been 

 made. It would be of the greatest interest to know the amount 

 of light at different depths in different seas, and thereby gain a 

 better understanding of the conditions of life, for instance, as 

 regards the development of the plankton, as the small plankton 

 algae need light for the processes of assimilation. 



Sea-water normally contains oxygen, nitrogen (with argon). Gases in the 

 and carbonic acid. These gases are absorbed at the surface ^^^• 

 from the atmosphere, and are carried by currents even into the 

 deepest parts of the ocean in varying amounts. A study of these 

 variations is of considerable interest, and may be briefly dealt 

 with here, although no gas-analyses were made during the 

 "Michael Sars " Atlantic Expedition. There are several good 

 methods of analysis. For the three gases named, the method 

 introduced by Bunsen, and further developed by Pettersson and 

 Fox, may be employed, the water-sample being boiled at a low 

 pressure, and the escaping gas collected and analysed. The 

 oxygen may be determined by a very simple titration, according 

 to Winkler's method, or Krogh's method of examining the 

 tension of the several gases in solution may be applied. 



Oxygen is not so readily soluble in salt water as in fresh ; Oxygen. 

 the higher the salinity the less the absorption of oxygen by the 

 water. It is also a well-known fact that cold water dissolves 

 more air than warm. This is clearly seen in the following 

 excerpt from Fox's tables, showing the cubic centimetres of 

 oxygen in i litre of water at different temperatures and sali 

 when the water is saturated with this gas : — /C^\ 



