34 



PROBLEMS OP LAKE BIOLOGY 



but according- to ZoBell and McE wen (1935) nnderstood ways call for exact measure- 

 virtnally no bactericidal radiations pene- ments of its rate of penetration. Yet seri- 

 trate sea water to as great a depth as 3 ons disagreement exists on the rate of 



a: 



UJ 

 UJ 



cr 



Q. 



o 



GQ 



a: 

 o 

 to 

 m 

 < 



UJ 



o 



< 



H 

 Z 

 UJ 



o 

 q: 



UJ 

 Q. 



3000A 



3500 ' 



5000 



5500 



4000 4500 



WAVELENGTH 



Fig. 9. Absorption of light by distilled water and by sea water at the ultraviolet end of the spec- 

 trum. Distilled water: Hulburt (1928); Dawson and Hulburt (1934); Sawyer (1931); and James 

 and Birge (1938). Sea water: Hulburt (1928), from calculated values at 3130a (87.8 per cent) 

 and at 3660A (25.9 per cent) and from data of Abbott, Fowle and Aldrich at 4360a and at 5460A 

 (sea water from Caribbean agreed with these values); Sawyer (1931), laboratory measurement of 

 water from Bay of Pundy off Head Harbor; Darby, Johnson, and Barnes (1937), relative values 

 obtained at sea near Tortugas Laboratory, Gulf of Mexico; Clark and James (1939), laboratory 

 measurement of water from Sargasso Sea after Berkefeld filtering. 



meters. The beneficial effects of ultraviolet 

 in vitamin formation* and in other poorly 



* Darby and Clarke (1937) report a relatively 

 high concentration of vitamin D in Sargassum and 

 they suggest that the vitamin finds its way into 

 cod liver oil from this source through the connect- 

 ing links of a food chain. The steps involved in 

 this suggestion should be carefully worked out since 

 it calls for a transfer through living agents of 

 material from the very warm Sargasso water to the 

 cold habitat of the cod. 



absorption of ultraviolet light even for 

 distilled water ! In Fig". 9 the measure- 

 ments of various workers at the short wave- 

 length end of the spectrum are compared. 

 Previously it had been supposed that ultra- 

 violet was absorbed very rapidly by pure 

 water itself, but the observations of James 

 and Birge (1938) indicate a much lower 

 absorption at least as far as 3 600 A. It is 

 essential to know whether this low rate 



