^ ULTRAVIOLET LIGHT MEIEE 357 



of freshly killed meat and hardening it before dessication. This 

 seems to be the only commercial use of the coagulating power of 

 ultraviolet exposure found by Dreyer and Hansen (1907). They 

 found that the ultraviolet coagulates the " coagulable " proteins, 

 some in neutral or even in alkaline solution while others require 

 acidification ; and in all cases the reaction is accelerated by 

 acidification. 



Experiments with microscopic green plants have also been made 

 in the ultraviolet. Gibbs (1926) studied filamentous green algae in 

 the short wave lengths. He noted that a latent period occurred 

 before death in his irradiated filaments of Spirogyra nitida aiftnis. 

 The limits of the toxic action were the wave lengths 3126 A and 

 2378 A. The chloroplasts (green pigments in the plant) were ob- 

 served to clump characteristically, owing to the great difference in 

 intensity of radiation reaching the " near " and " far " sides of the 

 filament. The behavior of the filaments was variable. Some died 

 while apparently perfectly normal in a2:)pearance, while in the others 

 there was coagulation of the protoplasm and a brown pigment that 

 exhibited Brownian movement was formed. 



Here at the Smithsonian Institution we have irradiated algao, 

 unicellular green plants, with ultraviolet raj's, and found that in the 

 regions where the ultraviolet waves shorter than 3022 A (the approx- 

 imate limit of ultraviolet irradiation in nature) were directed on the 

 culture, the green algal cells were killed. The certainty of the lethal 

 action as well as the quickness of the attack of the rays has also 

 been studied (Meier, 1934). Plate 1 shows a photograph of a typi- 

 cal agar plate that had been covered with green algae and then 

 exposed in the spectrograph for 74 minutes. The radiotoxic regions 

 are indicated by the decolorized sections where the algao were killed 

 at 2652, 2699. 2753, 2804, 2894, 2925, 2967, and 3022 A. This photo- 

 graph is superimposed on a diagram of the intensities of the wave 

 lengths. It should be remarked that a greater intensity of radiation, 

 according to thermocouple measurements, was directed on the cultures 

 at 3130 A, even though it did not prove lethal. Throughout the 

 ages the plants that have survived on the earth have become attuned 

 to solar radiation as it is received on the earth's surface and very 

 possibly with the same spectrum limit as at the present time due to 

 ozone. It is, therefore, not astonishing that the radiations shorter 

 than the solar limit produce unusual effects. 



In figure 2 are shown curves of reactions of different biological 

 materials to ultraviolet compiled by Dugi^ar and Hollaender (1934). 

 In comparison with their own curves for the lethal effect of the short 

 wave lengths on viruses and bacteria are the curves of protein coag- 

 ulation from Sonne (1928) ; lethal effect on paramoecium, Weinstein 



