320 



SCIENCE 



[N. S. Vol. XLVI. No. 1187 



In order to rule out the possibility that the 

 protective action might be indirect, i. e., not 

 attributable to mere absorption of the toxic 

 rays, and also to permit the employment of 

 toxic acids the following modified procedure 

 was employed: 



In a quartz beaker with a diameter of 32 

 mm. 2 c.c. of the given acid were placed, this 

 amount being just sufficient to completely 

 cover the bottom of the beaker. A square 

 piece of cardboard was placed on the Syracuse 

 dish containing the Paramecium suspension. 

 The quartz beaker was then placed over a cir- 

 cular opening in the cardboard, having a diam- 

 eter of 25 mm. By this means the organisms 

 were shielded from all ultra-violet rays except- 

 ing those which passed through the solution in 

 the quartz beaker. In order to fully expose all 

 of the organisms and to standardize the depth 

 of suspension, a paraffine mould was made in 

 the Syracuse dish by holding a No. 3 rubber 

 stopper in the center of the dish and pouring 

 melted paraffine around it. On cooling, the 

 stopper was withdrawn, leaving a depression 

 20 Trim, in diameter in which 0.5 c.c. of Para- 

 mecium suspension was placed. 



Somewhat over 100 exposures were made, 

 using this method with the following results: 



AVERAGE EXTERMINATION PERIODS 



(Paramecia not immersed in lest Solution) 



Water 130 sees. 



1 per cent, alanin 130 ' ' 



1 per cent, glyeocoll 130 ' ' 



1 per cent, aspartic aeid 130 ' ' 



1 per cent, glutamic acid 135 " 



1 per cent, leucin 250 ' ' 



0.5 per cent, tyrosin 420 ' ' 



1 per cent, amino benzoic acid 2400 " 



It wiU be noted that the results obtained by 

 this procedure confirm those previously ob- 

 tained by the method of immersion. 



In order to obtain 1 per cent, solutions of 

 tyrosin and cystin, which are very sparingly 

 soluble in water, slight amounts of alkali were 

 added to the test solution in the beaker and the 

 extermination-periods after passage of the rays 

 through alkaline solutions of these acids and 

 of certain of the acids enumerated above were 

 determined, with the following results: 



AVERAGE EXTERMINATION PERIODS 



(Paramecia not immersed in Test Solution) 



0.5 per cent. NaOH 150 sees. 



1 per cent. NaOH 170 " 



1 per cent, glutamic acid in 1 per cent. 



NaOH 200 " 



1 per cent, cystin in 0.5 per cent. NaOH. 1200 ' ' 

 1 per cent, tyrosin in 0.2 per cent. NaOH unaffected 



after 40 

 minutes 

 exposure. 



We may infer that solutions of gelatine, pep- 

 tone, amino-benzoic acid, cystin, tyrosin and 

 leucin detosicate ultra-violet rays which pass 

 through them, while solutions of the other sub- 

 stances investigated do not appreciably do so. 

 The protective action of tyrosin in alkaline so- 

 lutions is exceptionally marked, and in this 

 connection it is of especial interest to note 

 that Kober has found that an alkaline reaction 

 markedly increases the absorption of ultra- 

 violet rays by tyrosin solutions. 



The protective action of leucin, which does 

 not exhibit a selective absorption in the ultra- 

 violet, is at fijst sight somewhat puzzling. It 

 was noticed, however, that both tyrosin and 

 leucin solutions underwent a change of color 

 upon continued exposure to the ultra-violet 

 light. This change was especially marked in 

 the leucin solutions resulting after 40 minutes 

 exposure in closed quartz vessels in the pro- 

 duction of a dark brown fluid having a dis- 

 tinctly intensified odor. This solution had a 

 much greater protective power when tested in 

 the above maimer than leucin solutions which 

 had not been previously exposed to the light. 

 We may infer that ultra-violet light induces 

 chemical changes in a leucin solution resulting 

 in the production of substances having an en- 

 hanced power of absorbing ultra-violet rays. 



Our results are therefore decidedly in har- 

 mony with the view that the susceptibility of 

 protoplasm to ultra-violet light is conditioned 

 by the selective absorption of the toxic rays by 

 the aromatic amino-acid radicals of the pro- 

 teins. 



F. I. H.uiRis, 

 H. S. HOYT 



Univeesitt of California 



