§2] CHEMICAL ACTION OP LIGHT 165 



5. Changes resembling those brought about hj fermentation 

 are produced by light. Thus Nibpce db Saint Victor and 

 COEVISAET ('59) have found that a 0.1% solution of starch, 

 exposed during 6 to 18 hours to the summer sun, becomes trans- 

 formed into sugar, while in the dark no such change occurs. 

 The change is favored by a small quantity of uranium nitrate. 

 In a similar fashion glycogen is transformed into sugar more 

 rapidly in the light than in the dark. On the other hand, 

 Green ('94) finds that the ferment which normally transforms 

 starch into sugar is destroyed by subjection to a strong light, 

 the violet rays being especially active in this process. Like- 

 wise, ptyalin, the ferment of saliva, is destroyed by light. 



To sum up, light affects organic compounds in very varied 

 and important ways. We are, accordingly, prepared to find 

 that light exerts a very important influence on the activities 

 of protoplasm. Nor is the influence necessarily confined to 

 the surface, for most protoplasmic bodies are more or less 

 translucent. Thus Sachs ('60) found by looking through 

 a tube with one end fitted to the eye and the other directed 

 towards the sunlight, that considerable layers of plant tissue, 

 for example over 32 mm. of the tissue of the potato tuber, did 

 not cut out all the light, and that red had the greatest pene- 

 trating power, violet the least. Even the epidermis of man 

 permits light to pass; and Onimtjs ('95) asserts that light can 

 pass through the hand to such an extent as to affect during 

 26 to 30 minutes an orthochromatic plate kept in a tight wooden 

 box perforated only by the opening which is covered. by the 

 hand. Whether the " Rontgen rays," which have so striking 

 a power of penetrating organic matters, are more of the 

 nature of light than of other physical agents, is still a subject 

 of debate. Whether they produce any important chemical 

 changes in protoplasm has not yet been fully determined.* 



* During the six months which have elapsed since the ahove was written, 

 accounts of marked physiological effects of the Rontgen rays have heen pub- 

 lished. Thus, exposure of the skin to them for an hour frequently causes loss 

 of hair and finger nails, and produces symptoms resembling those of sunburn. 

 AxENFELD (Centralbl. f. Physiol. X, 147) finds that many insects and a crusta- 

 cean, PoTcellius, kept in a box only one-half of which is penetrated by the rays, 

 aggregated in this part. Several experiments upon the tropic influences of the 

 rays have resulted negatively. 



