294 BIOLOGICAL EFFECTS OF RADIATION 



Iodine and Ferrous Ion. — In an aqueous solution of iron salt and 

 iodine, an equilibrium is set up in the dark according to the equation: 



2Fe++ + I2 :e± 2Fe+++ + 21" 



When such a solution is exposed to visible light, a considerably different 

 equilibrium point is obtained. In the presence of light the equilibrium 

 is shifted (43, 27) so as to give a higher concentration of the ferric ion. 

 The photochemical reaction probably involves the following equation: 



2Fe++ + I7 + Aj' = 2Fe+++ + 31" 



The tri-iodide ion is formed in solution when iodine is mixed with an excess 

 of potassium iodide. Kistiakowsky found a quantum yield of unity at 



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all wave-lengths from 3660 to 5790 A under conditions where the reverse 

 thermal reaction was negligible. 



Mercury Vapor. — Mercury in the vapor state is monatomic, and when 



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the atoms of mercury absorb radiation at 2537 A from a mercury-arc 

 lamp they become activated to a high degree. The energy of this activa- 

 tion corresponds to more than 110,000 calories per mole. A great variety 

 of chemical reactions can be produced by collision of gaseous molecules 

 with these excited atoms of mercury. Hydrogen atoms are produced 

 when hydrogen molecules are mixed with a small amount of mercury 

 vapor and the mercury vapor is subjected to this light of 2536 A. The 

 atomic hydrogen, in turn, can produce a number of different hydrogena- 

 tion reactions. When ethylene, for example, is added to this system, 

 hydrogenation, decomposition, and polymerization all occur. A number 

 of different decompositions produced by excited mercury atoms have 

 been described by Bates and Taylor (3). 



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Cadmium vapor has a resonance line at 3262 A, and its atoms can be 

 excited in a similar manner by this radiation. The energy available at 

 this wave-length is considerably smaller, and atoms excited by this 

 radiation are not able to dissociate hydrogen, as is the case with mer- 

 cury (4). 



Monochloro-acetic Acid Hydrolysis. — When an aqueous solution of 

 monochloro-acetic acid is subjected to ultra-violet light of 2536 A, the 

 following reaction takes place: 



CH2CICOOH + H2O = HOCH2COOH + CI- + H+ 



The reaction goes on in the dark also to a slight extent, and it is acceler- 

 ated by hydroxyl ions. A quantum yield of practically one molecule 

 per photon is reported for the reaction (46). As the reaction proceeds, 

 the reaction diminishes in velocity at constant input of light, possibly 

 because of the increased acidity produced by the reaction itself. 



Nitrogen Dioxide. — Nitrogen dioxide is a brown gas. At ordinary 

 pressures it is partially polymerized to nitrogen tetroxide. Its absorption 



