316 BIOLOGICAL EFFECTS OF RADIATION 



tation or gelation. The effect of X-rays, beta rays, and gamma rays on 

 emulsoids may possibly be due to the same cause. 



In the case of emulsoids the fact that they are denatured irrespective 

 of their charge is evidence that an ionization of the colloid is not the 

 cause of denaturation. As water is not ionized except by radiation 



o 



shorter than 1700 A, denaturation by ultra-violet radiation is probably 

 not due to an ionization of the solvent. Unfortunately the sensitivity 

 curve of proteins to monochromatic ultra-violet radiation has not been 

 investigated. If it were known to be identical with the absorption 

 curve, one could be certain that the change produced by ultra-violet 

 radiation is a change in the protein molecule itself and not a change in 

 the solvent. Heat denaturation has been found to be a unimolecular 

 reaction. If radiation denaturation is also unimolecular, the change 

 is not, primarily, one of molecular aggregation. Unfortunately no 

 investigations have been made on the molecular weight of radiated 

 albumins to see if, after radiation, at a pH where visible aggregation 

 does not take place, the radiated albumin has the molecular weight as 

 well as the solubility and absorption characteristic of a globulin. The 

 aggregation seen under the ultramicroscope in globulins and the floccu- 

 lation at the isoelectric point in radiated albumins is not the primary 

 change produced by radiation but a secondary change taking place 

 between the denatured molecules. 



PRIMARY AND SECONDARY LIGHT REACTIONS 



In a primary light reaction a substance A is changed to form B on 

 the absorption of radiation and if B is stable, the amount formed is pro- 

 portional to the energy absorbed and i X t (intensity X time) is a con- 

 stant. Usually B is unstable. B may revert to A again with production 

 of fluorescence. Or B may react with other substances present {i.e., 

 B -}- C ^- D). In that case the amount of D formed is not proportional 

 to the amount of energy absorbed and it^ = const. In general this is 

 the type of reaction resulting from the absorption of radiation. The 

 amount of B formed may be proportional to the radiant energy absorbed 

 and independent of the temperature, but the final product measured 

 (D) is not proportional to the radiant energy absorbed and is not inde- 

 pendent of temperature, and the period of time necessary for the reaction 

 of B with C is responsible for the latent period which is such a well-known 

 characteristic of light reactions. 



The precipitation of negatively charged colloids by alpha rays may 

 possibly be a primary light reaction, though measurements to substantiate 

 this are lacking. 



The precipitation or gelation of positively charged suspensoids by 

 beta rays, gamma rays, and X-rays is a secondary light reaction as the 

 primary effect is apparently an ionization of the solvent. 



