EFFECT OF RADIATION ON PROTEINS 307 



tude, and the fact that the amphtudes are all nearly simple multiples of a 

 common factor, point to a similarity of constitution among these proteins 

 and to a variable "concentration" of the active group. 



X-rays and Radium Rays. — The absorption of X-rays and gamma 

 rays has been discussed in a previous paper and will be mentioned only 

 briefly here. The absorption coefficient ^ divided by the density p of 

 the absorbing screen = /z/p = the mass absorption coefficient. Dense 

 substances are more absorptive, mass for mass, than light substances 

 and m/p increases rapidly with the atomic weight of the absorbing mate- 

 rial. This increase is more noticeable with hard rays than with soft. 



No experiments have been carried out on the mass absorption coeffi- 

 cients of the various constituents of tissue, such as proteins, but the 

 absorption of blood and serum as a whole is not very different from that 

 of water. 



The beta rays of radium are more easily absorbed than X-rays and 

 gamma rays. Their penetrating power depends on the velocity of the 

 rays and the density of the medium. The heavier alpha particles are 

 absorbed more easily than beta rays. 



Ionization Due to Absorption of Radiant Energy. — The alpha, beta, and 

 gamma rays of radium and X-rays all ionize matter through which they 

 pass. Owing to their rapid absorption, alpha rays produce intense 

 ionization in a limited depth of material. The ionization produced by 

 the more penetrating beta rays is less dense and less concentrated. 

 X-rays and gamma rays are so penetrating that little energy is absorbed 

 by matter through which they pass so that they produce relatively little 

 ionization and the ions formed are more widely separated. 



The ionization of atoms and molecules, as the result of the absorption 

 of ultra-violet and visible radiation, is called the photoelectric efTect. 

 The majority of solids show this effect for wave-lengths shorter than 



o 



about 3000 A, although the threshold varies from element to element. 

 No liquids, either organic or inorganic, have been found to show a genuine 

 photoelectric effect with radiation longer than 1700 A except solutions of 

 ferrocyanide salts. Water shows only a very weak photoelectric effect 

 even at these very short wave-lengths. Photoelectric effects from organic 

 liquids are usually due to solid films or colloidal particles on the surface, 

 but apparently nothing definite is known in regard to the photoelectric 

 effect from colloidal solutions such as proteins. 



When atoms and molecules absorb energy from wave-lengths too long 

 to produce ionization or decomposition, they may become "activated" 

 and this activated state favors chemical reactions, such as oxidations and 

 reductions, and frequently brings about isomeric changes and polymeriza- 

 tion of molecules. 



There is also evidence of increased conductivity in some liquids 

 absorbing ultra-violet radiation (15) which may be due to the formation 



