X 



xanthophyll — A plant pigment. See nonastacin by the transitions of atoms from K, L, . . . en- 



carotenoid. ergy states to lower energy states. In nuclear 



X-rays— Electromagnetic radiation of wave- reactions it is customary to refer to photons orig- 

 in,, . ^, 1 . -.^^ s / . N inating in the nucleus as gamma rays, and to 

 lengths less than about 100 A (angstroms) , pro- ^ j.^ge originating in the extranuclear part of the 

 ducecl: (1) when electrons striknig a target lose atom as X-rays. These rays are sometimes 

 kinetic energy in passing through the strong called Roentgen rays after their discoverer, 

 electric fields surrounding the target nuclei ; (2) W.C.Roentgen. (41) 



185 



