Relative Efficiency of Roentgen Ray Tubes. 99 



portance for the proper understanding of the biological action of 

 the Roentgen rays. 



When the rays enter a plant or an animal body they injure 

 the cells of the organism through the biochemical action of the 

 rays on the protoplasm and mainly on the nuclei. This biological 

 action differs quantitatively in accordance with the amount of 

 rays absorbed by the cells and the susceptibility of the latter to 

 the action of the rays. When slightly injured a cell may com- 

 pletely recover, while if the injury is severe the cell dies. Every 

 cell of the organism may be killed by a sufficiently large quantity 

 of rays. Nevertheless the biological action of the Roentgen rays 

 must be considered selective in as much as quantities of the rays 

 sufficient to kill a certain kind of tissue may leave adjacent tissues 

 intact or only slightly injured. In order to obtain this selective 

 action the rays must be distributed as evenly as possible through 

 the organism. Such a distribution is possible only with the so- 

 called hard rays, whose penetration is much greater than ab- 

 sorption. 



At present there does not exist a direct method for measuring 

 the quantity and the penetration of the rays necessary to kill or 

 injure a given cell. All the measurements are indirect and are 

 based on the fact that the various chemical actions of the rays are 

 also in direct ratio to their quantity. A solution of barium-platin- 

 cyanide which has normally a green color becomes brown under the 

 influence of the rays, and the shade depends upon the quantity 

 used. A photographic paper in black covering will not be influ- 

 enced by rays of light, but will become blackened under the in- 

 fluence of the Roentgen rays. The shade of black deepens in pro- 

 portion to the quantity of rays used. On the basis of these color 

 reactions various apparatus are devised for measuring the quan- 

 tities of the Roentgen rays emitted by a tube in a unit of time. 



Before reporting the results of the present investigation a 

 brief outline should be made of the differences in the physical 

 characteristics of the two types of tubes used in the experiments. 



Roentgen rays originate on the surface of a metal which is 

 bombarded by the negative electrons of the kathode rays. The 

 greater the velocity of the kathode rays the harder, the more pene- 

 trating are the Roentgen rays. The tubes of the old type have an 



