The relative efficiency of the biological action of the Roentgen rays emitted by the Coolidge and the old type tubes

In accordance with the modern conception in physics the Roentgen rays present pulsations in the ether analogous to the rays of light. The waves of ether forming the Roentgen rays are considerably shorter than the shortest ultra-violet waves of light. The waves of the so-called soft Roentgen rays are about 1,000 times shorter than those of ultra-violet light, and the waves of the hard Roentgen rays are still shorter.

Any substance, solid, liquid, or gaseous, absorbs a part of the Roentgen rays which pass through it. The fraction of the rays thus absorbed depends upon the density and thickness of the substance. The remaining rays penetrate beyond the interposed substance. The relation between the quantity absorbed by the substance and that penetrating beyond it is of fundamental importance 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 completely 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.