1. When B. coli are irradiated by x-rays in a series of salt solutions of tenth molar concentration, the synergistic effect does not become appreciable until heavy salts are used.

2. When Pb(NO3)2 or KBr solutions are used in increasing concentrations, the synergistic effect is not appreciable until large concentrations or absorption coefficients are reached, whereupon the effect increases sharply. Thus the number of effective hits per bacterium per minute, α, is a function of the absorption coefficient, µ.

3. The sharp increase in α does not occur at the same concentration, or same µ, for Pb(NO3)2 and for KBr. Thus α is a function of the nature of the salt, or possibly of the penetration of the salt into the cell, some measure of which may be obtained from the initial toxicity of the solution.

4. For a given solution, α increases as the wave length λ of the x-rays decreases, although µ decreases by the same process as the cube of λ. Thus α is a function of λ to some power greater than the cube.

5. A possible mechanism whereby the synergistic effect takes place is discussed briefly, as is the possibility that the heterogeneity of the x-rays accounts for all or part of the increased bactericidal effect of the rays in the presence of heavy metal salts.

6. Results indicate that within the range investigated, short wave lengths of x-rays, in conjunction with sensitizers, are the more efficacious in producing lethal effects.

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