The survival ratios of colon bacilli subjected to several monochromatic ultraviolet radiations follow semilogarithmic straight lines. For each wave length approximate observations have been made of the energy involved in cell destruction. This energy varies somewhat with frequency in the ultraviolet region; it is furthermore nearly one hundred times as great as the amount of X-ray energy required to bring about the same killing.

Preliminary experiments show no measurable difference either in rate of killing or in lethal energy between B. coli and B. aertrycke. Parallel results have already been obtained with X-rays and electrons.

The data from colon bacilli are interpreted in terms of the assumptions employed for X-rays. They indicate that though bacterial death should result from a single quantum absorption, millions more such absorptions seemingly are without injurious effect on cell growth and multiplication. The "sensitive volume" within which, according to this picture, the lethal quantum must be stopped proves to be about the same as that of a single protein molecule. If this is the correct description of the phenomena of ultraviolet killing, it seems strange that the millions of non-deadly quanta absorbed per bacillus should not show themselves by altered growth rates or in other ways. That they apparently do not suggests the inapplicability of the statistical picture. The death rate under this kind of radiation then would be primarily an expression of the relative sensitivities of the bacterial population. Additional experiments are required to determine this question.

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