1919] Harris-lloi/l ■ Ad ion tif UUraviolet Light on Bacteria 2V.) 
are both absorbed, i.e. :i48U to 2710 A. II., should be tlie most 
toxic for protoplasm, whereas the region containing wave lengths 
shorter than 2300 A. U. should be relatively non-toxic. 
Our former experiments eonelusively demoiistrated that a 
solution of tyrosine will absorb practically all of the toxic rays, 
those getting through not being sufficiently toxic to kill paramecia 
after exposure for forty minutes, whereas if the rays absorbed 
by tyrosine were allowed to act the paramecia were killed in 
100 seconds. 
Conclusions 
1. The aromatic amino acid radicals are among the sub- 
stances in bacteria affected by the action of ultraviolet light, as 
was shown for paramecium in a previous report. 
2. The ultraviolet radiations produced by the mercury arc of 
wave lengths not aljsorbed by tyrosine and phenylalanin are 
relatively non-toxic. Therefore, using Kober's determinations 
for the wave lengths corresponding to these two absorption bands, 
the ultraviolet radiations which are toxic for protopla.sm are of 
wave lengths from 2480-2710 A.U. 
3. In the three types of bacteria studied capsulated organisms 
were found to be most susceptible and sporulating organisms 
most resistant to the action of ultraviolet light. The work sug- 
gests strongly that the protoplasm of Bacillus mucosus capsulatus 
contains greater amounts of the above mentioned substances than 
the non-capsulated staphylococcus and the sporulating hay 
bacillus. 
Transmitted Fehruanj 6, 1919. 
