246 University of California Publk-atioiis in Pathologi) [Vol. 2 
action of ultraviolet light obeys this law and is due to the absorp- 
tion of the rays by certain constituents of all living protoplasm, 
namely, the tyrosine and phenylalanin radicals of the protein 
molecules. These two acids are the specific absorbents in living 
protopla.sm for the rays of the mercury arc. 
The object of the present work was to confirm this work, 
using bacteria as a biological test, and further to study the rela- 
tive speed of absorption of the ultraviolet raj^s by the protoplasm 
of the various types of bacteria. 
jMethods 
Three organisms were selected as typifying thi-ee general 
groups of bacteria, which are classed on the basis of the posses- 
sion or lack of protective structures, spores and capsules. 
1. A non-sporulating, non-capsulated organism, Staphylo- 
coccus aureus. 
2. A sporulating. non-capsulated organism. Bacillus suh- 
tilis. 
3. A capsulated, non-sporulating organism, B. niucosus 
capsulatus. 
For the exposure of these organisms various methods were 
tried. The organisms were grown on agar slants for twenty-four 
hours, and then washed off with sterile 0.85% NaCl. A pre- 
liminary exposure was made with a given amount of each saline 
suspension exposed directly to the rays. As no consistent results 
were obtained by this method, due undoubtedly to the absorption 
of the rays by the upper layers of the bacterial suspension, this 
method was abandoned. 
A number of plating methods were then tried, ^lelted agar 
was poured at 42° C and allowed to harden in ten centimeter 
jictri dishe-s. To prevent condensation it was found best to cover 
with sterile tile covers. After hardening three methods of 
inoculation ■were tried. 
a. Two separate streaks were made with a loop full of saline 
suspension, one on cithei- side of the diameter of the plate. Half 
of the plate was then exposed, the other half being protected by 
a glass cover, covered with black paper. The plate was then 
