248 JJniversitij of Valifoviiia PuhUcaiionsin Paflioloyij [Vol.2 
These figrires represent the relative resistance of these three 
organisms. 
The protective action of tlie amino acids was then studied. 
The cover slips were exposed as before, but between the cover 
slip and the arc a quartz beaker containing the given amino acid 
was interposed, so that the rays before striking the organisms 
passed through the amino acids. The results obtained confirm 
our previous work. With B. suhtilis, whose normal extermina- 
tion period is 150 seconds, we found exposure for forty minutes 
to ultraviolet light passed through 1% tyrosin solution exerted 
no toxic effect upon the bacilli, a good growth being obtained in 
forty-eight hours. Similarly, Staphylococcus aureus gave good 
growth after forty minutes; B. mucosus cwpsulafus, though not 
tested after longer exposure, gave satisfactory growth after ten 
minutes. 
A good growth was also obtained with amino-benzoic acid 
after exposure of these organisms for 3200 seconds to ultraviolet 
light detoxicated by passing through this substance. Phenyla- 
lanin could not be secured, but there is little doubt that similar 
results could be obtained with it. 
These re.sults confirm those of our previou.s report*''' and 
indicate that the aromatic amino-aeid radicals are the absorbing 
substances in bacteria as well as in protozoa. Kober's" work 
placed the absorption band for tyrosin at 248 to 297/i, or 2480- 
2970 Angstrom units, of wave length, and for phenylalanin 236- 
27 V, or 2360-2710 Angstrom units. 
Therefore ultraviolet light, of wave lengths 2360-2970 Ang- 
strom units, should contain practically all of the rays toxic for 
protopla.sm. Two recent papers have appeared, however, which 
report different results. Browning and Russ" found the toxic 
action of ultraviolet light falling off sharply at 2960 A. U.. which 
would apparently correspond with one edge of the tyrosine band. 
The\' report, however, an apparently constant toxicity from 
2960-2100 A.U., and did not investigate below 2100 A.U. New- 
comer'*-" reported also a constant toxicity from 2100 A.U. up to 
a little less than 21)00 A.U., the toxicity falling off to practically 
zero at 2970 A.U. The region in which tyrosine and phenylalanin 
