1895.] Vegetable Physiology. 673 
a trace of any germicidal influence being visible. From the foregoing it 
is apparent that the red, orange, yellow, and true green rays of the spec- 
trum have no bactericidal action. Finaliy, portions of infected plates 
were submitted to the direct action of portions of the solar spectrum, 
passed through a grating as narrow as practicable (1 mm.) and through 
quartz plates instead of glass. These exposures confirmed the pre- 
ceding and show that the infra-red, red, orange, and yellow rays of the 
spectrum are absolutely without effect, the spores exposed to these rays 
germinating as readily as those on the non-exposed parts of the film. 
So far as could be determined by the methods used, the bactericidal in- 
fluence begins where the green shades into the blue, reaches its greatest 
intensity in the blue-violet in the vicinity of Fraunhofer’s line G, and 
fades out in about the middle of the violet, the more refrangible half 
of the violet and the ultra violet showing no influence. Subsequently, 
in conjunction with Prof. Oliver Lodge of Liverpool, many experi- 
ments were tried with a powerful are light. Even 8-12 hour expos- 
ures produced only a transient bactericidal effect when its rays had to 
traverse the glass covers of the Petri dishes, and in course of the ex- 
periments it was discovered that even the thinnest plate of glass is so 
obstinate a barrier to the bactericidal rays that it was not possible to 
use it and quartz had to be substituted. When this was done, 8-12 
hour exposures served to kill the spores of Bacillus anthracis, and 
even 6 hours exposure killed great numbers of them. Exposures of 
infected films to the spectrum of the arc light gave results in the main 
confirmatory of those previously obtained. Here again the infra-red, 
red, orange, yellow, and green rays were without perceptible effect, but 
the germicidal influence did not begin in the blue-green but just be- 
yond it in the blue, and its influence was visible into the ultra violet, 
the maximum effect being reached just beyond the violet. With both 
sun and arc light there is for a day or two after the colonies begin to 
appear a curious blurring of the margins of the insolated spots which 
gradually disappears as the colonies develop and which is attributed to 
halation. The germicidal etfect of the arc light is so powerful, when 
not destroyed by glass screens, that Prof. Ward thinks it might be 
turned to practical account in the disinfection of hospitals, cattle sheds 
and similar places. In these experiments the distance of the light 
was two feet. The author is inclined to think that not only the lower 
forms of life but also all protoplasm is sensitive to these rays of the 
spectrum and that the higher plants escape injurious effects by having 
provided themselves with natural color screens. Among other low 
organisms which he has found sensitive to direct sunlight are a violet 
