STERILIZATION BY LIGHT 129 
by the bacteriologist to sterilize media or apparatus. The work on the 
germicidal action of light is extensive and those wishing bibliographies 
on the subject should consult the papers of Dieudonne (1894), Ward 
(1895) and Weinazirl (1914). 
Ward (1895) using anthrax bacilli and spores found that exposures 
of one-half to one hour were necessary for inhibition of growth and 
that one and a half hours were necessary for sterilization. 
Dieudonne (1894) worked with pathogenic bacteria. With B. 
fluorescens puditus and B. prodigiosus he found that one and a half 
hours were required in March, July and August and nearly twice as 
long in November. 
Kruse found that an atmosphere of oxygen shortened the length 
of life by one-half over that of hydrogen when bacteria were exposed 
to sunlight. 
Weinzirl (1914) devised a new technique which involved exposure 
of the bacteria on paper directly to the sunlight. He stated that non- 
spore-forming bacteria were killed in from two to ten minutes. Such 
conditions are rarely secured in nature where the organisms are often 
accompanied with organic matter. For spores from two to eight 
hours were necessary. 
The various colors of the spectrum exert a different action on bac- 
teria. The intensity of this action increases as one goes from the red 
toward the violet. Apparent destruction is noticed only after the 
yellow is passed. From the yellow through the violet and ultra- 
violet, the germicidal action is at its maximum. In fact it is now well 
established that the shorter the wave length the greater is the bacteri- 
cidal action. Newcomer (1917) has reported experiments which indi- 
cate that X-ray fluorescence 1s very bactericidal. X-rays alone exerted 
a, partial bactericidal effect on suspensions of typhoid bacilli. Kempster 
(1917) exposed tubercle bacilli to X-rays and secured a reduction in 
their multiplication. Inoculations of tubercle bacilli, which have been 
exposed to X-rays, into guinea pigs caused no tubercular lesions to 
develop. He stated that the phagocytes were able to overcome the 
bacteria if their multiplication was reduced. 
Ultra Violet Light. This light on account of its short wave length 
possesses decided bactericidal effects. Ward (1892) in studying the 
effect of arc spectra on bacteria found that the infected plates were 
sterilized whenever exposed to the violet or ultra violet rays. He 
successively treated Thames River water by submitting it the effects 
of ultra violet rays. Houghton and Davis (1913) give the following 
conclusions to their work: 
