EFFECTS OF RADIATION ON BACTERIA 1131 



of artificial sources particularly rich in ultra-violet radiation. The rapid 

 rise of the curve of lethal action as wave-length decreases, in general, 

 from around 3000 A toward the region of 2500 A or lower, makes it also 

 particularly important and interesting to know precisely the absorption 

 values of the cells in this region of the spectrum. 



In the earlier work on the absorption spectra of bacteria crude 

 suspensions were employed, and, in consequence, the scattering effect 

 introduced an error interfering materially with accurate determinations. 

 In the work of Browning and Russ (19), a wide absorption band was 

 found beginning at 2960 A and extending over lower wave-lengths to 



o 



2100 A. This band accordingly coincided rather accurately with the 

 bactericidal range, and is of interest in spite of the fact that the con- 

 centration in bacterial cells is not stated. B. typhosus and Staphylococcus 

 pyogenes aureus were the organisms used. 



Working with three organisms, Bayne-Jones and Van der Lingen (10) 

 used a highly concentrated suspension containing five billion organisms 

 per cubic centimeter. With a quartz spectrograph and a zinc or an iron 

 spark source, they made photographs at intervals, comparing these 

 consistently with photographs of the emission spectra of the arcs 

 employed, thus determining approximately the amount of light absorbed. 

 The results with the several bacteria were practically identical. The 

 effect of scattering was recognized, so that the results are regarded as 



o 



relative. Up to 3000 A the suspensions used completely absorbed the 



o o 



ultra-violet, while at 3500 A absorption was still strong, and at 3700 A 



o 



the absorption was 50 per cent. Above 3900 A the "emulsion transmits, 

 but greatly diminishes" the radiation. It will be recalled that these 

 authors (see section of this paper on ultra-violet) were able to demonstrate 

 bactericidal action at wave-lengths somewhat longer than 3500 A (with 

 filters). 



In connection with his work on lethal effects of ultra-violet on B. coli, 

 it is interesting to note that Henri (73a), on the basis of the absorption 

 spectrum of egg albumen, advanced the view that the bactericidal effect 

 may be properly regarded as proportional to the coefficient of protoplasmic 

 absorption. 



Gates (60) varied the technique by scraping a mass of bacteria from 

 an agar slant and pressing this into a layer between quartz plates to 

 yield a film consisting primarily of bacterial cells and the accompanying 

 metabolic products. He adopted as a standard unit of thickness 0.8 m, 

 that is, the average diameter of Staphylococcus aureus, one of the organisms 

 employed. It is assumed that the bacteria thus form a homogeneous 

 medium and apparently that all are living. The original paper should be 

 consulted for the technique both in the absorption studies and in the 

 methods of determining the thickness of the bacterial film. Five series of 

 determinations were made with Staphylococcus aureus and with B. coli. 



