374 ULTRA-VIOLET LIGHT ON BACTERIA AND PRODUCTS 



other hand, allows the active rays to penetrate. Barnard and Morgan' state that four- 

 fifths of the active light will be absorbed by a layer of water 2.5 cm. thick. The short- 

 er the rays the less the dista^nce of penetration. Lyman found no transmission of 

 waves below 1,792 A through 0.5 mm. of water. Kreusler^ states that 14. i cm. of 

 water will absorb waves of 2,ooOvA. 



The influence of the H-ion coni,':entration has been particularly studied by Bayne- 

 Jones and Van der Lingen.^ An acdd medium is more favorable for the destructive 

 action than an alkaline one. A favorite'-.method of study has been by the use of a seeded 

 agar plate. A slight stimulating action bf the medium adjacent to exposed portions 

 has been reported. Coblentz and Fulton^ \.found that agar plates exposed for long 

 periods to ultra-violet light were incapable of supporting growth but that short ex- 

 posures had no effect. It is well known that colored or turbid waters are difficult to 

 sterilize. ■> 



Time. — The time required to kill bacteria varies f i'om a few seconds to several min- 

 utes, depending upon the wave-length of light used and the intensity of the light. Some 

 of the earlier workers maintained that increase of temperature accelerated germicidal 

 action, but Henri and Cernovodeanu^ disputed this claim.- Bayne- Jones and Van der 

 Lingen,'' using temperatures varying from 2° to 40° C, were able to detect only a 

 slight increase in the velocity of the reaction with increase iu temperature. It is not 

 impossible that differences in velocity may be masked by the irapidity of the reaction 

 itself. 



Discontinuous exposure. — The total time required to kill bactveria on an agar plate 

 or in suspension is not changed by intermittent exposure. 



Residual action. — It has been claimed that water exposed to bactericidal rays will 

 retain its activity for some time. This was denied many years ago by Cernovodeanu 

 and Henri^ but the claim was revived in 1921 by Walker and Pryer.^ Their results, 

 using Bacterium paratyphosum B, are somewhat startling and have never been ac- 

 cepted by sanitary engineers. Theoretically, a residual germicidal action is not an 

 impossibility since we know from the work of Steenbock and Daniels,' Hess and Wind- 

 aus," and others that foods containing fresh fats and oils and lipoids such as cholesterol 

 can be activated for the prevention of rickets by exposure to ultra-v'i.olet light. It is 

 not outside the range of possibility that water, particularly after contamination by 

 man, might contain absorbing substances. 



I 



APPLICATIONS 



Practical use has been made of the bactericidal properties of ultra-violet light in 

 connection with the purification of drinking water." Although there are a number of , 

 ■ Barnard, J. E., and Morgan, H.: lac. cil. ^ Kreusler: Ann. der Physik., 6, 412. 1901. 1 



5 Bayne-Jones, S., and Van der Lingen, J. S.: loc. cit. 



1 Coblentz, W. W., and Fulton, H. R.: loc. cil. ' Cernovodeanu, B., and Henri, V.: loc. cil. 

 * Bayne-Jones, S., and Van der Lingen, J. E.: loc. cit. 



7 Cernovodeanu, B., and Henri, V.: loc. cil. 



8 Walker, W. F., and Pryer, R. W.: Am. J. Pub. Health, 11, 703. 1921. 

 ''Steenbock, H., and Daniels, A. L.: J. A.M. A., 84, 1093. 1925. 



"Hess, A. F., and Windaus, A.: Pyoc. Sac. E.xpcr. Biol. 6" Alai., 24, 171. 1926; 24, 369. 1927. 

 " Water Works Practice, p. 264. Philadelphia, 1925. 



