50 LAWRENCE T. FAIRHALL AND PAUL M. BATES 



that turbid and colloidal solutions are difficult to sterilize, al- 

 though in small quantities the presence of a colloidal substance 

 does not interfere seriously. The particles present in a turbid 

 solution cast shadows and consequently cut down the effective- 

 ness of the rays (Oker-Blom, 1913). Other limitations imposed 

 are due to the thickness of the liquid films and the distance 

 from the source of the light. The presence of coloring matter 

 does not decrease the effectiveness of the rays to any great 

 extent (Houghton and Davis, 1914). 



The intense activity of ultra violet rays has also been shown 

 in a number of other ways. It has been shown, for instance, 

 that the amoebae in a water supply, whether motile or encysted, 

 are killed by a comparatively short exposure to ultra violet rays 

 (Chamberlain and Vedder, 1911), hemolytic amboceptors di- 

 luted 1 : 100 and exposed for 10 minutes are destroyed (Stiner 

 and Abelin, 1914), diphtheria toxin is weakened by exposure 

 (Hartoch, Schurmann and Stiner, 1914), amylase and invertin are 

 sensitive to ultra violet rays, and albumin is coagulated by their 

 action (Chaulpecky, 1912-13). 



The activity of ultra violet rays as a bactericide is indicated 

 by the fact that exposure of a fraction of a second (in some 

 cases one-twentieth) close to the lamp will result in the death of 

 microbes (von Recklinghausen, 1914). The abiotic power 

 diminishes about as the square of the distance from the lamp. 

 It has been found that the relative sensitivity of different germs 

 does not vary as much as in the case of heat and disinfectants. 

 For instance, spores are often twenty times as resistant as the 

 unprotected germs against chemicals, while against ultra violet 

 rays they are only 1.5 to 5 times as resistant as ordinary unpro- 

 tected water bacteria. 



The abiotic action of ultra violet rays is independent of the 

 temperature between and 55°C. Thus, it has been found that 

 the effect is the same in clearly frozen ice as in water. It is 

 scarcely probable that by the action of the rays during the short 

 time necessary to kill the bacteria the entire bacterial cell should 

 be chemically changed, coagulated or otherwise modified. The 

 opinion has been ventured (von Recklinghausen, 1914) that 



