38 



The Germicidal Action of Ultra- Violet Radiation. 



to the correspondence between our result and that found by Grotthus in 

 1818* for photo-chemical action, which he expressed in the law " that only 

 those rays which are absorbed can produce chemical change." 



The Bearing of the Ohser vatioiis on the Clinical Uses of Ultra- Violet Radiation. 



The preceding data indicate that, from the clinical point of view, there are 

 two distinct regions of ultra-violet radiation : — 



Gron-p 1. — A portion which begins where vision fails, namely 3800, and 

 extends to 2960 A.U. These rays have no marked germicidal action ; they 

 are capable, however, of penetrating a considerable thickness of human skin. 



Group 2. — A portion which extends from 2960 to nearly 2100 A.U. 

 These rays have very marked germicidal action, the region of maximum 

 effectiveness being between 2800 and 2540 A.U. The penetrating power of 

 these rays is, however, very small ; they are completely absorbed by as little 

 as 1/10 mm. of human skin. 



It remains to consider to what degree these two regions of the ultra-violet 

 radiation contribute to the beneficial effects resulting from the clinical use of 

 these rays. 



If a powerful source of ultra-violet radiation be directed upon an infected 

 wound, the result of an adequate exposure will be that the pathogenic 

 organisms on the surface will be directly killed ; it is, of course, not 

 determined what action these particular rays may themselves exert on the 

 protective mechanism of the living tissues. Organisms at a depth cannot be 

 killed directly, for the germicidal rays do not reach them. The passage 

 through the tissues of rays constituting Group I, together with the luminous 

 portion of the rays, may not, however, be without effect upon them. It is 

 well to bear in mind the possibility of the luminous rays having definite 

 physiological effects, for it must be remembered that such rays penetrate to a 

 greater depth of the tissues than does even Group I of the ultra-violet rays. 



It has been stated by several writers that deep-seated conditions have 

 been benefited by exposure to some strong source of visible and ultra-violet 

 radiation. It remains to be found to what extent the different parts of these 

 two octaves of radiation are concerned in such clinical results, although, as 

 has been already stated, there is no evidence that deeply situated organisms 

 can be affected by those ultra-violet radiations which are known to possess 

 powerful bactericidal action. 



We have pleasure in recording our thanks to the British Thomson- 

 Houston Company for their gift of the tungsten used in this investigation. 

 * See Gilbert's 'Ann. d. Phys.,' vol. 61, p. 50 (1819). 



