BANISHING THE PLAGUES 



berculosis by direct sunlight, as carried out in hospi- 

 tals established at high altitudes in the Alps, owes its 

 efficacy to the ultra-violet rays coming directly from 

 the sun. These rays, however, are obstructed by the 

 air, and they have largely disappeared from sunlight 

 at the sea level. They are altogether obstructed by 

 glass; so the sun treatment of tuberculosis must be 

 carried out in the open air. It may be added that the 

 children who receive this treatment in the Alpine hos- 

 pitals are gradually inured to the cold until they can 

 without discomfort take their sunbaths in the open 

 air stark naked even in the coldest weather. 



Practical use has been made of the bactericidal 

 properties of ultra-violet light in the purification of 

 water. But the application of the method to the puri- 

 fication of milk has hitherto proved impracticable be- 

 cause, owing to the opacity of milk, the rays do not 

 penetrate its substance to any considerable distance, 

 and therefore leave the main bulk of the germs un- 

 touched. 



It would appear, however, that a method has now 

 been found to overcome this difficulty. The new 

 method consists of the use of plates of quartz placed 

 very close together, so that the milk passes between 

 them in a thin film, and is thus sufficiently transparent 

 to be suffused with the light of a mercury vapor 

 electric lamp, the rays of which after passing through 

 quartz are very rich in the short waves that char- 

 acterize the spectrum beyond the violet. 



The apparatus in question is merely a modified 

 form of Mr. Peter Cooper Hewitt's quartz lamp; but 

 rather curiously the experiments in sterilizing milk 



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