378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



Sim baths are certainly not likely to cause cancer in humans. 

 Cancer is produced in rats after prolonged exposure to ultraviolet 

 either from the sun or a quartz lamp. But rats and man do not re- 

 spond in the same way to sunlight. The normal habitat of the rat 

 is darkness; therefore it is more sensitive to ultraviolet light than 

 man. One year in the life of a rat is comparable to thirty years in 

 the life of a man. It required on an average about 7 months of con- 

 tinuous irradiation to produce cancerous changes in the rat which 

 would be equivalent to 20 hours of daily ultraviolet irradiation for 

 about 18 years in the case of man. 



Of the diseases of the skin, lupus vulgaris is the only one on which 

 ultraviolet rays have a specific curative action. 



The lethal effect of the ultraviolet on the lower plants such as 

 bacteria, fungi, and algae has been studied by a vast number of 

 scientists. The result of their research has been utilized for prac- 

 tical purposes in food preservation, milk pasteurization, sterilization 

 of operating rooms, the elimination of micro-organisms in drinking 

 water, and for partial sterilization of swimming pools. The wave 

 lengths of the ultraviolet having this lethal effect are those of very 

 short range, from 2950 A to 2200 A, or those wavelengths which are 

 filtered from the sunlight that reaches the earth's surface. 



Seed plants, because of their complex organization and their need 

 of visible light for normal growth, are still puzzling botanical work- 

 ers desirous of learning the true response of the plants to ultraviolet 

 light. The amount of the various wave lengths which plants and 

 plant parts absorb increases the uncertainty of knowledge of the sub- 

 ject. The injurious and destructive action of ultraviolet light on 

 plants has, however, been accurately determined for the wave lengths 

 shorter than those present in solar radiation. 



The effect of ultraviolet light on the different physiological proc- 

 esses of plants is being studied just as it is on those of animals. The 

 ultraviolet causes a temporary acceleration of respiration in higher 

 plants. A marked acceleration in ferment action is caused by irradi- 

 ating yeast cells. Results of investigation on chlorophyll formation 

 in green leaves have so far been somewhat contradictory. For all 

 this work such a specialized development of apparatus and technique 

 is required that progress is made very slowly. 



The ultraviolet possesses what seems to be an almost magical power 

 to transform an ordinary, somewhat drab object into a thing of 

 breath-taking beauty. A small stone of calcite subjected to ultra- 

 violet becomes a living rose color ; hyalite becomes sea green ; fluorite, 

 brilliant blue; aragonite, shell pink; wernerite, bright yellow; and 

 willemite, a dark green. All these stones that we might trample over 

 casually with no thought to their appearance, because ordinarily 



