INFLUENCE OF LIGHT AND OTHER RAYS. 165 



during the day and during the night accounts for the concentric rings 

 which are quite commonly found in older mold colonies, and which in- 

 dicate the age of the culture (Fig. 54). Similar rings are occasionally 

 found with yeast and bacterial colonies, and are possibly due to the same 

 influence of light. 



X-RAYS. Of other rays, the invisible X-rays and the radium rays 

 have attracted the attention of bacteriologists and physiologists. It 

 is known that the X-rays will destroy living tissue by long exposures; 

 microorganisms cannot be considered less resistant. X-rays are used 

 in the treatment of microbial diseases of the scalp and skin. 



RADIUM RAYS are not so well known, and their bactericidal action is 

 doubtful. The treatment of certain bacterial diseases has been attempted, 

 but it has not been applied as generally as yet as the X-ray method. The 

 sterilization of milk and possibly other foods by this method has been 

 suggested, but the practical application is at present quite improbable be- 

 cause of the cost and the uncertainty of the results. 



