SUNSHINE AND LIFE 75 



turn sewage into our rivers and so dispense 

 with the cost and labour of its treatment and 

 purification. 



This was actually suggested in a proposal made 

 for dealing with the sewage of the city of Cologne. 

 Fortunately further investigations have removed 

 these most erroneous and dangerous ideas ; and 

 whilst all due credit may be given to sunshine 

 for what it really does accomplish in the destruc- 

 tion of bacteria in water, there is now no doubt 

 as to its potency being confined to the superficial 

 layers of water. 



Perhaps Dr. Procacci's experiments will most 

 clearly convey some idea of this limitation, for 

 he made a special study of this particular 

 phenomenon. Some drain water, containing, of 

 course, an abundance of microbial life, was placed 

 in cylindrical glass vessels, and only the per- 

 pendicular rays of the sun were allowed to play 

 upon it. The column of water was about two 

 feet high, and whilst a bacteriological examination 

 at the commencement of the research showed 

 that about two thousand microbes were present 

 in every twenty drops of water taken from the 

 surface, centre, and bottom of the vessel re- 

 spectively, after three hours' sunshine only nine 

 and ten were found in the surface and centre 

 portions of the water, whilst at the bottom 



