26 



SULPHUR BACTERIA 



when the sand is allowed to dry ; it then assumes a grey 

 or sometimes a golden-brown colour, similar to the normal 

 sand in the same neighbourhood. When it is again wetted 

 the black colour returns. The following facts have been 

 determined : — 



(i) Number of Bacteria. — The following figures give the 

 " total count " of bacteria in sand selected from different 

 parts of the Clyde : — - 

 No. of Sample. 



Calculated on sand with- 

 >. out freeing it of its water 

 content. 



It was found that at least nine-tenths of these bacteria 

 liberated hydrogen sulphide under appropriate conditions of 

 cultivation. 



(2) Organic Matter, Total Iron, and Moisture. — The amounts 

 of these constituents are given as percentages in the following 

 table : — 



Two other samples examined independently by workers in 

 the author's laboratory gave 0-187 pcr cent, and 0*189 per 

 cent, respectively for the total iron content. 



* Subsequent counts on other samples from the same district gave the 

 figures 600,000, 650,000, 400,000. 



