PLATE 11 



EXPLANATION OF FIGURES 



ViewB of specimen no. 9 : a, on dark, mixed sand (after two days) ; b, in jar having 

 black walls and gravel bottom (after two days); c, in jar having white walls and 

 black bottom (after five days); d, in jar having black wails and black bottom 

 (after one day) ; (Comparison between the last two shows plainly the effect of the 

 white walls in the former case). 



500 



