Chap. V. AND DEN UDATION. 241 



sand, consisting of minute particles of silex 

 coated with the red oxide of iron ; and the 

 burrows, which the worms made through this 

 sand, were lined or coated in the usual manner 

 with their castings, formed of the sand mingled 

 with their intestinal secretions and the refuse 

 of the digested leaves; and this sand had 

 almost wholly lost its red colour. When 

 small portions of it were placed under the 

 microscope, most of the grains were seen to 

 be transparent and colourless, owing to the 

 dissolution of the oxide ; whilst almost all the 

 grains taken from other parts of the pot were 

 coated with the oxide. Acetic acid produced 

 hardly any effect on this sand ; and even 

 hydrochloric, nitric and sulphuric acids, 

 diluted as in the Pharmacopoeia, produced 

 less eifect than did the acids in the intestines 

 of the worms. 



Mr. A. A. Julien has lately collected all 

 the extant information about the acids gen- 

 erated in humus, which, according to some 

 chemists, amount to more than a dozen 

 different kinds. These acids, as well as their 

 acid salts (i.e., in combination with potash, 

 soda, and ammonia), act energetically on 



