56 DR. 11. ANGUS SMITH ON THE EXAMINATION 



The oxidation is assisted by the pressure to which the gas 

 is exposed by being brought in contact with a great amount 

 of surface^ as well as of numerous surfaces contributing to 

 the result. For these reasons nitrates are found best in soils 

 through which azotized matter in solution is slowly passed. 



An interesting question arises — Does the nitric acid 

 indicate the amount of organic matter which was previously 

 in solution? 



If nitrates are put on land they are decomposed by 

 vegetation, and the nitrogen is retained. These salts act 

 both as food and air to plants. The water which flows 

 from drained land may contain nitrates, but they are not 

 a measure of the amount put on the land ; and if they 

 result from the organic matter there, they are still not in 

 proportion to its amount, as, even after they are formed 

 from vegetation, they may be decomposed by it. 



Nitrogen, therefore, may be removed from water either 

 as ammonia, or organic matter, or nitric acid, every trace 

 of it disappearing. Those nitrates, however, which do 

 remain indicate that at least an equivalent of albuminous 

 matter or sewage-matter did exist. 



Another interesting question occurs — Is the nitric acid 

 removed without vegetation? I believe that it is so. 



In water from drained fields vegetation is generally 

 found, though frequently in very small quantities. In the 

 soil around drains it is found in considerable quantities ; 

 but after putrefaction has occurred in the Medlock, at 

 Manchester, I have not found it. The oxygen seems to 

 be removed as the oxygen of the air is, probably leaving 

 nitrogen to pass ofi' as gas. 



As the nitric acid indicates an equivalent of albumen, I 

 have put it down as telling us of the previous presence of 

 organic matter, in other words, of old organic matter. 

 The word '' old " has no relation to time except so far 

 as to mean older than that from which the nitrites 



