80 DR. R. ANGUS SMITH ON THE EXAMINATION 



frequently see. This decomposition is performed by 

 living plants apparently ; but some observations seem to 

 indicate that the effect may be produced by the organic 

 dead matter in water. The nitrites found in plants by 

 Schonbein may possibly have been formed from the nitrates 

 by deoxidation.^' 



Animal and Vegetable Matter — The Common Salt of 



Sewage, 



This subject deserves a further separate treatment. 



As animals are formed out of vegetables, the elements 

 which compose each are of course the same ; and even the 

 proximate organic principles are difficult to distinguish, 

 especially in a state of decomposition. Still it isilremark- 

 able what a clear insight is given into the quality of 

 water by simply boiling down a few thousand grains and 

 burning the residue. We can by the eye and the smell 

 detect humous or peaty acids, nitrogenous organic sub- 

 stances and nitrates, and estimate their amount to a very 

 useful point of accuracy. There may be times when this 

 is the only experiment that can be made. After doing 

 this, and trying other methods many hundred times, I still 

 return to it as delicate and little liable to fail. The want 

 of numerical results is a serious objection, rendering the 

 plan unfit for use when public reports are to be made of 

 the analyses ; but it is an excellent guide for the chemist 

 in his laboratory. We may even decide by it the animal 

 or vegetable origin of the matter. 



This, however, is not enough ; we require to obtain 

 some knowledge of the quantity as well as of the quality, 

 and we even require to know something of the previous 

 history, of the water. 



Supposing decomposition to have gone so far that 

 nothing^ definite can be affirmed as to the organic sub- 



