No. XXV.] APPENDIX. 331 



The doctrine of " Pre-existing Sewage " has for some time occupied 

 my attention, because as now understood it is one of the most dangerous 

 fallacies of the day. 



The question resolves itself into two parts: — Ist. The question of 

 pre-existing sewage contamination, as infen-ed from matters containing 

 nitrogen in organic matters in the act of change, or undergoing oxidation. 

 2nd. The question of pre-existing sewage contamination, as inferred from 

 the presence of nitrates which are assumed to h.ave arisen from the final 

 oxidation of organic matters. 



On the first part of the subject I have a little to comment, because all 

 organic matters in the act of change are bad, though doubtless sewage is 

 materially worse than other foi-ms of changing organic matters. The 

 dangerous part of the doctrine is, the inference of pre-existing sewage 

 from the presence of nitrates. 



As a matter of fact nitrates may be present without any pre-existing 

 organic matter, and every flash of lightning causes the union of the 

 elements of the air, and the production of nitrates without any previous 

 sewage contamination. 



The originators of the doctrine of pre-existing sewage say that at any 

 rate the presence of nitrates sbows the possibility of antecedent sewage 

 contamination. 



The fallacy of the doctrine consists in assuming a possibility as a 

 probability, and acting upon it as a reality, which in practice in this 

 metropolis may be followed by the most disastrous consequences. 



A very considerable quantity of water is supplied to London from 

 the overflow of water from the great chalk hills which act as a perpetual 

 storehouse. 



The water from the chalk deposits, both in this country and abroad, 

 contains a very appreciable quantity of nitrates, which does not vaiy in 

 any very important manner. 



Chalk water by running over water weeds loses its nitrates and 

 greatly stimulates their growth, and especially it may be noted that the 

 best watercresses are grown in water which has lately emanated from the 

 depths of the chalk formation. 



In consequence of this result, the metropolitan waters contain 

 more nitrates the less they have been exposed to the contaminating 

 influence of i-ivers. 



The New River Company is mischievously returned by the authorities 

 aa supplying more pre-existing sewage to its customers, in proportion to 

 the quantity of water which tbe company pumps from its deep springs and 

 the less it supplies from the river. In this way, the purer the water 

 which it supplies to its consumers, the more pre-existing sewage is 

 officially returned by the authorities. 



The origin of the nitrates in chalk waters has not been satisfactorily 

 discovered. I myself have made many investigations on the subject, and 

 the day before my recent indisposition believed that I had obtained a clue 

 to the solution of the mystery, which demands full inquiry. I have con- 

 sidered the question under four heads : — 1st. Do the nitrates come from 

 the nitrogen and oxygen of the atmosphere ? 2nd. Are the nitrates fossil 

 products of the animals which formed the chalk deposits ? 3rd. Ai-e 

 the nitrates the products of animal matter superimposed upon the chalk ? 



