330 APPENDIX. [No. XXV. 



the mantelpiece in a warm room, and in two or three days, notwithstanding 

 the filtering processhad removed suspended matter, it would begin to change, 

 and give unmistakable evidence to the olfactory nerves of the presence of 

 putrefying organic matter. Everything tended to show that animal matter 

 in a state of decomposition was to be feared. In seasons of epidemic it 

 was impossible to pass the excretae of one town to another in rivers without 

 great danger of propagating disease ; and for that reason water ought never 

 to be taken from such a source. Now, if it was matter in a state of change 

 which was injurious, there came the consideration whether the sewage 

 was presented as a totally changed matter in river waters ; in other 

 words, whether the sewage assumed a totally different form. Suppose 

 sewage ^was put upon the ground and absoi-bed by vegetables, such as 

 cabbages, or was absorbed by weeds in rivers, it was no longer sewage ; but 

 notwithstanding, there were several cases on record which showed that it 

 was not perfectly safe to manure gardens by pumping sewage : under 

 these circumstances, aa the plants grew up they would quickly decompose 

 after being cut for use, and would not be as wholesome for food as those 

 manured with sweet and fresh fertilizing matter. But if organic matter 

 assumed another form, it was really a new substance and harmless. The 

 question of changed matter was brought forward every month by the 

 Registrar-General and fallaciously estimated as pre-existing sewage, which 

 had caused some persons to be misled as to the wholesomeness of perfectly 

 unobjectionable waters. The matter must be utterly changed before it could 

 safely be used, and that change could be effected on the strata of the earth 

 by long-continued contact with mould and air. There was reason to 

 believe it was so with chalk. Whei-ever the water percolated through 

 chalk strata it was deprived of organic matter, — perfectly deprived of 

 that changing organic matter to which he had referred as beiug noxious, 

 and which was converted into nitrites and nitrates. No doubt animal char- 

 coal could do a good deal artificially ; but while a great and perfect filter- 

 bed existed in Nature, he held it was right and proper to get for a large 

 town, especially for London, such an amount of water perfectly filtered 

 by Nature as to extract all organic matter from the water, whether in the 

 original or the changing state. In that way alone could wholesome 

 water, wanting no artificial filtration whatever, be supplied to the com- 

 munity. 



Lettee of Alfeed Smee read at a meeting of Medical Men convened to 

 consider the Paper read by the late Dr. Letheby, ' On the Methods of 

 estimating Nitrogenous Matters in Potable Waters, and on the Value 

 of the expression " Previous Sewage Contamination," as used by the 

 Registrar-General in his Monthly Reports of the Meti-opolitan 

 Waters.' 



Deae Sia, — I regret that recent indisposition will prevent me from 

 accepting the invitation to be present this evening at the reading of 

 Pr. Letheby's paper. 



