Trans. N. V. Ac. Set. 112 Apr. g, 



stability of much of the organic matter so great, that the best we can 

 do is to make a tolerably accurate estimation of the total amount, and 

 endeavor to find out what portion of this is safe and what is possibly 

 dangerous to health. To make this scientific guess, so lo speak, various 

 expedients have been hit upon, and a vast deal of thought, controversy 

 and time have been expended. The mode most in vogue is to heat the 

 drinking-water with an alkali and a powerful oxidizing agent, and to 

 find how much ammonia distils off from it. The ammonia thus ob- 

 tained is regarded as a measure of the putrefiable organic matter 

 present. Another way is to find out just how much nitrogen and how 

 much carbon are present, and, from the fact that animal substances 

 have a much higher proportion of nitrogen to carbon than vegetable 

 substances, to conclude whether any dangerous bodies of animal origin 

 are present. These two ways are greatly in use in England. The 

 Germans place great confidence in a third method, which is to find how 

 much of the organic matter is capable of being oxidized by per- 

 manganate of potash when the water is being boiled with this strong 

 oxidizer, and to regard this oxidizable fraction of the organic matter as 

 the part dangerous to health, and of importance therefore to consider. 

 In France, again, much stress is laid upon finding out how large an 

 amount of oxygen is dissolved in the water, on the ground that if there 

 is much less present than is always found in really pure water, there is 

 reason for believing that the deficiency is due to the absorption of 

 oxygen by bodies undergoing decay and putrefaction. 



The new method, I am about to propose, depends upon the fact that 

 compounds of silver are not decomposed by light, when they are in 

 solution in water, unless organic matter is present in the water also. 

 If sufficient care is taken to exclude every trace of organic matter, even 

 such as might accidentally enter from the dust of a room, silver 

 solutions may be kept in the sunlight for years without change. 

 Another fact lying at the foundation of this new process is, that stable 

 organic bodies, like sugar, starch, gum, etc., have very little influence, 

 while decomposing substances, like excreta ot all kinds, throw down 

 the silver very rapidly. 



[Here a bottle was shown in which some sewage-water had been 

 added to a solution of nitrate of silver, and then exposed to sun- 

 light. The whole interior of the bottle was covered with a bright 

 mirror of metallic silver. Another bottle was shown containing 

 some Croton water drawn at the Christopher St. Ferry last Saturday. 

 It was covered at the bottom with a black deposit of metallic silver, 

 nearly but not quite so great in amount as a similar sample taken from 

 the Passaic River (the water-supply of Newark and Jersey City) a 

 week before.] 



