58 Ml'. II. Medlock on the Reciprocal Action of Metals 



there is frequently a black residue which is extremely difficult to 

 burn off, even by very lengthened ignition. 



This difficultly combustible carbonaceous matter appears to 

 occur only in waters which are mixed with the sewage of towns, 

 since in s]iring-waters I have never observed it. When the eva- 

 porated residue of urine is burned^ it leaves with the mineral 

 constituents a quantity of black, lustrous carbon, which requires 

 an intense heat, long continued, before it is entirely consumed. 

 As the carbonaceous matter left with the ignited residue of 

 Thames water bears a remarkable resemblance to that of urine, 

 it is probably in this common admixture with Thames water we 

 must seek the origin of the almost incombustible carbon which 

 remains with the ignited residue. 



The residue of Thames water, when dissolved in a dilute acid, 

 evolves a large quantity of sulphuretted hydrogen. The organic 

 matter is commonly sujiposed to I'educe the sulphates to sul- 

 phides, and these of course evolve sulphuretted hydrogen when 

 acted upon by a dilute acid. Having frequently noticed the 

 large quantity of sulphuretted hydrogen evolved from the solid 

 residue of Thames water, I tested the residues of six separate 

 gallons of water by placing them in Florence-flasks^ and adding 

 to each 10 cubic centims. of water, and then, through funnel- 

 tubes, dilute hydrochloric acid. The slips of ])aper moistened 

 with acetate of lead previously ])ut into the necks of the flasks, 

 became instantly black, and the quantity of sulphuretted hydrogen 

 evolved was sufficient to taint the entire atmos))here of the labo- 

 ratory. The residues of three other gallons of water which had 

 been in contact with iron wire for twenty-four hours, when simi- 

 larly treated, gave no indication of sulphuretted hydrogen. 



The white cloud produced in Thames water, after it has been 

 in contact with iron for from five minutes to an hour (according 

 to the temperature of the day), bears a remarkable resemblance 

 to finely-divided sulphur ; but the quantity even from ten gallons 

 of water, when tested, was too small to permit me to pronounce 

 it to be suljihur with positive certainty, although a portion of it, 

 w'hen burned, evolved an odour resembling that of sulphurous 

 acid, and another portion, when boiled with strong alcoholic 

 potassa, and tested with lead, gave a dark-brown coloration. 



With the view of determining whether sulphides are usually 

 present in Thames water, I evaporated six separate gallons, 

 covering each dish over with white blotting-paper, moistened 

 here and there with a drop of dilute solution of acetate of lead. 

 As soon as the water became warm, every lead spot was slightly 

 discoloured from the evolution of sulphuretted hydrogen or a 

 volatile sulphuret. Three other gallons of water were now placed 

 in contact with iron wire (two ounces of iron wire in each gallon). 



