Selvage Precipitation. 59 



dyeworks, a bleach and sizing works, a paper mill, and a 

 rubber factory will demonstrate the mode of procedure by 

 which they can each be clarified. A drop or two of the iron 

 salt is added to each. There is no apparent effect on the dye- 

 water, which remains like ink. The bleach and paper 

 works' effluents are coagulated, but no effective precipitation 

 takes place. The rubber works' effluent is very satisfactorily 

 clarified. We now mix a portion of the three non-pre- 

 cipitable effluents with varying proportions of sewage, 

 taking preferably the very filthy one from Clayton. When 

 one or two drops of iron salt are now added there is an 

 immediate coagulation and separation of solid matters, and 

 we get a very complete subsidence of the mixture carrying 

 down the former obstinate impurities. In the case of the 

 dye-water, filtration through a proper filter at once shows 

 the " ink " converted into clear water. 



This property of many kinds of sewage to coagulate 

 under the action of the iron salt, and carry down even a 

 soluble impurity, is strikingly shown by the addition of a 

 quantity of sewage to a very strong solution of aniline blue, 

 which is, of course, quite unaltered by subsidence or filtra- 

 tion, until, by the addition of two or three drops to the 

 mixture, a clarification is at once effected, and, after subsi- 

 dence, the former densely blue liquid becomes like town's 

 water, the nitrogenous matter having, as a matter of fact, 

 abstracted the dye just as wool exhausts a dye beck. 



The foregoing experimental illustrations will, I think, 

 tend to show the great part which properly chosen iron 

 salts will play in clarifying and disinfecting our sewers and 

 rivers, and they certainly show that a somewhat intimate 

 study of the chemical condition of the sewage and effluents 

 is necessary at the time the purifying agents are added, if 

 anything like uniform success is to be attained. 



