92 Proceedings of the Royal Physical Society. 



are still comparatively pure, while others again are certainly 

 foul enough. The Eiver Esk, which, as every one knows, has 

 been the source of much contention, occupies a sort of mid- 

 way position. By the time it passes Lasswacle, it is not 

 exactly so pure as we like to see a river, but it is certainly 

 nothing like the Water of Leith, as it is seen passing 

 Canonmills. The chief manufacture existing on the banks of 

 the Esk is paper-making, and when one remembers that 

 there are on this river no less than eight large paper mills, 

 the marvel is that the water is not more filthy than it is. A 

 great deal has been done of late years with the view of im- 

 proving the condition of the Esk, much of which has been 

 very successful. It is wished, however, to introduce if 

 possible, still less foreign matter into the river ; and with the 

 object of rendering the effluent water from the mills purer 

 than it is at present, I have lately been making some experi- 

 ments on the large scale with different processes ; and as many 

 of our trials were fairly successful, I thought it might be 

 interesting to the members of the Society to have an oppor- 

 tunity of witnessing these processes in operation. 



In No. 1 Process, iron is the active ingredient, Mr Mackay 

 of Leith, who has proposed this process for the purification of 

 effluent water from paper mills, uses the iron in the state of 

 per-chloride, but for a reason which will be obvious when 

 you see the process performed, I prefer to use the iron as 

 per-sulphate. This process, as all these processes should be, 

 is very simple, and consists in adding to the dirty water, 1st, 

 a small quantity of an acid persalt of iron, and 2d, a 

 sufficient quantity of lime to render the mixture strongly 

 alkaline. In a few minutes all suspended matter settles to 

 the bottom leaving the water as clear as if it had been filtered. 

 By using ferric- chloride, a large quantity of calcium chloride 

 is produced, which being readily soluble in water, is carried 

 into the river, whereas by using ferric-sulphate, as I recom- 

 mend, calcium sulphate is formed and this being a difficultly 

 soluble compound, it is in great part precipitated, and so kept 

 out of the river. The sludge or precipitate which is thus 

 produced, I should mention, has been worked up with great 

 success in the manufacture of coarse paper. 



