152 THE SALMON RIVERS OF SCOTLAND 



than any other, is responsible for the perfecting or adapting of 

 bacterial nitration in the treatment of pot ale, Dr. Cowie of Mortlach 

 Distillery having first carried out a number of valuable experiments 

 as to this particular method. The process is quite simple and quite 

 scientific, being in essence Nature's method carried out in a conveni- 

 ently concentrated manner. After a certain amount of precipitation 

 by lime, the pot ale is diluted and sprinkled by an automatic arrange- 

 ment on the surface of a high percolating filter. The bacteria, 

 which soon multiply enormously in the filter, devour the noxious 

 properties in a few minutes, just as in a garden bed the bacteria 

 consume and convert into ammonia and nitrates the manure bene- 

 ficial to the surrounding plants, and which is in this form trans- 

 mitted to the plants. The resulting effluent gives an excellent 

 analysis, and may be run into any stream quite safely. The degree 

 of filtration can be regulated and refined to a very considerable 

 extent. The effluent, for ordinary practical purposes, is very slightly 

 cloudy, but no longer resembles pot ale ; it has no smell, does not 

 putrefy, and is well supplied with dissolved oxygen. A feature of 

 the effluent, which has caused a certain amount of apprehension to 

 one or two eminent chemists who have examined it critically, is that 

 it certainly contains a considerable amount of nitrogenous matter. 

 Under ordinary circumstances this is naturally regarded as impurity 

 of a harmful kind. It has to be specially noticed, however, that the 

 nitrogenous matter in the effluent referred to is reduced to a stable 

 or unchangeable condition, so that when carried off from the filter it 

 does not cause pollution ; it is carried off in the fluid in its stable 

 condition. Had the effluent been harmful in any way, it could not 

 have stood the tests to which it has been subjected, and to which I 

 shall presently refer. 



I cannot but regard this purification of pot ale as a notable 

 achievement of the greatest importance to those interested in the 

 welfare of our salmon rivers, as well as to many others. Moreover, 

 the method, as compared with other plans which have been tried, is 

 comparatively inexpensive. Bacterial treatment is the form which 

 future purification works must adopt to obtain satisfactory results 

 from all impurities of an organic nature. 



In its application to salmon fisheries, I may mention that I was 

 asked by Lord Iddesleigh's Commission to supervise some experiments 

 designed to test practically the effect of the purified effluent upon fish 

 life. For this purpose a hatchery and tank-house was erected at 

 Coleburn Distillery, where the latest pattern of filter had been 



