160 THE SALMON RIVERS OF SCOTLAND 



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 operating for two years. In this, 

 in the winter of 1907-1908, a series of four double tanks were 

 placed. Three of these were for testing the effluent, the fourth 

 being a control experiment in pure water. Through the kind- 

 ness of the Duke of Richmond and Gordon we obtained sal- 

 mon ova and salmon fry of two ages. Each pair of tanks was 

 provided with, in one case, a glass grill of ova, and in the other 

 a number of fry. One pair of tanks contained the undiluted 

 effluent as it came from the filter, another the effluent of 40 

 per cent, strength, another the effluent reduced to 4 per cent. 

 The first and second were regarded as quite exceptionally severe 



