•2in 



REPOnr OF CHEMICAL LAnOnATORY 



Colour ol 

 ■■ Blue" anil 

 "White" 

 Nile waters 



Cause of 

 difference in 

 colour 



While practiciilly ilt'iir tlnring tlu' months iiu'iitioiu'd above, the Blue Nile water was 

 never found iilisolntclv frei- from suspi'iuli-ii mutter. The lii|^liest proportion found was on 

 August I'Jtli, l,l(i(i parts per million, and the lowest t/u January Tith, 4 parts per million. 

 When nearly clear, the suspended matter was not elay, as in the ease of the White Nile, but 

 coarser partieles, probably sand and other niiitrriiil Miiwii into the river iiy the hij^h winds 

 whicli prevail during the winter and spring months. 



rite colour of llie " /i/ite" ami tlie " W lii/e" Xili- ini/er. — It has been mentioned that 

 the White Nile water, while it carries a comparatively small amount of suspended matter, is 

 never clear. The material in suspension is in great part very fine clay, and, especially after 

 the larger partieles have settled, the water when examined in a glass vessel has a slight 

 milky-white colour or opalescence. This clay remains in suspension a very long time : 

 samples which remained undisturbed in the laboratory for weeks were still distinctly opalescent. 

 The Blue Nile water, on the cmitrary, especially in the season of lower water (from 

 January till June) is comparatively — for the greater portion of the time, almost (piite — clear, 

 and like other clear water containing little organic matter, has a blue colour when viewed in 

 deep layer. 



The reason for the different behaviour of tlic two waters becomes apparent by reference 

 to their composition. The White Nile water contains but a very small amount of calcium 

 and magnesium compounds, and a comparatively large amount of sodium and potassium 

 carbonates and bi-carbonates. Blue Nile water contains on the one hand, more calcium and 

 magnesium compounds, and on the other much less of the alkali carbonates. 



In order to appreciate the bearing of tlie above upon the condition of the water, it will 

 be well to rehearse some of the facts which have been determined as to the suspension of 

 clay in water, and its precipitation by various agents. 



Ordinary clay contains a certain proportion of wliut lias been called true or " colloid " 

 clay. When clay is mixed witli pure water, this jelly-like colloid clay remains permanently 

 suspended in it, no matter how long the mixture is allowed to .stand at rest. Certain clas.ses 

 of bodies have been found to cause the precipitation of this clay, while others have a 

 tendency to favour its ditfusion. Among the bodies which cause coagulation and 

 precipitation from suspension, are acids, calcium and magnesium compounds, and certain 

 salts — especially common salt. Alkaline substances, on the other hand, favour diffusion 

 of the clay in water. Ammonia is especially active in this respect, but sodium carbonate is 

 also very effective. Schloesing has pointed out that the clearness or turliidity of river water 

 depends essentially on the proportion of calcium and magnesium compounds present. 

 According to him, for a stream to be able to clear itself of the clay brought into it, the water 

 must contain 7U to SO jjarts of lime and magnesia per million. 



Both Blue and White Nile waters contain a higher proportion of soluble matter during 

 the .season of lower water, especially from March till June. This is due not so much to the 

 evaporation and concentration of the water as to the fact that at this time the river derives 

 its supply in part from the water which has passed into the soil during the period of flood. 

 At this latter season, both the waters contain leas caleium and magnesium compounds, and in 

 addition, the White Nile contains a comparatively high proportion of sodium an<l potassium 

 bicarbonates and carbonates. Neither river is therefore able to free itself from the clay 

 brought into it, and their svaters remain turbid, even though allowed to stainl at rest for a 

 long time. When, later, the iiiHux of subsoil water takes i)lace, the proportion of lime and 

 magnesia increases very appreciably. Thus in June the sum of the lime and magnesia in 



