•il-J 



UErOnT OF CHEMICAL I.AIIORATORY 



Cause of 

 absence of 

 sul|>ha(es 



Precipitation 

 of clay by 

 organic matter 



Till- cxiiliinaliiiM (if tlic ilisaiiinaraiicc uf siil|>liiitcs in tlic swamiiy ri'i^'idiis is, no iloulit, 

 till- siinie us that wliii-li ai-<iiUMts for tlic formation of sodium I'arlioiiatc from soiliiim sulplmtu 

 in tlie alkali lakes which arc coniiiioii to I'l^^yjit ami similar Imt, dry countiics. In the 

 presence of a larj^e excess of organic matter, uiul the consetiuent limited supply of oxygen, 

 sulphates are readily reduced to sulphides umlcr tlie influence of micro-organisms. These 

 sulphiilcs are converted into carlionatcs hy the carbonic acid formed at the same time, and 

 the hydrogen sulphiile is set free. Otlur decompositions also take place, and inflaniniaMc 

 gases in considerable amount are constantly given otf in these swampy districts. 



A sample of the mud taken from the bed of l?ahr-El-Zcraf by a dredger, was recently 

 submitted by Mr. Dupuis, Inspector-(Teneral of Irrigation, for examination. When freshly 

 brought to the surface the mud had a greenish-black colour, due to the presence of iron 

 sulphide, formed as a result of the reduction of the sulphates. On exposure to the air 

 oxidation (>f the iron sulphide takes place, and the mud becomes reddish -yellow. 



The Sobat Hiver water is also .seen to be free from sulphates. This is to be expected, 

 as the river derives its water from extensive swamps in wliiili all its tributaries meet. 



Since the above was written, the following analyses of waters from the White Nile, in 

 the Sudd region, have been made. The point furthermost south from which it was possible to 

 take samples was at Bor. This is in the beginning of the marsliv regions, and it will be seen 

 that the sulphates have already been reduced to a very low proportion — about one part per 

 million. A little further on, and the last trace of sulphates is found to disappear. The 

 amount of " oxygen absorlied," which is a nieasure of the organic matter present, is seen to 

 increase steadily during the river's passage througli the Sudd, up to the point at which the 

 Sobat River joins the Nile, where there is a sudden marked fall. The effect of the decom- 

 position of the vegetable matter and the solution of its mineral constituent is also traced in 

 the proportions of potassium and sodium, whi(di are subject to a steady rise, and a fall at 

 the point mentioned above. It will be noticed that these waters, when they had arrived at 

 Khartoum, with the exception nf the first, were all able to clear themselves; a certain 

 amount of sediment, largely organic, was present, but tlie water itself was clear. The organic 

 matter present had eviilcntly acte<l as a precipitant to the clay, just as do acids, calcium and 

 magnesium salts, and those already mentioned. After the river passes a point beyond Kodok 

 the large amount of alkali carbonates, resulting from the decompositions in the Sudd region, 

 cause clay to be taken up from the soil in the river bed. The river again iieeonies turl>id, 

 and remains so for the rest ofitscour.se, except at the time of lowest water and after its 

 junction with the Blue Nile, by which the proportion of alkali carbonates are rc(iuced, by 

 dilution, and the relative proportion of calcium and magnesium increa.sed at the same time. 



