36 



KEPORT OF CHEMICAL LAHOKATORY 



No attempt was made to carry the agitation in the horizontal shaker beyond six hours, 

 since it was considered that this length of shaking is as long as can be conveniently carried 

 out, a longer one entailing either two or more periods of shaking, or its continuance during 

 the absence of the analyst, which always introduces an undesirable uncertainty into the 

 determination. 



Soils of arid regions usually contain notable proportions of calcium and magnesium 

 compounds, especially calcium carbonate and sulphate. It had already been noted by 

 Briggs, Martin and Pearce' that ammonia, when applied to the treatment of such soils, 

 appears rather to flocculate the clay than to break up the flocculations, and its addition in 

 such cases is therefore not desirable. They further note that certain soils (containing an 

 excess of magnesium and calcium carbonate) and which presumably were shaken with 

 water only, were badly flocculated at the beginning of the mechanical analysis ; but 

 that after two or three decantations had been made, the tendency to flocculate disappeared 

 and the final separation was made without difficulty. These soils were found to contain 

 as much as 25 and 42 per cent, respectively, of carbonate, calculated as calcium carbonate. 



Although Sudan soils rarely, if ever, contain as much as ten per cent, of carbonates, 

 calcium sulphate is frequently present, more especially in the subsoils, and the two 

 Dertocculation together cause these -soils to flocculate persistently in spite of the absence of ammonia. 

 We have found, however, that trouble of this kind usually disappears on the addition 

 of sodium carbonate. As we have abandoned the needlessly laborious and no more 

 accurate method of determining the clay by direct weighing, the presence of sodium 

 carbonate introduces no difficulty. Deflocculation is so effectually aided by the sodium 

 carbonate that only moderate agitation in the mechanical shaker is i-equired, and either of 

 the two forms of machine mentioned above may be applied to the purpose, the one figured 

 on page 35 being, however, preferable by reason of its far greater convenience in use. 



The use of sodium carbonate in the analysis of these soils is, it appears to us, all the 

 more indicated in view of the fact that it may be formed in the soil as the result of 

 inefficient drainage. The indications furnished by its use in the laboratory are of direct 

 practical bearing aside from the fact that it appears to be the only agent which will enable 

 us to obtain an idea of the true mechanical constitution of the soil, without the use 

 of acids which, for the reason already given, are wholly inadmissible. 



A number of experiments was made in order to determine the effect of varying 

 amounts of sodium carbonate. From fifty to one hundred milligrammes to 100 c.c. of 

 water appears to be a suitable proportion. If the amount of sodium carbonate be 

 materially increased, the reverse effect of floceulation results. The yield of clay with the 

 higher proportion of carbonate was nevertheless, in the few experiments made, slightly 

 lower ; which is satisfactory in that it goes to show that there is no danger of solvent 

 action on the fine siliceous silt in the soil. The results were as follows : — 



the presence 

 of sodium 

 carbonate 



U.S. Bureau of Soils, Bulletin 24, p. 24 



