320 Mechanical Aualiisia of Soils and other Dispersions 



Conclusion. 



The method of mechanical analysis above described has two recom- 

 mendations. It is more expeditious and economical than the standard 

 method. Granted that the new method gives reliable results, its adoption 

 would remove one of the (gravest objections against the jiresent method, 

 namely its laboriousness. With the new method it has been found 

 possible in the writer's laboratory to carry out six mechanical analyses 

 in a day. With pro])er organisation there should be no difficulty in 

 carrying out 35-40 analyses in a week. Anyone familiar witli the routine 

 of the older methods will realise that it would be impossible to deal witii 

 such numbers single handed. There is the further consideration of 

 economy in the use of beakers, filters, etc., to say nothing of the dis- 

 tilled water, ammonia and hydrochloric acid required. On the other 

 hand the suggested method requires careful manipulation. Working 

 with 2 per cent, suspensions, an error of 1 mgm. in weighing corresponds 

 to -25 per cent, in the result obtained. It is doubtful if the method would 

 commend itself for teaching purposes. 



The new method mav be used within limits for obtaining continuous 

 curves such as are obtained by the Oden and Wiegner methods. Pro- 

 vided temperature conditions are controlled it is easily possible to carry 

 the distribution curve for a soil or clay considerably beyond the limits 

 of clay as defined by the standard method, and the constitution of the 

 finer fractions can thus be investigated over ranges hitlierto scarcely 

 explored^. An application may also be found in the investigation of 

 changes in degree of dispersion consequent on manuring, cultivation 

 and season. 



With regard to the errors of the method it may be remembered that 

 the experiments recorded were carried out in ordinary measuring 

 cylinders. These are rarely uniform in cross section. With perfectly 

 cylindrical columns, no doubt, better results would be obtained. With 

 regard to temperature effects, no attempt was made to secure rigid 

 control of temperature conditions. As in the ordinary method, the 

 settling took place in an ordinary laboratory \vith its unavoidable 

 vicissitudes of temperature. Care was however taken to avoid the 

 proximity of sources of heat. In certain cases where the settling columns 

 were in the vicinity of radiating surfaces, unsatisfactory results were 

 obtained. Generally speaking for the longer periods the columns were 



' A considerable number of clays have, in fact, been followed by this method as far 

 as log 0= 7, which appears to be near the lower limit. 



