(1. W. Robinson :119 



Table II. Comparison of Results hi/ Old and New Methods. 



Silt 



The agreement between the two series of results is generally close 

 and not unsatisfactory when it is considered that mechanical analysis 

 by the older method is hable to considerable errors. On the average, 

 putting the figures obtained by the old method as 100, the new method 

 gives 102-7 for the clay, 100 for the fine silt and 99-6 for the silt. It 

 would thus appear that there is no appreciable constant error in com- 

 paring results by the two methods. In view of the large number of 

 manipidations recj^uired in the old method, there are more occasions for 

 error than in the new , and any serious disagreement is at least as likely 

 to be due to errors in the former as in the latter method. 



It will be noticed that the most serious disagreements are in the case 

 of light soils. To secure a perfect comparison it would be necessary to 

 secure that the prehminary dispersion is exactly the same in both 

 methods. In the standard method the material is repeatedly triturated 

 after each pouring off. The method provisionally adopted for dispersion 

 in the new method, namely a 2 to 4 hour shaking in an end over end 

 shaker would appear to give a comparable degree of dispersion. The 

 longer period is apparently necessary in the case of soils rich in organic 

 matter. Dispersion by sodium carbonate^ gave good results with raw- 

 clays but was unsatisfactory with soils containing much organic matter. 



1 Cf. .Joseph and Martin, ./. Agnc. Sei. 1921, 11, 29,3-303. 



