364 



H. A. Baker — Loose Arenaceous Sedionents. 



The assumption is made that the surface-areas of two powders, 

 derived from a unit volume, are in inverse ratio to the average 

 diameter of their grains, and the reciprocal of the average diameter 

 is taken as the factor. The assumption is now known to be 

 erroneous, but as the factor affords a convenient approximation it 

 is often used. This factor was proposed by Jackson, and amended 

 by Purdy, and goes under the name of the " Purdy fineness 

 factor". It was first propounded to meet the requirements of the 

 ceramic industry, but it has been admitted that since the difference 

 between the true surface-factor and that calculated from the average 

 diameter increases as the diameter decreases, more confidence may 

 be placed in the Purdy factor as calculated for sediments coarser 

 than the fine material used in the ceramic industry. The method 

 of obtaining the Purdy fineness factor will be clear from the 

 following : — 



Suppose the sample to have been separated by elutriation into 

 four grades, thus : — 



Then the Purdy fineness factor = total surface factor = 



X 



A\ 



a + h 100/ ^ \& -f c 100/ 



B \ 



d ^ 100/ + U +^e ^ 100> 



Tlie total surface-factor is obtained by multiplying the surface- 

 factor of each grade by the percentage-weight of that grade, and 

 finding the sum of the products thus obtained. 



Before proceeding to test the utility of the Purdy fineness factor 

 in relation to arenaceous sediments, it is legitimate first to make 

 the criticism that the true average diameter of the grains in any 

 separated grade is likely to be seriously different f lom the half-sum 

 of the limiting sizes. 



Bearing in mind that the Purdy fineness factor should show 

 larger numbers for finer sediments, the above formula was appfied 

 to the results of a large number of mechanical analyses carried out 

 on arenaceous sediments by the writer. The result was to show the 

 complete failure of this factor to serve any useful purpose as a means 

 of comparing arenaceous sediments from the point of view of fineness 



