382 Allen and Crenshaw — Stokes Method for the 



Then 72,4*89 -^" R, 8 = 0-423 and ?* 2 5'03 -^R 2 3 = 0*468. 

 o 3 



«,4-89^R 1 ' 



3 __ -423 , n^irR^ _ 2-832 _ 49*0 



n,5-03— R t " _ ^ " W » 4,rR "' " ^^ ~ "* 



o 



So while the pyrite in a 50 per cent mixture by weight 

 would possess only about 49 per cent of the surface; after the 

 oxidation by ferric sulphate, it would possess about 51 per 

 cent. The average variation of the surface percentage from 

 the weight percentage in the course of the reaction is there- 

 fore slight. By the same mode of reasoning we find that in 

 a mixture of the sulphides containing 90 per cent by weight 

 of marcasite, the surface of the latter would be 90*2~ per cent 

 before, and 89*7 per cent after the oxidation. Here also the 

 average variation of the surface per cent from the weight per 

 cent would be small. In a mixture containing 90 per cent by 

 weight of pyrite, the pyrite surface would vary from 89*8 per 

 cent to 91 per cent. Here the average variation of the sur- 

 face from the weight per cent is also comparatively small, viz. 

 0*4 per cent. 



The assumptions regarding the size, shape and uniformity 

 of the particles are of course only approximations ; still no 

 other error has been found which could compensate any con- 

 siderable error here and the conclusions deduced from the 

 assumptions are in accord with the experimental results. 



The plan naturally suggests itself, of taking the density of 

 the two minerals into account in making the mixtures, and 

 then increasing the quantity of the sulphide to be oxidized, so 

 that the ratio of the surfaces of the two at the beginning 

 should remain sensibly constant throughout the process of oxi- 

 dation. Small quantities, however, are far more readily 

 purified and otherwise handled, and it should also be noted 

 respecting the application of the method to synthetic products 

 that the latter are troublesome to prepare in quantity. 



Although a careful study of other sources of error was 

 made before the disturbance caused by flocculation was entirely 

 understood, none of them proved of any great importance. 

 It may be wise, however, to discuss them as briefly as 

 possible. 



(b) Loss of water during the oxidation process. — Stokes 

 asserts that if the reaction flask is provided with a reflux con- 

 denser, and if the carbon dioxide which passes through the 

 liquid during the process is first bubbled through water, there 

 is no appreciable change in the concentration when a blank 

 test is made with the standard solution. We can confirm this 

 assertion as far as the water is concerned, but find a slight pre- 



