R. N. Maxson — Small Amounts of Gold. 467 



With N/1000 solutions and with the application of this correc- 

 tion, ten experiments gave a mean error of less than 4/1000 rag. 

 between extremes of +2/100 mg. and — 29/1000 mg.; while 

 in another series of experiments in which the start was 

 made with pure gold foil, dissolved in chlorine water and 

 treated by a process carefully described, the average error of 

 fourteen determinations was +2/1000 mg. between extremes of 

 + 1/100 mg. and —8/1000 mg. This is not theory : it is experi- 

 ment. Here are volumetric solutions capable of showing 

 1/100 mg., or less, of gold. Is it any more absurd to deter- 

 mine gold in this manner than to weigh that element upon 

 an assay balance sensitive to 1/100 of a milligram ?. To be 

 consistent, Rupp should include all processes, gravimetric as 

 well as volumetric, in his sweeping declaration. 



The fact that large percentage errors occur in certain deter- 

 minations made by Grooch and Morley, upon small amounts of 

 gold, is again made the subject of criticism by Rupp. 



As I have previously stated, every analytical process has a 

 certain inevitable error, and because of this fact as we approach 

 the limit of accuracy of the process the percentage error will 

 increase rapidly. This increase can, however, under no cir- 

 cumstances be considered a reason why small quantities of sub- 

 stances should be disregarded in analysis, either volumetric or 

 gravimetric. Because the ordinary analytical balance is sensi- 

 tive only to 1/10 or 1/20 mg. is no reason for declining to 

 weigh tenths or twentieths of a milligram, although the per- 

 centage error in weighing such amounts may be very large. 



Besides these wholly unreasonable criticisms, reference is 

 made by Rupp to some matters of scientific fact, and to these 

 I next propose to give attention. 



To Rupp's statement that with dilute solutions the starch 

 indicator is no longer reliable, it is sufficient to reply that experi- 

 ence has shown that one small drop of -N/100 iodine solution 

 developes the starch color at once, in solutions of the volume 

 and concentration described ; and if N/1000 solutions be used, 

 a perfectly definite, and not excessive allowance of 0*l cc , equiva- 

 lent only to 1/100 mg. of gold, is all that is needed. 



Further, Bupp claims that the decomposition of aurous 

 iodide is the greatest error of the process of Gooch and Morley, 

 and refers to the handbooks of chemistry to substantiate this 

 opinion. No handbooks are known to me which discuss the 

 deportment of aurous iodide under exactly the conditions of 

 this analytical process, but a study of the reaction, of which 

 I made mention in a former paper,* showed that aurous iodide 

 possesses stability sufficient for the purposes of the analytical 

 process. 



* Loc. cit. 



