CHEMICAL CONTROL OF DIPPING TANKS. 



149 



ditions obtaining in a dipping tank. Blank estimations by the 

 iodine method were made on this solution at monthly intervals, 

 and the results calculated to the equivalent amounts of arsenious 

 oxide in each case, the same procedure being followed exactly as 

 with dip fluid. 



It will be noticed that the addition of a large excess of con- 

 centrated acid when estimating the total arsenic gives a smaller 

 blank figure than in the estimation of the arsenite. 



In a five-day dip fluid the proportion of arsenious oxide is 

 016 per cent., so the error due to absorption of the iodine by 

 extraneous matter is reasonably low. It may be also pointed out 

 that by the use of a fairly close filter paper the amount of iodine 

 absorbed in the blank estimations is slightly less than when the 

 ordinary quick paper is employed. 



To Test Effect of Excretory Matter in Dip Fluids. 



It has been stated by more than one authority on this subject 

 that the frequent addition of fresh excretory matter to the arseni- 

 cal dip fluid will not only retard the oxidising action that usually 

 goes on in such fluids, but will even cause the speedy reduction of 

 the oxidised arsenic (as arsenate) back to the unoxidised form (as 

 arsenite). In order to test this contention under laboratory con- 

 ditions the following series of investigations were carried out, the 

 solutions in each case being kept in stoppered Winchester quart 

 bottles. 



1. A solution of 4 grams of sodium arsenite in 2 litres of dis- 

 tilled water was made up and analysed immediately after. Then 

 10 grams of fresh dung were added to the bottle, and the contents 

 were analysed at fortnightly intervals. The results were as 

 follows: — 



First 

 Analysis 



After 

 2 weeks 



After 

 4 weeks 



After 

 C weeks 



After 

 8 weeks 



After 

 10 weeks 



a. Arsenite (as As 2 3 ) 



b. Total Arsenic (as As 2 3 ) ... 



per cent. 

 0151 

 0155 



per cent.|per cent. 

 0-137 0-097 

 0-154 0-154 



per cent. 

 0-037 

 0-152 



per cent. 

 0023 

 0-152 



per cent. 

 0020 

 0154 



