140 CHKMICAL COXTROL OF DIPPING TANKS. 



It will be noticed in most cases that the amount of oxidation 

 in the tanks containing a disinfectant in addition to the arsenical 

 substance is strikingly larger than in the tanks, containing plain 

 arsenite of soda, although in a few instances the amount of oxida- 

 tion in the former group is unusually low. But these low results 

 can be explained by either of the following reasons: — 



1. The tank had only been filled with fresh dip a few weeks 

 previously, as is the case with tanks E and J. Under such cir- 

 cumstances bacterial activity would not have had sufficient time 

 to progress very much. 



2. The addition of a large ainount of fresh raw dip to make 

 up the strength of the liquid already in the tank, and also a pro- 

 portionate amount to the fresh water added to make up the 

 volume in the tank. This explains the sudden drop in the 

 percentage of oxidation in tanks A, B and F. 



3. It will be noticed that in the case of tanks C, D, Iv and L, 

 the total amount of arsenic in each tank is consistently low 

 (approximately 0"05 per cent, in each case), and it would appear 

 that under such conditions the activity of the oxidising organisms 

 is approached by that of the reducing ox'ganisms. This might be 

 expected, for under these conditions the amount of disinfectant 

 jjresent would also be very small, especially where no addition of 

 fresh dip has been made to a tank for a considerable length of 

 time. 



Leavixig out the abr)ve-mentioned anomalous cases, then, we 

 obtain the following striking results: — 



Average percentage of oxidation in 

 the private tanks using the proprie- 

 tary dip containing coal tar deriva- 

 tives 41"6 per cent. 



Average percentage of oxidation in 

 the private tanks using plain 

 arsenite of soda 23 per cent. 



This marked difference in the average amount of oxidation 

 in these two groups of tanks can only be explained by the presence 

 of the coal tar derivatives in the proprietary dip. The amount 

 present is not enough to keep the fluid in the tank sterile, and 

 the conjecture naade in my last paper concerning the probable 

 explanation of the continuous oxidation during the previous years 

 in the tanks at Cedara seems to be justified, namely, that the 

 reducing organisms are rendered more or less inactive in a par- 

 tially sterilised medium and that the oxidising organisms are 

 comparatively unaffected. 



It was also mentioned in the same paper that recent investi- 

 gation has shown that the reducing organisms require the presence 

 of a certain amount of fresh urine in the arsenical medium for 

 their growth and well-being. This probably explains the appar- 

 ently anomalous phenomenon met with in the case of a tank in 

 the Pietermaritzburg district. This tank had only been in use for 

 six or seven months, the arsenical material used being plain 



