CHEMICAL CONTROL OF DIPPING TANKS. 141 



arsenite of soda. When the contents of the tank were tested the 

 amount of arsenite (arsenious oxide) was only 0'062 per cent., 

 but the total arsenic (calculated as arsenious oxide) was 0"300 

 per cent. The fluid in this particular tank was exceptionally 

 clean and free from organic matter, as only seven head of cattle 

 were put through it every 'week, so it is likely that it was not a 

 suitable medium to enable the reducing organisms to flourish. 



It need hardly be pointed out that the continued general use 

 in dipping tanks of arsenical fluids containing also an appreciable 

 proportion of a coal tar disinfectant is somewhat of a menace to 

 the stock industry, and will always be a source of anxiety to the 

 conscientious farmers and to the officials of the Veterinary Divi- 

 sion, who have to see that the regulations governing the dipping 

 of stock are properly carried out. This seemingly rather sweeping 

 statement is justified by the following facts: — 



In the first place it is very difficult to keep the tank using such 

 dips up to proper strength, which is essential for the effective 

 control of the ticks. The constant addition of raw dip that is 

 necessary to make up for this continuous oxidation also makes 

 it a very expensive dip to use. This is especially the case when, 

 owing to extensive oxidation and consequent uncertainty as 

 regards the insecticidal value of the fluid, the farmer has to empty 

 his tank oftener than he otherwise would have to. 



Secondly, it may be pointed out that farmers and veterinary 

 officials, as a rule, test the strength of a dip in a tank by means 

 of the so-called " field-testers," which only give the amount of 

 unoxidised arsenic in the tank. Now it is usually reckoned that 

 the oxidised arsenic has almost half the insecticidal effect on the 

 tick and also toxicological and excoriating effect on the beast that 

 the unoxidised arsenic has. It therefore happens very often that 

 the " tester " may show a dip to be apparently very much below 

 strength, whereas the total amount of arsenic present may be 

 even more than what is safe for regular dipping. Consequently, 

 the making up of the strength of such a dip, according to the 

 tables accompanying the instructions for the use of the " tester," 

 to the standard strength of unoxidised arsenic, might conceivably 

 have disastrous results. To take the example of the tank in the 

 Pieterinaritzburg district previously mentioned, in which the 

 amount of unoxidised arsenic was 0"062 per cent., and of the 

 oxidised arsenic, 0'238 per cent. If this had been made up to 

 standard strength (0'16 per cent.) in unoxidised arsenic as a result 

 of the field test, it would be found (taking the strength of the 

 oxidised as half that of the unoxidised arsenic) that the equivalent 

 amount of arsenic in that tank now would really be about 0-28 

 per cent., which would certainly be too strong for the five or 

 seven-day dipping of stock, especially calves. 



