6 BULLETIN 259, U. S. DEPARTMENT OF AGRICULTURE. 



send samples, many of this set of experiments were discontinued 

 at just about the time when the second stage probably had been 

 reached, but the phenomenon will be better illustrated in the series 

 of experiments in 1914. 



In explaining the occurrence of the second stage in the history of 

 the vats which show it, it is not enough to say that the considerable 

 degree of usage to which the vats were subjected and the consequent 

 copious renewal with fresh bath allowed no time for the arsenic to 

 become largely oxidized before it had been carried out by the cattle, 

 for in several cases it is very plain that the lines tend to converge 

 under a figure of use that is no greater but even decidedly less than 

 the figure of use under which they had previously diverged, a fact 

 explicable only by the existence in the former case of some agent 

 which positively prevents oxidation. During the examination of 

 samples from vat A, it happened several times that the field test made 

 at the vat by the field agent of the bureau would be reported as 

 showing a figure for actual arsenious oxid decidedly lower than the 

 figure obtained through laboratory analysis, while in some such cases 

 laboratory analysis showed no difference at all between the results 

 for actual and total arsenious oxid; that is, the bath appeared to 

 be entirely unoxidized. While the laboratory was puzzling over 

 this annoying phenomenon and contemplating active steps to inves- 

 tigate the cause, the previously noted article by Laws came to its 

 attention. If Laws's conjecture of the action of reducing bacteria is 

 correct, then it was possible that in the case of these doubtful sam- 

 ples the analyses of the field agent and the laboratory were in fact 

 both correct; that is, the composition of the sample had changed en 

 route. Therefore, laboratory experiments were undertaken to deter- 

 mine the truth of Laws's conjecture, which was not supported by any 

 direct experimental evidence. At the same time it seemed desirable 

 also to run some experiments on oxidation. 



LABORATORY EXPERIMENTS. 



The plan of investigation was simple. Strong stock solutions of 

 sodium arsenite and sodium arsenate were prepared by boiling the 

 corresponding oxids of arsenic with sodium carbonate in amounts 

 proportionate to the regular bath formula. Nutrient broth was 

 prepared according to the formula, 5 grams solid meat extract, 10 

 grams Witte's peptone, and 2h grams sodium chlorid per liter, but 

 sterilization appeared unnecessary for the work in view. For the 

 tests the stock arsenic solutions were diluted, mixed with broth 

 (about 30 per cent of the final volume), antiseptic added if desired, 

 and the mixture inoculated with a portion of used dipping-bath 

 from the field. No tar was added. The freshlv made mixtures 



