ASSAY OF ARSENICAL DIPPING FLUIDS. 7 
usually fades out in a minute. In the case of very dirty baths the 
color may appear simply as a violet, reddish, or brownish deepening 
of the naturally dark color of the bath itself. In such cases it is 
well to have two flasks at hand, both containing the measured por- 
tions of bath and starch solution. Then by running iodin solution 
into one of the flasks and constantly comparing the color with the 
color of the liquid in the other flask, the point at which the change 
occurs may be more easily distinguished. Since the color is not at 
all permanent in the case of old and dirty baths, and since slight 
changes of tint are impossible to distinguish in such baths, it is good 
practice to add the iodin solution in quantities of 0.5 c. c. at a time 
when it is suspected that the end point is close at hand, then to mix 
thoroughly and immediately observe the color. This quantity of 
iodin solution is usually sufficient to produce a very pronounced 
change of tint if the end point of the titration really has been reached. 
The final reading should then be corrected by subtracting 0.25 or 
0.50 c. c, whichever is judged nearest correct. The number of cubic 
centimeters of iodin solution needed to produce the blue color, 
multiplied by the strength of each cubic centimeter in terms of 
arsenic, and this again by 4, will give the grams of arsenic per 100 c. c. 
of bath — that is, the percentage. For instance, suppose that each 
cubic centimeter of iodin solution was found to be equivalent to 
0.00239 gram arsenious oxid, and that 18.3 c. c. of iodin solution were 
employed in the titration of 25 c. c. of a bath under examination. 
Then the bath contains 0.00239X18.3X4 = 0.175 per cent of actual 
arsenious oxid. • 
Theoretically, method "A" can not be applied with perfect accu- 
racy to dipping baths, on account of the possibility that substances 
other than arsenious oxid — such as tar and organic matter derived 
from the cattle — may absorb some iodin and thus lead to false results. 
Practically, however, this method has been shown by many tests to 
give useful results on dipping baths of all ages prepared after the 
''standard formula" recommended by the Bureau of Animal Indus- 
try. (See p. 1.) That is, the various ingredients of the tar and the 
organic matter derived from cattle actually do not interfere fatally, 
but absorb iodin so slowly that the end point with starch is obtainable 
without difficulty, although it usually fades out in a brief time. 
Therefore, method "A" is suggested for use in the practical testing 
for ordinary purposes of baths prepared after the "standard formula." 
It must not, however, be employed on baths prepared from any 
proprietary dip unless it is certainly known that the particular dip 
contains no substance which can interfere. Since it is the present 
policy of the bureau to permit for use in official dipping only such 
proprietary preparations as may be satisfactorily tested by means of 
the field outfit later to be described, and since the field .test is practi- 
