10 BULLETIX 
to about 200 c. c, nearly discharge the color of iodin by sodium 
tliiosulphate, add a little starch solution, and continue the cautious 
addition of thiosulphate solution until the blue color disappears. 
Then add standard iodin solution from a burette until the faintest 
possible persistent blue remains. Whichever method is used, the 
solution at this stage should be in a capacious flask, and should be 
diluted to about 300 c. c. Add a few drops of methyl orange, then 
render alkaline by the cautious addition with a spatula of anhydrous 
sodium carbonate. When the solution is clearly alkaline wash down 
any solid adhering to the neck or sides of the flask and add dilute 
sulphuric acid until the reaction is clearly acid, taking care that no 
particles of sodium carbonate remain undissolved. Then add a 
liberal excess of sodium bicarbonate, 10 c. c. more starch solution, 
and titrate with standard iodin in the usual manner. The final 
titration should of course be corrected for the adsorption of arsenious 
oxid by blood-charcoal. Xo additional correction for adsorption of 
arsenic oxid appears necessary. Xo extra addition of potassium 
iodid appears to be necessary in case cresylic acid, etc., is present. 
The above-described methods for actual arsenious oxid and for total 
arsenic have proved reliable for the examination of all products thus 
far permitted for use in official dipping, and it is probable that they 
will be found applicable to any proprietary products which may 
receive such official recognition in the future, though obviously a 
positive statement on this point can not be made. The experienced 
chemist will naturally think of other methods which might be applied. 
For example, the acidified and filtered bath may be treated in a sepa- 
rating funnel with ether or some other appropriate organic solvent 
to extract cresylic acid, fatty acids, etc.; total arsenic may be deter- 
mined after the destruction of organic matter by appropriate diges- 
tion or incineration, or it may be separated by distillation; while it is 
known that some chemists prefer to determine directly arsenic 
existing as arsenic acid by titration with uranium acetate. 
FIELD METHODS OF ASSAY. 
A field method of assay is generally based upon a laboratory 
method; apparatus, reagents, etc., being simplified to the extreme 
in the interests of durability and portability, and the operations 
being reduced to the fewest and simplest, that they may be success- 
fully executed by persons wholly unacquainted with chemistry as a 
science. But a skilled chemist in the laboratory to supervise the 
field tests is even more necessary than if all the work were actually 
performed in the laboratory itself, for upon his experience and care 
reliance must be placed to standardize the methods in such a way 
that useful data may actually result from the manipulation of inade- 
quate apparatus by unskilled hands. Of course, it is only excep- 
