346 ON A DIRECT METHOD OF OBTAINING IODINE. 
starch is only l-1000th, that of wheat 1 -500th, whilst that 
of the potato 1 -200th of an inch in diameter. By taking 
advantage of this observation, even considering it as a mere 
approximation (for the granules of the same species vary 
exceedingly in size,) the facility of precipitation is much in- 
creased, favored as we are by the additional circumstance, 
that this is the very species of starch which may be obtain- 
ed at the least, and in fact almost nominal cost. But, fur- 
ther, by adding a solution of ammonia to a solution of the 
neutral acetate of lead, we obtain, as is well known, the 
tribasic acetate of lead, PbO(C 4 H 3 3 )-f 2PbO. By adding a 
solution of this compound to starch, we form an insoluble 
compound of starch with oxide of lead ; and by making use 
of this substance, which can be obtained with great facility 
and at a trifling expense, we can in a few seconds precipi- 
tate any quantity of iodine from a solution ; by taking ad- 
vantage also of the weight and denseness of the precipitate, 
we can decant thejsupernatant liquid, and, if necessary, re- 
peatedly wash the precipitate without loss. A few precau- 
tions will still better adapt the above plan to the end design- 
ed, and facilitate manipulations considerably. Let the po- 
tato-starch be formed as usual, by grating and washing, 
and then strained through a sieve or coarse linen, to sepa- 
rate it from the broken-down cellular tissue ; the turbid 
liquid should now be allowed to repose for a few minutes, 
and the supernatant milky fluid be poured off; the residuum 
is again washed, and treated in the same manner, until the 
precipitate subsides rapidly, and the liquid clears in a few 
seconds; the smaller granules are thus removed. Having 
previously prepared a strong solution of acetate of lead, with 
the addition of caustic ammonia, this is now added, and the 
the compound sought after immediately prepared, which is 
then ready for use, or may be strained through flannel, dried, 
and kept ready for application. 
We now revert to the course which we propose for the 
adoption of the agriculturist. 
