ON IODIDE OF POTASSIUM. 
289 
as the equivalent of the acid tartrate of potash is 189.11, one 
pound of this on being ignited cannot produce more than 
5.86 ounces. Now, since 11.04 ounces of carbonate obtained 
from sixteen ounces of the bicarbonate cost Is., which is 
about the price per pound of the latter, a pound will cost Is. 
5±d.; whereas one pound of cream of tartar at lOd. per 
pound, producing only 5.86 ounces of pure carbonate, each 
pound of it obtained in this way must cost 2s. 3d., being 
9hd. per pound dearer than that obtained from the bicarbon- 
ate; besides, the time and trouble required in preparing a 
pure salt from cream of tartar is very much greater than 
when the bicarbonate is used. For these reasons, when 
we can obtain a pure bicarbonate, we use it in preference. 
Having proved its purity, we put 1% ounces troy, reduced 
to coarse powder, into a crucible, and heat it for about half 
an hour at a low red heat, and, after it has cooled sufficient- 
ly, weigh out four ounces, and dissolve in the water poured 
from the flask into the bason, and the rest in two or three 
ounces of hot water in a test glass or phial. 
Previous to this we have prepared the solution of iodide 
of iron; for this purpose we add nine ounces troy of iodine 
to the water in the flask, and then three ounces troy of pure 
iron filings, the iron being in the same proportion to the 
iodine as in the formula of the London college; frequently 
a much larger proportion than this is recommended: so 
much even as double the weight of iodine: but the above 
proportion is fully sufficient, and no useful end can be ob- 
tained by using more. 
The action resulting from the contact of the iron and 
iodine having ceased after brisk agitation, we take advantage 
of the heat produced, and immediately raise it to the boiling 
point over a rose gas-burner, or in any other convenient way: 
we stop the ebullition when the froth becomes white, and al- 
low the liquid to cool down to about 100° Fahr., when we 
immediately pour in the solution of carbonate of potash con- 
vol. ix — no. iv. 
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