2S8 
ON IODIDE OF POTASSIUM. 
but this is an element in the process which, as it adds very 
materially to the cost, and is to a great extent unnecessary, 
will not be generally adopted. 
For the preparation of the compound under consideration, 
we use a glass flask, capable of containing six imperial pints 
and having filled it to within a sixth of its capacity with dis- 
tilled water, previously boiled and cooled, pour out into a 
bason so much as to leave the flask half full; our reason for 
this is, that we may have a measure of the quantity of water 
for dissolving the alkaline carbonate, so that we may be able 
to pour the solution into the flask at once, and without hesi- 
tation. 
The carbonate of potash we use is prepared by heating to 
redness crystallized bicarbonate of potash, so as to expel the 
excess of carbonic acid; but as it is most important that we 
should use a pure salt, we test its purity, before considering 
it fit for our purpose ; having, therefore, dissolved thirty or 
forty grains in distilled water in a test glass, and acidulated 
with an excess of nitric acid, we add first nitrate of silver, 
and then nitrate of baryta, or chloride of barium. If these 
merely produce an almost imperceptible haze, we do not 
consider the salt unfit for the end in view; but, if the reac- 
tion be more decided, we reject the specimen as altogether 
unfit for our purpose; if the solution of a fresh portion stand 
these tests, after neutralizing with ammonia the excess of 
nitric acid previously added, it will be still more suited to 
the proposed end. 
The Edinburgh College orders a pure carbonate prepared 
from cream of tartar by ignition; and Dr. Christison, in his 
Dispensatory, supports it in this preference, for the alleged 
reason that the carbonate prepared in this way is cheaper 
than that prepared from the bicarbonate; but this is not the 
case, as may be easily proved by calculation. Thus, the 
equivalent of bicarbonate of potash is 100.39; that of the an- 
hydrous carbonate is 69.27; consequently 16 ounces of the 
first by ignition produce 11.04 ounces of the second. Again, 
