292 
ON IODIDE OF POTASSIUM. 
A moment's consideration will show that the value of 
this could never compensate for the time spent and fuel con- 
sumed in filtering and evaporating three pints of liquid, 
which is the amount ordered by the London college for the 
quantities used in our process. 
To return from this digression: having concentrated the 
solution to the extent mentioned, it will be found almost in- 
variably that the liquid shows a strong alkaline reaction; for 
however carefully the alkaline solution may be added, it is 
almost impossible to avoid an excess; this excess must 
therefore be remedied, and the method we adopt for this end 
is the following: — 
We prepare, in a Florence flask, a solution of iodide of 
iron, from 3iij. iodine, 3j. iron filings, and 3vj. of water; 
with this proportion of cold water, the action goes on with 
perfect safety, but, with very little less, the heat developed 
is so great that the liquid almost instantaneously enters into 
uncontrollable ebullition, and the^whole contents of the flask 
are expelled with a sort of explosion. Infusing the propor- 
tions we have adopted, the heat produced is nearly sufficient 
to bring the solution to the boiling point, so that a few mi- 
nutes' application of the gas-flame renders the liquid suffi- 
ciently clear. 
We do not think it useless to remark here, that the cau- 
tions commonly given, not to apply more than a very gentle 
heat, do not appear to us to be well grounded, and proceed 
from a needless fear of the loss of iodine; the attraction of 
iron and iodine in the presence of water is so energetic that 
they combine almost instantaneously; and, after the action, 
the quantity of iodine, in a free state in the liquid, must be 
very small; and we should conceive that there is less likeli- 
hood of a loss of iodine by applying the heat rapidly than 
otherwise, inasmuch as the currents in the liquid are ren- 
dered more rapid, and thus every point is brought succes- 
sively in contact with the iron diffused in this way through 
the fluid; and, in our opinion, the clearing of the liquid re- 
