_The Double Iodide Solution--Purity of Photographic Chemicals._--The
observations of MR. LEACHMAN upon the solvent powers of iodide of potassium
(Vol ix., p. 182.) are perfectly correct, but I believe our photographic
chemicals are often much adulterated. The iodide of potassium is frequently
mixed with the carbonate. DR. MANSELL writes me word, in a comment upon
your note upon his communication, "What I used was _very_ pure, having been
prepared by Mr. Arnold with great care: it was some that had gone to the
Great Exhibition as a sample of Guernsey make, and obtained a medal." I
have this day used exactly seven ounces avoirdupois to make a pint of the
iodizing solution, which, within a few grains, agrees with my former
results. Nitrate of silver, I am informed upon a most respectable
authority, has been adulterated thirty per cent., and without careful
testing has eluded detection; but I am inclined to think our cheapest
article has come in for its largest share of mixture. I have lately
perfectly failed in the removal of the iodide of silver with a _saturated_
solution of what I purchased as hyposulphite of soda, but which could have
been little else than common Glauber's salts; for upon applying a similar
solution of some which was made by M. Butka of Prague, and supplied me by
Messrs. Simpson and Maule, the effect was almost immediate, demonstrating
how much we are misled in our conclusions, from believing we are
manipulating with the same substances, when in fact they are quite
different.