"When thus renovated, the collodion will be found as sensitive and good as it was originally."
This plan is certainly more simple than any that has yet been recommended. The action of the silver being its mere combination with the free iodine, thereby producing the reduction of the collodion to its original colourless condition, I would venture to put this question to MR. CROOKES (to whom the readers of "N. & Q." are already under great obligations): Does he consider that it is the mere presence of free iodine which causes the want of sensitiveness in the collodion? This is all which appears to be accomplished by the process which Mr. Crookes recommends.
Now, as one who has had some experience, both in the manufacture and uses of collodion, such a view does not agree with my practice and observation. Occasionally, upon sensitising collodion, I have found it assume a deep sherry colour a few hours after being made. This must have depended upon the free iodide it contained, and yet such collodion has worked most admirably. I have now before me a large body of collodion almost red, and which has been made some three or four months; yet the last time I used this, about a week since, it was just as good as when it was first made. Undoubtedly collodion does more or less deteriorate with age; but here I would observe, that there is an immense difference in the different manufactures of collodion, and which can be ascertained by use only, and not by appearance.
But Mr. Hennah, who has had much practical experience, recommends the collodion to be made sensitive merely by the iodide of potassium; and he said, "if it did not work quite clearly and well, a little tincture of iodine brought it right." Here, then, is added the very thing which MR. CROOKES proposes to abstract.
Again, MR. CROOKES considers the free iodine to be the cause of the colouring of the collodion; will he then kindly explain its _modus operandi_?
As he has on several occasions given your readers the benefit of his great chemical knowledge, I trust they may be favoured by him with a solution of these difficulties, which have puzzled
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