C. E. F.
Since I wrote to you last, I have tried a solution of twelve grains only of nitrate of silver to the ounce of distilled water, for the paper albumenized, as mentioned in my letter of the 13th of February, and have found it to answer perfectly. The paper I used was thin Canson, floated for one minute exactly on the solution; but I have no doubt the thick Canson will succeed just as well; and here I may observe that I have never found any advantage in allowing the paper to rest on the solution for three or four minutes, as generally recommended, but the contrary, as the paper, without being in the least more sensitive, becomes much sooner discoloured by keeping. My practice has been to float the thin Canson about half a minute, and the thick Canson not more than a minute.
C. E. F.
Mr. Crookes on restoring old Collodion.—I am happy to explain to your correspondent what I consider to be the rationale of the process.
The colour which iodized collodion assumes on keeping, I consider to be entirely due to the gradual separation of iodine from the iodide of potassium or ammonium originally introduced. There are several ways in which this may take place; if the cotton on paper contain the slightest trace of nitric acid, owing to its not being thoroughly washed (and this is not as easy as is generally supposed), the liberation of iodine in the collodion is certain to take place a short time after its being made.
It is possible also that there may be a gradual decomposition of the zyloidin itself, and consequent liberation of the iodide by this means, with formation of nitrate of potassa or ammonia; but the most probable cause I consider to be the following. The ether gradually absorbs oxygen from the atmosphere, being converted into acetic acid; this, by its superior affinities, reacts on the iodide present, converting it into acetate, with liberation of hydriodic acid; while this latter, under the influence of the atmospheric oxygen, is very rapidly converted into water and iodine.
I am satisfied by experiment that this is one of the causes of the separation of iodine, and I think it is the only one, for the following reason; neither bromised nor chlorised collodion undergo the slightest change of colour, however long they may be kept. Now, if the former agencies were at work, there is no reason why bromine should not be liberated from a bromide as well as iodine from an iodide; but on the latter
supposition, could take place, the affinities of acetic acid being insufficient to displace hydrobromic acid.
A great many experiments which I tried last autumn, for the express purpose of clearing up this point, have convinced me that, cæteris paribus, the addition of free iodine to the iodizing solution, tends to diminish the sensitiveness of the subsequently formed iodide of silver. On paper, this diminution of sensitiveness is attended with some advantages, so that at present I hardly know whether to introduce the free iodine or not; but in collodion, as far as my experience goes, I see no reason for retaining it; on the contrary, everything seems to be in favour of its removal.
I can hardly imagine that the increased sensitiveness mentioned by Mr. Hennah is really due to the free iodine which he introduces. Such a result being so contrary to all my experience, I would venture to suggest that there must be some other cause for its beneficial action; for instance, commercial iodide of potassium is generally alkaline, owing to impurities present; the tincture of iodine in this case would render the collodion neutral, and unless a very large excess of iodine were introduced, its good effects would be very apparent. This, however, involving the employment of impure chemicals, is a very improbable explanation of a phenomenon observed by so excellent an operator as Mr. Hennah: there is most likely some local cause which would be overlooked unless expressly searched for.