[The aceto-nitrate _may_ be used, but in our own practice we do not do so. It is apt to give an unpleasant brownish colour. The solutions of silver, {650} whether used for albumenising or otherwise, being reduced to a state of chloride by the addition of common salt so long as any precipitate is formed: fine silver may then be readily obtained by heating a crucible, the chloride consisting of three-fourths of pure metal. It is a false economy to use dirty or doubtful solutions, and by adopting the above course the pecuniary loss is very trifling. Our ordinary stoves will not always give a sufficient heat, but any working jeweller or chemist having the ordinary furnace would accomplish it.] _On the Restoration of old Collodion._--Many plans have been suggested for the restoration of collodion when it has lost its sensitiveness by age. In the last Number of the _Photographic Journal_, p. 147., MR. CROOKES proposes "to remove the free iodine from the collodion by means of a piece of pure silver. For two ounces of liquid I should recommend a sheet of stout silver foil, about two inches long and half an inch broad. It will require to remain in contact with the collodion for about two days, or even longer if the latter be very dark-coloured; and in this case it will sometimes be found advantageous to clean the surface of the silver, as it becomes protected with a coating of iodide, by means of cyanide of potassium or hyposulphite of soda. "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.