_Filtering Collodion._--Count de Montizon, in his valuable paper on the
collodion process, published in the second number of the _Journal of the
Photographic Society_, objects to filtration on the ground that the
silver solution is often injured by impurities contained in the paper.
It may be worth while to state, that lime, and other impurities, may be
removed by soaking the filter for a day or two, before it is used, in
water acidulated with nitric acid; after which it should be washed with
hot water and dried.