(1) A sponge employed in photographic manipulations for a few months loses all its valuable qualities, becoming black, hard, and greasy, and contaminating anything which it touches. To clean it, a solution of permanganate of potash in water is prepared of such a strength that it appears of a wine colour, and into this the unserviceable sponge is immersed, and allowed to remain for some time. When taken out and squeezed, it is next put into a diluted muriatic acid of ordinary commercial quality, being immersed and kept saturated therein for sometime as before. The most appropriate strength of this acid solution is about 10 parts water to 1 of acid. The sponge is taken out after sufficient treatment, squeezed well to free it from the acid, and then washed well in good spring water. When taken out, it will be found to be quite clean, to have again assumed its light colour, and to be free from all foreign matter. Sponges treated in this way become like new sponges, and can be used without any fear of their contaminating, even if employed for the filtration of neutral liquids.
The main thing to be attended to in this plan of purifying sponge is to see that it is thoroughly saturated both by the permanganate and the acid solutions, which should be allowed ample time to soak through the mass; care must also be observed to wash the sponges thoroughly with plenty of water at the end of the operation. (Dr. J. Stinde.) (2) When sponges get greasy, let them dry, and then work them with a small quantity of turpentine, and after a few minutes wash them with warm soap-and-water with a little bit of soda. This will get them quite clean with very little trouble. (E. T. Scott.) (3) Put a handful of salt on the sponge, and rinse the salt well through the sponge. Let the sponge dry in a thorough draught of air. The latter precaution alone will keep sponges free from slimi-ness, unless they become saturated with soap. (4) I tried the effect of sulphuric acid as follows: - In a large basin mixed about a pint of water and two table-spoonfuls of sulphuric acid (common oil of vitriol), then steeped the sponge about 2 hours, wrung it out several times in the acid, and finally well washed out the acid in clean water; it was then just like new, having regained its former size, colour, and elasticity, with not the slightest trace of its former sliminess.