(40) W. P. S., Jr., says: Can you give me a recipe for making _papier maché_? A. _Papier maché_ is obtained from old paper and the like made into a pulp by grinding with milk of lime or lime water, and a little gum dextrin or starch. This pulp is then pressed into form, coated with linseed oil, baked at a high temperature, and finally varnished. The pulp is sometimes mixed with clay (kaolin), chalk, etc.; and other kinds are made of a paste of pulp and recently slaked lime. This is used for ornamenting wood, etc. (41) M. P. B. says, in reply to a correspondent who asked how to prevent his water filter from getting choked up: Fit in the filter, on the top of the charcoal, a piece of board having in the center a circular hole from two to four inches in diameter, according to the size of the filter; force in this a sponge so tightly that all the water has to pass through it first, but not so as to prevent its free passage. This sponge will absorb readily the gross impurities of the water, and can easily be taken out and cleaned once or twice a week. (42) A. C. S. asks: 1. Is there any reason why lightning rod points should always be bright, if the points are kept sufficiently sharp? A. It makes no difference if the points are not bright. 2. If lightning rods put up in the ordinary way above the roof terminate in the eaves' spouting of the house, and said spouting had good ground connections, would this not be equal to the best lightning rod, and thereby save many feet of rod and many dollars of expense? A. The arrangement you suggest would be good. Make a thoroughly good ground connection with leaders, have all joints well soldered, and you may dispense with the rod as you propose. (43) J. A. W. says: Having occasion to do some copper plating some time ago, I dissolved sulphate of copper in water in a glass jar. I then poured it off into my battery, and there was some left in the jar. I threw a small piece of iron into it and left it for some days. I then took it out; and to my surprise, I found that it had been perfectly plated with copper. Please let me know the cause? A. The reaction you note is taken advantage of to cheaply copper plate small articles of cast iron. See answer to J. O. M., p. 347, vol. 36. In the presence of water, the reaction is as follows: CuSO_{4} + Fe = FeSO_{4} + Cu. Sulphate of copper. Iron. Sulphate of iron. Copper. As the iron is a more positive metal than copper, it displaced the latter in combination with acids, the remaining portions of the iron becoming coated with the precipitated copper.