(23) H. P. writes: Please give me the advantages and disadvantages of substituting a galvanized iron tube 18 inches in diameter and 20 feet high for a wood tank, 5 feet wide and 6 deep, as a container of water in a dwelling house in the country. Would the narrower body of water keep fresh or sweet longer, etc.? Also the thickness of iron necessary to safety, and the number of gallons of water this tube would hold. A. The advantages are in favor of the wooden tank; zinc lined vessels (galvanized) are unsuitable for reservoirs for potable water. See p. 369, vol. 36, SCIENTIFIC AMERICAN. 0.3 inch iron would be stout enough. A pipe of the dimensions specified would contain about 327 gallons when full. (24) F. L. M. asks: 1. What is the process by which wire is given a copper finish? A. Clean the wire by pickling it for a short time in very dilute sulphuric acid and scouring with sand if necessary. Then pass the clean wire through a strong bath of copper sulphate dissolved in water. 2. Can wire be thus finished and also annealed? If so, how? A. The wire should be annealed first. 3. What other finish can be put on iron wire (annealed), and by what process? A. Zinc--by passing the clean wire through molten zinc covered with sal ammoniac; tin--by drawing the wire through a bath of molten tin covered with tallow. MINERALS, ETC.--Specimens have been received from the following correspondents, and examined, with the results stated: J. H. McF.--A fine quality of kaolin.--F. C. H.--The floury powder consists chiefly, if not altogether, of calcium carbonate.--C. L. G.--They are all silicious limestones. We cannot judge fairly of their value for building purposes from the powders sent.--D. K.--Ferruginous earth or marl.--A. E.--It is a partially decomposed feldspar. The white powder is for the most part an impure, silicious, kaolin.--E. H.--It consists chiefly of basic carbonate and hydrated oxide of lead--poisonous.--J. B. V.--It is a fair quality of pipe clay--impure silicate of alumina--probably worth about $2 per ton in New York. * * * * * =COMMUNICATIONS RECEIVED.= The Editor of the SCIENTIFIC AMERICAN acknowledges with much pleasure the receipt of original papers and contributions on the following subjects: Religion. By W. M. E. Cause of Explosion in Flouring Mills. By G. M. * * * * * [OFFICIAL.] INDEX OF INVENTIONS FOR WHICH =Letters Patent of the United States were Granted in the Week Ending= =May 28, 1878,= =AND EACH BEARING THAT DATE.= [Those marked (r) are reissued patents.]