SPECIAL NOTE.--This column is designed for the general interest and instruction of our readers, not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however, when paid for as advertisemets at $1.00 a line, under the head of "Business and Personal." All reference to back numbers should be by volume and page._ * * * * * C.H.G., of N.Y.--To make pure nitrate of silver, dissolve pure silver in pure nitric acid, evaporate the solution to dryness, or, if crystals are preferred, evaporate until the solution is sufficiently concentrated to form crystals. If you can not get pure silver, you may purify it by dissolving coin in nitric acid, filtering the solution and precipitating the silver in the form of a chloride by hydrochloric acid. Next wash the precipitate with hot water until the washings cease to redden litmus paper. Next mix the pure chloride of silver while yet moist with its own weight of pure crystallized carbonate of soda, place the mixture in a covered porcelain crucible and heat very gradually until the fusing point of silver is reached. The reduced silver will be pure and may be removed by breaking the crucible. Wash the button thoroughly with hot water to remove the flux. In dissolving the pure silver thus obtained in nitric acid, it is better to use an excess of acid; the excess will be driven off by heat in evaporation. G.B., of Iowa.--Nominal horse power is merely a conventional expression for diameter of cylinder and length of stroke, and does not apply to the actual power of the engine. It is found by multiplying the cube root of the stroke in feet by the square of the diameter in inches and dividing the product by 47. This rule is based upon the postulate established by Watt, that the speed of a piston with two feet stroke is 160 feet per minute, and that for longer strokes the speed varies as the cube roots of the length of the stroke. It is needless to say this rule is not observed in modern practice, yet the expression, nominal horse power, is like many other relics of past time still retained. The above rule does not apply to high pressure engines. For such engines Bourne has given the following rule: Multiply the square of the diameter of the cylinder in inches by the cube root of the stroke in feet, and divide by 15.6. The real power of an engine is estimated from the mean effective pressure in the cylinder--not the boiler--and the speed of the piston. Your data are insufficient to determine the horse power of your boiler. The horse power of boilers is estimated from the extent of heating surface when the grate and all other things are correctly proportioned, but with them as with engines, only actual test will positively determine it. The pipe you mention ought to be enlarged as proposed.