QUESTION 450. _What considerations should determine the source from which a supply of water should be drawn?_ _Answer._ The first must of course be its convenience to the point where the water is to be used; but more attention should be given to the quality of the water than it ordinarily receives, as the use of impure water, or that which contains a considerable amount of mud or solid matter mixed with it, or in suspension as it is called, or has lime or other mineral substances chemically combined with it, will very soon coat the inside of the boiler with a covering of scale, which is a very bad conductor of heat, and consequently the boiler is much less efficient and much more heat is wasted than if the heating surfaces were clear. Besides this loss of efficiency, when boiler plates are covered with non-conducting scale, they are much more liable to be injured by the action of the fire than when the water comes directly in contact with the metal of the plates. Some water, too, has a corroding effect on the metal of the boiler which is very destructive. QUESTION 451. _How can the relative amount of incrustating substances in different kinds of water be determined?_ _Answer._ The relative quantity of solid matter or mud which is held in suspension can be at least approximately determined by simply filling vessels, say large clear glass bottles, with different kinds of water and letting them stand for some time until the solid matter settles to the bottom. An easy method of precipitating the lime and some other salts which are held in solution and which will not settle until they are converted into a solid form is the following: Dissolve in a goblet of pure water (distilled or freshly caught rain water) two or three teaspoonfuls of the _oxalate of ammonia_. Have equal quantities, say a goblet-full of each of the waters to be tested, ranged side by side and marked so as to be identified. Into each of these goblets stir equal quantities of the solution mentioned--about three teaspoonfuls will be enough--and let them stand for a day. The lime and some other salts will be precipitated and fall to the bottom as a powder; and the quantity of this precipitate in each glass will form a very good index of its relative injuriousness in the formation of scale. When the oxalate of ammonia cannot easily be procured, an experiment may be tried, in the same way, by dissolving common white soap, or other pure soap, in a goblet of pure water, and then stirring into the glasses of water to be tested a few teaspoonfuls of this solution. The comparative amount of lime in the water will be shown by the amount of coagulated matter which will be thrown down.[104] [104] Correspondent of the Railroad Gazette. QUESTION 452. _How are locomotives turned around on the track?_