The desired qualities in the water-supply must be the first consideration. While in general it may be said that water which is suitable for drinking purposes is suitable for locomotive-boilers, even this statement cannot be taken absolutely. A catalogue of the desired qualities in the water-supply can best be stated by describing the objectionable qualities. It is popularly supposed that an absolutely pure distilled water would be the ideal type of water to use. Apart from the high cost of distillation, distilled water is actually objectionable, since it attacks the iron of the boiler quite rapidly and causes it to rust. The purer the water the more quickly will it attack the iron. A second objectionable quality of water is that caused by the presence of carbonates of lime or magnesia. When water containing these carbonates is boiled, the carbonates are deposited on the surface of the boiler, and since they are poor conductors of heat, the efficiency of the boiler is reduced on account of the lack of heat-conductivity of the scale. This form of scale, however, is not hard and is readily washed out, but such impurities in the water cause trouble and expense in proportion to the amount of them. The sulphates of lime and magnesia are far more dangerous. They deposit on the surface of the boiler in a very hard scale, which is removed with great difficulty. The difficulty of removing them may cause the washing-out process to be ineffective, unless very thorough, and the hard scale may be allowed to accumulate until it becomes very thick. In such a case the situation becomes actually dangerous, since the scale is a very poor conductor of heat. Since the intense heat of the fire is not readily transferred to the water, the iron will become overheated until it may actually soften and give way, causing an explosion. The dangerous effect of such water, and the great difficulty of removing the scale formed by these sulphates, render such waters very undesirable.

Water sometimes contains sulphuric acid, especially if it has drained out of a coal-mine. Other acids may occasionally be found, owing to the contamination of the source of supply by some industrial works. These acids will corrode the iron in the boiler and soon cause deterioration.

One of the most common difficulties with boiler-water is that caused by the presence of the sulphate or chloride of sodium or the chloride of calcium. The presence of these chemicals produces what is known as "foaming." Although the steam-pipe running from the boiler to the cylinder is led from the dome of the boiler, which is purposely made as high as possible above the surface of the water, the foaming of the water will carry more or less liquid water into the steam-pipe and from thence to the cylinders. This results in broken cylinder-heads and pistons, broken valves, and many forms of destruction of the machinery. To avoid this effect when using foaming water, the engineer or fireman will keep the water as low in the boiler as he dares, in order that the surface of the water shall be as far as possible below the dome. In the endeavor to accomplish this, too little water may be left over the crown-sheet, which becomes overheated, and the fire-box is ruined even if the boiler does not explode.

On account of the many evils resulting from impurities, as described above, railroads now generally follow the policy of submitting all proposed sources of water to a thorough chemical analysis, in order to determine their qualities. The actual evils resulting from the use of impure water, which show themselves in the expense accounts in excessive repair charges for the repairs of boilers, fire-boxes, leaky tubes, with an occasional boiler explosion, justify the expenditure of considerable sums of money to obtain suitable water-supplies. The very large increase in recent years in the number of small municipalities which have constructed water-works for their own use, has resulted in many railroads relying on such water-supplies for the supply of their local water-stations. Although it is generally possible to obtain by pumping from a private well or a stream a sufficient quantity of water at a much lower price per gallon than would usually be charged by the municipality, nevertheless it is generally advisable, especially in view of the danger of the future contamination of a well or stream, which may at th present give a suitable supply, to utilize these municipal supplies.

The above paragraphs describe the evils of a contaminated water-supply. The requirements of operation, which necessitate the location of water-stations at approximately fixed places on the line of the road, leave no alternative but to use such water as is obtainable and, if necessary, to treat it chemically before it is used, so that it will not be injurious. In the early history of the Southern Pacific Railroad there were stretches of several hundred miles where water was unobtainable, or else was so alkaline as to be unfit for use. For a considerable time after the road was built it was considered necessary to haul with each train one or two large tank-cars carrying water in order to supplement the supply carried in the tender. Later the same railroad incurred considerable expense, after seeking the most expert geological advice obtainable on the probabilities of obtaining water by sinking Artesian wells, and thereby succeeded in locating water-stations approximately where they were desired by the operating conditions. In ordinary cases a comparatively simple treatment of the water with chemicals will so modify the injurious ingredients that they are virtually rendered harmless, so far as boiler use is concerned, although the water is still very far from being pure water. Of course the method of treatment depends entirely on the nature of the chemical present in the water. A treatment suitable for one kind of water would only render another kind of water still more harmful.