They are based upon the principle of mixing chemicals with the water to be treated, so as to precipitate the matters in solution and give a boiler feed-water free from carbonates and sulphates of lime and magnesia. The chemicals are added in the form of solutions of carefully regulated strength to the water, which flow in a continuous stream into a tank. The flow of the water and chemical reagent is adjusted by previous analysis. The various machines differ in details of construction, and in the methods by which the mixing of the water and reagents is effected. The object to be achieved is the complete precipitation of the dissolved salts and the production of a clear water, free from sediment, in an apparatus that will treat a maximum quantity of water at a cheap rate per 1,000 gallons. The process needs proper attention. The addition of reagents in wrong proportions will do more harm than good, and possibly result in hardening the water instead of softening it. The following may be quoted as an example:-- ----------------------+-----------+-----------+------------ Composition of Water. | Before | After | Change. | Treatment.| Treatment.| ----------------------+-----------+-----------+------------ Calcium carbonate | 13·863 | 38·920 | 25·057 gain Calcium oxide (lime) | 0·0 | 14·300 | 14·300 " Calcium silicate | 2·062 | 3·591 | 1·529 " Calcium sulphate | 1·625 | 2·121 | 0·496 " Magnesia | 0·0 | 0·266 | 0·266 " Ferric oxide, etc. | 0·447 | 0·987 | 0·540 " +-----------+-----------+------------ Scale forming mineral| 17·997 | 60·185 | 42·188 gain +-----------+-----------+------------ Calcium chloride | 1·331 | 2·114 | 0·783 gain Magnesium chloride | 0·672 | 0·0 | 0·672 loss Sodium chloride | 0·478 | 0·476 | 0·003 " +-----------+-----------+------------ Soluble salts | 2·482 | 2·590 | 0·108 gain +-----------+-----------+------------ Total mineral matter | 20·479 | 62·776 | 42·297 gain +-----------+-----------+------------ Carbonic acid gas | 9·71 | 0·0 | 9·71 loss Oxygen gas | 0·66 | 0·66 | 0·0 " ----------------------+-----------+-----------+------------ Treatment required: 1·8 lbs. of lime, 0·2 lbs. soda ash per 1,000 gallons. Apparently 5·5 lbs. of lime were being used and no soda (Stromeyer).