Having diagnosed this case of the lighting system, starting with the symptoms of a dim light in a residence some miles away, and having located the difficulty inside of the boiler of the power-plant, we desire to treat it. The boiler can be "killed," and the scales removed by going into the boiler. It can then be revived by refilling it with water and rekindling the fire. Then, too, let us assume that there are two boilers, and that we can keep the plant alive with one; a low ebb of life, to be sure, but not dead. We will then cool one boiler at a time, go into it, and remove the scale, thus restoring the plant to full efficiency. This method can be used where the boiler may be cooled, but as this cannot be done with the human power-plant, for the sake of our analogy, let us suppose that the steam boiler, like the human body, must always be kept under pressure that it cannot be "killed" and revived. What, then, shall be done? [Sidenote: Removing the cause of the scaly deposit] It is evident that the first thing to do is to cease the use of water containing the solution of mineral, which causes the scaly deposit. This will prevent the condition from growing gradually worse, and may be accomplished by distilling the water before introducing it into the boiler, or, by using rain-water. As to the scale already in the boiler, it must be dissolved, and gradually eliminated, or remain there. There are many so-called "boiler compounds" for the purpose, and every well-informed man in charge of such a "plant" knows how important it is to avoid using a compound that may cause damage to the boiler itself. A "compound" that would attack the steel, as well as the scale, would be a desperate remedy indeed. [Sidenote: One degree of variation in temperature indicates dis-ease] [Sidenote: The marvelous economy of Nature]